What Size Tankless Water Heater Do I Need? (2,3,4,5,6 Family)

When it comes to the tankless water heater, one of the bigger mistakes is buying a unit that is too weak to satisfy all our hot water needs.

You don’t want one that is too small, nor do you want a tankless heater that’s too big and will needlessly spend energy. Tankless water heater size should be as appropriate to your household hot water needs as possible.

What size tankless water heater do I need? How many GPM tankless water heater do I need for the gas unit and how many kW for the electric unit?

Here’s the deal:

To properly answer what size tankless water heater do you need, you have to figure out two things beforehand:

  1. What are your maximum hot water needs?
  2. How much water per minute (measured in Gallons Per Minute or GPM) can a certain tankless water heater heat up, and by how many degrees?

To correctly size the tankless water heater, we need to make a rough estimation of our maximum hot water needs at any given point.

Most households have the highest hot water needs from 9 PM to 11 PM. That’s when we shower, brush our teeth using a hot faucet, and might even have a dishwasher running.

We need to tally all this hot water needs up. Here’s a useful table of how much GPMs do different water fixtures require:

Fixture Gallons Per Minute (GPM)
Shower 2.0 – 3.0 GPM
Faucet (kitchen, bathroom) 1.0 – 2.0 GPM
Dishwasher 1.5 – 2.0 GPM
Washing Machine 2.0 – 2.5 GPM

Example: If you’re taking a shower (100% flow and 110˚F hot water) and simultaneously use two faucets (100% flow and 110˚F hot water), you will need at least 5 GPM tankless water heater.

Tankless heaters can deliver anywhere from 2 GPM to 12 GPM of hot water. How many GPM do you need? The 5-10 GPM ones are most appropriate for the majority of households. Of course, the cost of a tankless water heater increases with capacity as we have written here.

Note: For low water needs up to 8 GPM, the electric tankless hot water heaters are appropriate. For bigger needs (8+ GPM), you should choose one of the best gas tankless hot water heaters here.

Difference Between Maximum Water Flow And Realistic Maximum GMPs

When looking at different tankless heaters’ specs, you will notice they note the maximum GPMs. In practice, the maximum GMP your tankless heater will realistically achieve can be much lower.

Why the discrepancy?

Because the maximum water flow in GMP is measured by warming up 77˚F water. The inlet temperature of water currently in your pipes matters quite a lot.

In south Texas, for example, inlet water temperature is 77˚F. In Minnesota, for example, the inlet water temperature can be as low as 37˚F. That is an additional 40˚F difference a tankless water heater needs to overcome.

Short calculation: Let’s say we have a tankless heater with a maximum water flow of 10 GPM. In Texas, we can actually get 10 GPM of 110˚F water because the inlet temperature is 77˚F. The heater has to heat water from 77˚F to 110˚F; that’s a 33˚F difference.

On the other hand, the inlet water temperature in Minnesota is 37˚F. To heat water to 110˚F in Minnesota, a tankless heater has overcome a temperature difference of 73˚F degrees instead of a 33˚F difference in Texas.

You’re not from Minnesota or Texas? Here’s an infographic created for Rinnai RU160iP SE+ Series 9 GPM tankless water heater that will give you an idea of what is the maximum water flow in your state (valid of USA):

different ground water temperatures in order to calculate how big of an tankless water heater you need

Here’s another example based on the infographics above: If you live in Florida (77˚F inlet temperature), the Rinnai RU160iP SE+ Series tankless heater will have a maximum water flow of 7.1 GPM. That’s enough to run several showers simultaneously.

On the other hand, if you live in New York (52˚F inlet temperature), the same tankless water heater achieves a maximum water flow of 4.5 GMP. That is the direct result of the difference in inlet temperature.

In New York, the heater has to overcome an additional 25˚F. With the same heater and the same energy expenditure, you will be able to run 2 or 3 showers simultaneously.

Especially with the bigger units, it’s worth noting the running costs as well. You can check how much electricity do bigger electric tankless water heaters use here. Another good informational article about propane units is how much propane do these on-demand hot water heaters use.

What Size Tankless Water Heater Do I Need For A Family Of 2, 3, 4, 5, Or 6?

One of the most questions in tankless water heater sizing is how big a unit you need for a family of several people. Obviously, a family of 3 will need a smaller tankless water heater than a family of 5. But what are the exact GPM (for gas-powered) or kW (for electric) numbers?

Given the variable water inflow temperature, it’s rather hard to pinpoint exactly how big a tankless hot water heater you need. Here is a table with estimates; the intervals are, regrettably, quite large. Households in the Northern US will need bigger units than households in the Southern US, for example:

Number Of Family Members: Gas Tankless Heater Size (GPM) Electric Tankless Heater Size (kW)
What size tankless water heater do I need for a family of 2? 6-8 GPM 10-18 kW
What size tankless water heater do I need for a family of 3? 7-9 GPM 15-23 kW
What size tankless water heater do I need for a family of 4? 8-10 GPM 20-28 kW
What size tankless water heater do I need for a family of 5? 9-11 GPM 25-34 kW
What size tankless water heater do I need for a family of 6? 11+ GPM 34+ kW

These figures are for reference only. Practically, there are several important factors like inflow water temperature and acute hot water needs that determine the exact size of tankless water heater you need.

Here is an expert tip from a San Diego HVAC company: “For a family of four, an ideal gas-powered tankless water heater size typically ranges between 8 and 10 gallons per minute (GPM). This ensures an ample and consistent supply of hot water to meet the demands of simultaneous activities like showering, dishwashing, and laundry.

It’s crucial to consider the climate in your region and the desired water temperature rise to determine the specific GPM capacity required.

In terms of electricity-powered tankless heaters, a suitable range is typically between 24 and 27 kilowatts (kW). Electric units may require a higher power input compared to gas to achieve similar flow rates, so it’s essential to assess the electrical capacity of your home to ensure seamless operation.” (Reuven Adam, Managing Director, HVAC San Diego)

Looking At Specifications Sheets

On specification sheets, you will notice that producers either give a maximum water flow number in GMP or maximum electric power in kW. Generally, gas-powered tankless water heaters come with a GMP number, while electric tankless hot water heaters come with a kW number.

As we’ve seen, the maximum GMP is relative. It depends where in the US you live (because that affects the inlet water temperature). Power (measured in kW), however, is absolute. We can compare how powerful different tankless heaters are by comparing their maximum wattage (as we’ve done in the table of the best tankless heaters below).

All in all, you also have to take into example some financial calculations. You are recommended to check if a tankless water heater is really worth it (we made some calculations) here.

Let’s look at two examples. The first one concerns what size of a tankless water heater you need if you want to replace, let’s say, the 50-gallon water heater. The second one is based on the number of people living (and using hot water) in your household. For example, how large a tankless water heater does a family of 5 need.

What Size Tankless Water Heater Do I Need To Replace A 50 Gallon Water Heater? (Example #1)

Here is how this situation goes: You currently have a 30, 40, 50, or even 80-gallon water heater and want to replace it with a tankless water heater.

The main difference, obviously, is that with a water tank, you have, let’s say, 50 gallons of hot water, and with a tankless, you have on-demand heating of water.

During an average 10-minute shower, for example, you spend about 10 gallons of hot water. If 3 people have a shower, you run a faucet or two, a dishwasher, and so on, you can quickly spend all those 50 gallons.

However, the case with the tankless water heater is different. You don’t have hot water in storage; the powerful heating exchanger in the tankless heater heats the water when you need it with a certain maximum GMP limit.

To replace a 50-gallon water heater, you would, roughly speaking, need:

  • 10 GPM gas tankless heater or at least a 27 kW electric tankless water heater if you live in the northern part of the USA.
  • 7 GPM gas tankless heater or at least an 18 kW electric tankless water heater if you live in the southern part of the USA.

That’s why Rinnai, the world’s best gas tankless heater makes, offers a wide array of models – from 7 GPM to 11 GPM:

choosing size of tankless gas water heater with 7 gpm, 9, gpm, 10 gpm and 11 gpm
Rinnai offers its gas tankless models in several sizes. They are considered the best gas tankless water heater brand as noted here.

Be mindful that this is only a rough estimate. The smart thing to do is buy a tankless heater that is a bit more powerful than the estimated needs. Better be safe than sorry.

What Size Tankless Water Heater Do I Need For A Family Of 5? (Example #2)

If 5 people live in a single household, they can use several faucets or showers simultaneously. When picking the size of a tankless water heater, we have to take this into account.

A household hot water consumer that requires hot water fastest is a shower. 5 people can also simultaneously run several hot water faucets, a dishwasher, doing laundry, and so on.

In short, a family of 5 would need a 10 GPM gas tankless heater or 27 kW electric tankless heater if you live in the northern part of the USA, where the input water has a lower temperature. The tankless heater has to work extra hard to bring the water temperature up to 110˚F or 120˚F.

However, if you live in the southern part, the capacity of the tankless water heater can be reduced by as much as 30%. So, for a family of 5 in the southern part of the USA, that would mean a 7 GPM gas tankless heater or an 18 kW tankless heater should be more than enough to satisfy all hot water needs.

Do be aware that, especially with bigger units, you can save a lot on hot water with tankless water heater circulation pumps. These pumps are quite useful if you want hot water from a faucet or shower quickly. You won’t have to wait for hot water to start flowing, wasting all that cold water during that time.

How Many Tankless Heaters Do I Need?

This is quite a common question, especially for bigger houses.

Here’s the deal:

In most cases, you need only 1 tankless heater for an entire house. However, if you really have a big house (2+ bathrooms), even the big 11 GPM Rinnai gas tankless water heater will not be enough to satisfy all your simultaneous hot water needs.

In these cases, it makes sense to install 2 tankless heaters. The usual combo is this:

Biggest Gas Unit (11 GPM, 199,000 BTU) + Electric Unit

The big tankless heater takes care of the most hot water needs. On occasions when we need a tremendous amount of hot water, the electric tankless water heater is switched on to help out.

Another example is if you have one unit for one part of the house and another unit for the other part of the house. In this case, the piping for both units is separate.

Hope all of this helps.

130 thoughts on “What Size Tankless Water Heater Do I Need? (2,3,4,5,6 Family)”

      • My house has two 50 gallon tanks one on either side of our upstairs in closets. Do I need to get two tankless heaters because of the lines or just one? What sizes would I need?

        • Hello Sherry, you don’t really need 2 tankless units because of the lines; you usually need 2 of them because you need enough hot water. To replace a 50-gallon tank, you need a 7 GPM to 10 GPM tankless heater (depending on where you live). So, the easiest solution would be to buy two 9 GPM units and install them on the existing lines. The best ones are Rinnai gas tankless heaters, you can check them here.

  1. Hi there.
    I live in South Jersey where it looks like the ground water temp is 50F, according to the map.
    It’s a small house with just two people. We use the hot water for showers, dishes and the occasional bath. We do not yet own a washer machine but when we do we will occasionally use hot water in that as well. It will be a small machine, maybe 3cf.

    We want a tankless, electric hot water heather. Any advice?

  2. We have 4 kids, oldest beginning to go into teenage years. So HUGE water needs for a family of 6. We have a traditional water tank now and run out of water if we have 3-4 people showering around the same time. We are in a very cold climate. I am assuming we’d need the largest unit?

  3. We live in Central Florida and are building a two-story, 4200 s.f. house for a family of four (2 adults, 2 tweens). We will have 4 showers in the house but only 3 of them will be used on a daily basis. Right now, the plan is to have a Rinnai RL94E tankless gas water heater.

    Recently, we were told by one of the vendors that we may need two. Is that really necessary? Seems like an unnecessary expense.


    • Hello there, the Rinnai RL94E has a maximum how water flow of 9.8 GPM. Given that you’re located in Florida, you don’t really need 2 of those. In Boston or New York, maybe, but 9.8 GPM should be enough for a 4-person family.

      • Thanks! We asked them about adding a recirculation system and they recommended the RUR 199E, which already has a built-in pump as well as adding a 3/4″ return line. I think having that configuration should be more than sufficient. I have no interest in maintaining 2 water heaters!

        • I question the pairing of tankless, on-demand hot water with a recirculation system.

          Tankless heaters mean that you heat water ONLY while you use it. This ‘demand’ is identified by the heater when water flows through it’s inlet and stops once the water flow stops.
          The idea of recirculation is to maintain hot water in the pipes throughout the house, so that opening a tap provides immediate hot water. This is convenient, yes, but it keeps your “on-demand” water heater in a constant state of flow (in demand). Essentially, you traded maintaining hot water in an insulated storage tank to maintain hot water ‘stored’ in your uninsulated pipes.

  4. What size of electric water heater do I need for 2 persons, washing machine, dishwasher and kitchen sink. I live in the Caribbean.

  5. Hi, I live in the norther zone of illinois. We have 5 people living in a 1800 square foot home. Would be no more than a shower, dishwasher, and efficient washing machine at same time. I assume an 8-9 gpm heater will suffice. Not sure of gas vs electric. What do you suggest?

    • Hello Bert, you’re right, for your situation 9 GPM would be enough. Usually, gas is recommended because it saves costs in the long run (price of gas is much lower than electric) and they are more reliable in general. You can check out the best gas tankless heaters here; Rinnai unit is the best choice.

  6. I have a family of three and use hot water for showers, dishwasher and washer as well as vanity. According to the map my inlet water is 52 degrees so what size do you thank I need. I’m in north east kentucky.

    • Hello Aimee, if you don’t use a lot of hot water simultaneously, 7 GPM would be suffice. However, the safer option is 9 GPM. Hope this helps.

  7. Hi Bert, what size gas tankless wh, exterior mount, Southern California. 2 people, 1 shower, 1 tub, dishwasher, washing machine. And should recommend a specific manufacturer

    • Hello Charles, for a 2-person household with a high initial water temperature (Southern California), you really need a small tankless heater. 7 GPM would more than suffice, for example. About the manufacturers, you really can’t go wrong with Rinnai. They produce the best gas tankless heaters; you can check more about Rinnai and other best gas units here.

  8. Hello Charles. Building a 5200 sq ft home in central Oklahoma with a total of 6 full baths for family of 5 with guest room that is occasionally occupied by in-laws/parents. Additionally have two dishwashers and a washer that are primarily run throughout the day/overnight and don’t anticipate running either during typical shower times. That said could easily foresee 3 showers running simultaneously during morning/evening. Planning on largest tankless hot water heater with recirculation but curious if it would be better to have 2 tankless water heaters. Thanks!

    • Hello Tarek, this is definitely a case for the biggest gas tankless water heater – 11+ GPM. It makes sense to ask about 2 tankless heaters. In general, Oklahoma has a higher-than-average inflow water temperature, and 1 really big heater might be enough. It’s really hard to estimate this. The safe version would be to install 1 unit and see if you have an adequate hot water supply. If not, you can always install 2nd unit; electric ones are much easier to install as auxiliary units. Hope this helps.

  9. I want to replace a 40 gallon water (gas) heater with a tankless water heater. I live in Albuquerque, New Mexico (just the 2 of us). My wife has Alzheimers and I bought a walk in bath tub for her for easy access in and out. The tab capacity is 67 gallons of water. I don’t run any other water at the time she bathes. I usually take a shower after I get her bathed and dressed. What do you recommend? Thanks, Ken…

    • Hello Ken, thank you for describing your situation so thoroughly. New Mexico has a high input water temperature and with just 2 people, you are looking for one of the smaller gas-powered tankless water heaters. A Walk-in tub does require quite a lot of hot water but with a 7-8 GPM heater that shouldn’t be a problem (if a 40-gallon heater was enough, the 7-8 GPM will certainly be enough as well). The best choice, in this case, would be the Rinnai V75IN 7.5 GPM you can get here. It has an indoor installation, it is the right size, and Rinnai is considered the best brand for gas tankless water heaters.

  10. I have family of 6 in Chicago area with 3 bathrooms. I have the opportunity of getting an 8.4 tankless and an 6.4 tankless. Can I add the two different sizes together for one system?

    • Hello Mike, you do have high hot water needs and it makes sense to use 2 units. You can add the two of them together for one system, yes.

  11. Hi! I have a Southern California 2918 sq foot house with three bathrooms. 2 adults and 4 kids. We just purchased our home and are remodeling it, and i am thinking of replacing a traditional water heater with a natural gas Rinnai 199eN. We don’t currently have a dedicated return line, but I was wondering if the 11gpm and the use of the built-in circ pump and thermal bypass that is included would be enough for our family?

    • Hello Elaine, as a family of 6, you do have high hot water needs 6 but Southern California has a very high ground water temperature (up to 77°F, actually). 11 GPM tankless water heater would be more than sufficient. We’re talking about Rinnai 11 GPM RU199en-Natural Gas/11 (this one), right? That’s the perfect choice in your situation. Hope this helps.

  12. Hi, I live alone in Austin Texas. I have an 800 sq/ft home with one shower, one bathtub, one dishwasher, one washing machine, and two hot water sinks. Can you tell me what I need in an electric tankless water heater?
    Thanks so much,

    • Hello Helen, given you have very low hot water needs, you need one of the smaller electric tankless heaters. 10-12 kW should more than suffice. You can check some examples of great electric tankless heaters here; Stiebel Eltron is pretty much the best brand. Check out their 12 kW Stiebel Eltron – Tempra 12 Plus model, that would be a good choice in your situation.

  13. Thank you, this is the most helpful site I have visited concerning this topic. I am building a 3200 sq ft home in southern Virginia with inlet of 52F. Family of 4 using typically dishwasher, washing machine and maybe shower or 2 and one faucet between 6pm and 9pm Builder offers a 7.2 or 9.5 Rinnai but not sure if its worth the expense for the 9.5. Would that be sufficient or should I get the 7.2 and change to 11 gpm later.

    • Hello Alexander, for a family of 4, 9.5 GPM would be a safe choice. The 52˚F inlet water temperature is right in the middle; at 60˚F, for example, 7.2 GPM would be a viable choice. However, given your water requirements (2 showers + 1 faucet max.) you can get away with 7.2 GPM. For 3 simultaneous showers, for example, 7.2 GPM would not be enough. Hope this helps a bit.

  14. Hello l live in far west Texas and need to replace a tank water heater with a tankless water heater. I have a family of three and use dishwasher daily and 2 and half bathrooms. Would a Rinnai V75IN 7.5 GPM be sufficient ??

  15. Hi! I live in Southern California and I have 1 shower ,1 tub (that doesn’t get used) and a smaller bathroom with sink as well as the kitchen sink, no dishwasher. 3 adults live here and take daily showers. What size tankless electric outdoor water heater would I need?
    I would really like to have 2, one by the kitchen and the other on the other side of the house for the shower.
    Thanks Don

    • Hello Don, electric units are not as powerful as gas units, and you’ll probably be better off with 2 tankless electric heaters if you find that more convenient. For 1 shower, 1 rarely used tub and so forth, you would require no more than 20 kW (given that Southern California has a high input water temperature). For specific recommendations, you can check our article about the best electric tankless hot water heaters here; #1 Stiebel Eltron is arguable the best unit, and offers 12kW to 36kW units. The optimum choice would be 1 20kW unit or 2 12kW units (for the kitchen and other side of the house). Hope this helps a bit.

  16. Hello!
    Thank you very much for this article! Definitely the most helpful resource I could find on that topic so far.
    We live in San Diego, a family of 3, 2 bathrooms, dishwasher, situations with 2 showers running at the same time are very rare. We’re eyeing Rinnai RU130iN vs Rinnai RU160iN. What would be your recommendation between the 2? Or even maybe you can suggest a totally different model…
    Again, thank you for this great resource!

    • Hello Oleg, for your situation the 13 GPM will be more than enough. Rinnai does produce the best units, and natural gas is pretty much the most economic way of heating up water. Rinnai RU130iN would be an adequate choice. Thanks for the compliment, we do try to help everybody out as best we can.

      • Sorry, I think there is a typo there, Rinnai RU130iN has 7 GPM rating.
        Another interesting point is most individual contractors and water heater installation companies recommend Navien and Norits over Rinnai, at least in our area. I suppose, it’s because of simpler installation, maybe?

        • Hello Oleg, you’re completely right; 130iN has a 7 GPM rating. Typing too quickly, sorry about that. There are many factors that come in why an installation company recommends a certain product; Noritz units, for example, are easier to install, that’s true, and that may be the main factor.

  17. This has been a really informative article! I just want to make sure we’re not going overkill on our tankless water heater though… We’re in Canada (eastern Ontario), two people, but would likely end up using washing machine and either bathtub or shower at the same time, possibly also running the tap in the kitchen sink. Or substitute dishwasher for washing machine. I was looking at ordering a Rinnai RU199iP. Is this recommendable, or would you suggest stepping down to the RU180iP?

    • Hello Cindi, thank you. In Canada, you’ll like have the ground water temperature of 35°F or even lower. That means you’ll have to have a powerful tankless water heater to increase the water temperature adequately; gas-powered units are the right choice and you can’t go wrong with Rinnai. It’s very likely that RU180iP will be sufficient for two people.

  18. Hello, we live in north Texas in an old farm house. We are in the process of adding an addition onto the house ,which is a large bathroom. Is it possible to get a tankless system to to supply that room only? There are only 2 of us , but we have a large bathtub that holds 50 gallons of water.

    • Hello Kim, it’s possible for a tankless water heater to only service one room. You have to talk with the professional installer to get the plumbing correct, and that can be quite costly.

  19. Hello I am looking to upgrade to an electric tankless water heater. I own a mobile home. One bathroom with one faucet and one shower. Kitchen has one faucet and a dishwasher. And an washer. Just me and my wife. Now all signs point to low volume but we use a lot of water . We do a lot of laundry and dishes and long showers. Basically what i am tryin to figure out how much 2 people could possibly use. Thank you do or any input you could offer.

    • Hello Robert, the key to sizing a tankless heater is to figure out how much hot water you need simultaneously. So, even if you use a lot of water, but you use it throughout the day, you can get a very small electric tankless heater. The major component is also the input water temperature. Considering all the available data here, the 20 kW electric tankless heater should be enough. You can check our post about tankless electric heaters here; #1 Stiebel Eltron is available in 20kW capacity, for example.

  20. Very useful site, thanks! I’m considering a seasonal outdoor 80 gallon whirlpool bath installation. 55°F water temperature. I already have a Rennai propane tank for the house. Do you think I can use some of the “camp” propane models just for the tub? What would be the lowest gpm I should consider?

    • Hello Roger, well, a lot depends on the GPM of the faucet that will be used to fill the whirlpool bath. For smaller hot water needs, it’s advisable to install an electric tankless heater (cheap but can spike electricity costs). If you opt for a gas tankless heater, the smallest Rinnai propane unit has is a 7 GPM model. In most cases that should suffice, but again, it does depend on what kind of faucet you have (in terms of GPM) for filling the bath. Hope this helps.

  21. Hello. I live in upstate NY (42F water intake zone). My house has one bath, dishwasher, washing machine and my personal sanitation needs. My house could accomodate 3 people for the same usages. My tub faucet outflow is 4 GPM.What size tankless gas unit do I need for BTU’s and GPM to satisfy that potential occupancy?. Thank you.

    • Hello Mike, the intake temperature (42F) is quite low. The tub faucet outflow of 4 GPM is quite high. If you want to use some of these appliances simultaneously, you would require quite a sizable gas tankless heater. Something in the neighbourhood of 9-11 GPM. You can check which are the best gas tankless heaters here; Rinnai is universally considered the best gas tankless heater brand.

  22. Hi,
    I have a 2 bed/2Bath short term vacation rental in Key West. Sleeps 6 max. It has a washing machine and dishwasher. It currently has a 80 gal. electric water heater and we have never had a complaint regarding compacity. It is very important that a tankless unit handle everything running at the same time as it is quite likely that will happen regularly. Based in the article above and using some approximate percentages, I calculated that a 30kW electric unit would be effective. Do you agree or have a recommendation?

    • Hello Kevin, you have quite high hot water needs. The real advantage you have is that you’re located in Key West, Florida, where the ground water temperature is relatively high. 30 kW might be enough. A yet better would be a 10+ GPM gas tankless water heater (you can check some of the best gas tankless heaters here; Rinnai is the #1 brand). Gas units might cost a lot more than electric units but the low cost of gas compared to electricity will save you a lot more on electricity costs in the next 10-20 years. Hope this helps.

  23. I am adding a bathroom to a barn. I am considering using 2 tankless heaters. One for the shower and bathroom faucet. Would one that heats 2.5 GPM be enough for that?
    The other would just heat the water for a kitchen sink? How much for that. This is a room that would not get much use but I do like to have hot water. Thanks.

    • Hello Vickie, a lot depends on your ground water temperature. But since the you have very limited needs, a small electric tankless heater would be sufficient.

  24. Hi, this site provides the best information. Thank you!

    I have 2 50 gal water heaters . They all need to be replaced. The plumbing company recommended one Navien 190k BTU. We have 3 showers and a jacuzzi which is rarely used. Winter ground water temp is 47F. Peak demand could be 2 showers, 1 faucet, 1 washer and 1 dishwasher at the same time. Is one tankless enough. Do I need a recirculating unit? My house was built in 2002, plumber said the current vent pipes are cellular core PVCs which are not up to code any more, so pipes need to be replaced as well. Is it necessary?

    Thank you!


    • Hello Helen, thank you. For residential use, the maximum allowed tankless water heater size is 199k BTU. You have considerably hot water needs; that’s why the recommendation is the 190k BTU Navien unit. You need that size if you want to have only 1 unit. The real question is, as you have neatly figured out, do you need a 2nd unit. Given your needs, 1 big 190k BTU unit should be enough. Filling up a jacuzzi might cause some problems but it’s rarely used. The best solution here is to get a 190k BTU Navien unit. It should be enough; if you see that you would need a second unit, you can always get it (a smaller one). Hope this helps.

  25. Hi,
    I have two gas 50 gal water heaters. They are side by side and connected so that when we use the first 50 gals., we still have another 50. 3500sf house in Reno, NV with d/w, washing machine and large bathtub (72 gals). At peak usage we had four kids with washing machine and d/w in constant use. Never ran out of hot water even with insanely long teenage showers. What do you recommend for a gas, tankless heater to replace this set up?
    Thank you.

    • Hello Kerry, ‘insanely long teenage showers’ just made my day. 🙂 On a serious note, given very large hot water needs, you should be aiming for the biggest allowed residential gas tankless water heater (199,000 BTU unit). Rinnai makes the best ones, these are 11 GPM units. You can check our article about the best gas tankless heaters here for some suggestions. Basically, you can check the #1 Rinnai unit and get the 11 GPM unit; they have natural gas and propane option, choose the one that works best for you. Hope this helps.

  26. hi there trying to hook up a hydronic heating system. I have it all hooked up and running of a 4 gal hot water tank coming out of the tank at about 125%F Everything works great however at 3-4 gal a min cycle the pipes cool in 3-4 min. Can one get a instant electric heater to tie into this system to raise the temperature and give me a longer time to heat. the temperature ingoing say about 90 and outgoing 140

    • Hello Kim, if you need extra water heating, you can install an instant electric heater to do that job. You’ll have to consult an HVAC expert and get the on-spot assessment for the details.

  27. Thank you for this article, i learned a lot by reading it, plus questions posted.

    My question in on the recirculation of hot water and how that works.

    I have a large older home with hot water lines going in various directions from the existing source.
    Will i need all of those lines to be connected to a manifold system for recirculation to properly work?

  28. I’m trying to size a gas tankless water heater for my girlfriends house. It’s about 2,500 sq ft with 3 bathrooms, dishwasher, and washer. There are 4 adults living there with similar showering schedules. We live in Victorville California. I know it’s Southern California and the water inlet temperature is good, but it gets freezing in the winter here. The main problem is in the master bathroom it doesn’t seem to get hot enough when showering. I turned the water heater all the way up and it doesn’t seem to help. Any insight would be helpful. Thank you in advance.

    • Hello Doug, those ‘similar showering schedules’ will increase the size of a gas tankless water heater substantially. Given higher-than-average simultaneous hot water needs, you would need a big unit – 10 GPM might be too small, 11 GPM would be most adequate. If you need some suggestions, you can check our post about the best gas tankless heaters here; the Rinnai units are available in a big size range, including the 11 GPM unit. Hope this helps.

  29. Hi, thank you, your website is very helpful.
    I live in Santa Fe, New Mexico, I need to change my 40 gallon water heater tank and I am thinking about electric. Usage is for 1 person with the occasional friend visiting. My home is 600 square feet, with one kitchen faucet, one bathroom faucet, one low-flow shower head, one bath, one small Miele washer. I use the shower everyday but take a bath maybe once a month, not even. I love to cook though!
    I am looking at the Stiebel. Which one would be good for me?

    For gas tankless, what do you recommend? I was told that it has to be sized properly and that if I go too high (199,000BTUs) this might damage the pipes and appliances.
    Thank you for your help.

    • Hello Brigitte, for a 1-person household, you would require a small electric tankless heater. Gas units are usually meant for bigger houses; 199,000 BTU is, in fact, the biggest available tankless unit for 3,000+ sq ft houses.

      Now, for the choice of a small tankless electric unit: Stiebel is actually the best brand for electric tankless heaters. You can check our article about the best electric tankless heaters here and you’ll see that Stiebel Eltron is at the #1 position. Given the 1-person 600 sq ft home, you should choose either a 15 kW or a 20 kW unit. That will be more than adequate size for your hot water needs.

  30. 1700 sq ft house w/ two bathrooms, standard showers in both bathrooms and a Koehler walk in tub in the master bathroom. Typically, there are two of us but occasionally up to 4 persons. High efficiency washer and dishwasher. Gas is preferred but I don’t know the difference between recirculator and pump. Help

    • Hello there, the recirculation pump doesn’t really decrease your GPM needs; its purpose is to decrease the time the hot water reaches a faucet, shower, and so on. The size of a tankless water heater you need depends on where you live (or the temperature of the intake water). For a 1,700 sq ft house with up to 4 persons, you’re looking at an 8-10 GPM unit. If you need some recommendations, you can check the best gas tankless water heaters here; Rinnai is usually the #1 choice.

  31. Hello! I have a 8 mo old home in Louisville, KY, area with an essentially brand new 75 gal tank w/ recirculating pump. Family of 3 with 2 guest rooms, total of 4 bathrooms potentially used for bathing. Master shower is sized for 2 with a rain head, 2 standard shower heads, and a hand wand…each on 1/2” pipe with 2x 3/4” valves supplying the water (one pair off each valve). We, of course, have a washing machine and dish washers, but can easily deconflict their use times. Currently have issues running out of water toward the end of a shower in the master, or if we have house guests. Looking to be able to run the equivalent of 3 showers simultaneously with additional showers in rapid succession. Don’t necessarily want to throw away the existing tank, so was thinking of adding a high-gpm tankless upstream of the traditional water heater to backfill as water is used. Thoughts?

    • Hello Steven, thank you for all the details. The 2 guest rooms here are tricky as far as adequately determining how many GPM tankless water heater you need. When you have guests, you will have an acute need for higher GPMs; without the guest, those higher GPMs are unnecessary.

      Not throwing away the existing heater is a good choice. What you most probably need is an additional smaller tankless heater (electric heater might be a good choice) that can bridge that extra GPMs gap when you have higher-than-average hot water needs (guests). For 3 showers simultaneously in Kentucky you will need quite a lot of GPMs. The best choice here is to call a hands-on HVAC guy to estimate the size of the second tankless water heater on the spot. They will also help you with the new unit and installation. Hope this helps.

  32. Hello, thanks so much for this site!

    I’ve had a Robinson tankless gas heater for 18 years, not sure of the GPM rating, but was told it was a “3 appliance”. I’ve never serviced the unit, didn’t know I should have been. This is a small 2 person household in South Dakota, with 2 bathrooms (only one with a tub that’s never used for baths), kitchen, washer and dishwasher, none of which are usually ever run simultaneously, but lately my shower temp fluctuates wildly, making me shout out to my brother to “stop running the sink!!” It has always taken soooo long to get hot water in the kitchen, which is directly above the heater, and now seems worse. When the water finally gets hot, you rinse a dish, turn it off. 2 minutes later you want to wash your hands, the water is cold again, and you wait again. I’m also wondering if my “water efficient” washing machine has ever washed clothes in hot water, since it uses so little water. After reading through these articles, I’m thinking that a recirculating pump could help.
    Whoever installed this heater did not give us the shut-off valves to run cleaner through the unit, (which I didn’t know I was supposed to be doing anyway) so I’m thinking the scale build-up is to blame for the longer wait times, probably the fluctuations, too. (Not my brother, haha)

    If I were to buy a new unit, with a recirculating pump built in, can you still turn off the pump? Would the filtered water at the kitchen sink be warm now, too? Would I not be able to wash clothes in cold now? Just not sure which direction to go, or what size heater I actually need.

    • Hello Cory, there is quite a lot to check here. Yes, the recirculating pump does help you reduce the time you wait for the water (you can read more about it in the article about tankless water heater recirculation pumps here). However, in your case, it might not be helpful because the scale build-up might be the main culprit here.

      Now, tankless water heaters have to be serviced from time to time to prevent this scale build-up, temperature fluctuations, and so on. Before you go ahead a get the recirculation pump installed, check with an HVAC technician for a hands-on look at the whole heater. They will be able to give you an insight into if the pump is actually the solution.

      As for the size, it’s best you check your current tankless water heater GPMs. Check the manual, the model number, and so on. You’ll be able to figure that out on your own. As a general rule, it’s best to get the heater looked at, and go from there. Hope this helps.

  33. Hello, we are replacing our 40gal gas watertank, thinking about tankless. We are confused as to getting electric or gas, giving the fact that gas is super expensive now and we also have solar. We are a family of 4 in a smaller less than 1,200sqft home. We have electric washer & dryer and don’t ever use the dishwasher, but we like to take somewhat longer showers. Which would be more cost efficient? Gas or electric? Thank you!!

    • Hello Kristie, gas vs electric tankless water heater is the standard dilemma. In general, gas tankless units cost more to buy and install, but have lower operating costs due to the lower price of gas. Electric units are cheaper to buy and install but electricity is more expensive than gas.
      In your case, however, you have solar power, right? That means that the electricity cost for you is lower or zero. That would be an argument for electricity. You have to be aware, however, that electric tankless water heaters require very high wattages. A standard electric unit can run on 20 kW of electricity. If your solar system can provide that, go with electricity. If not, you are in a dilemma again; it usually comes down to cost of electricity vs the cost of natural gas/propane in your area. Hope this helps a bit.

  34. Hello we live in Pittsburgh, PA and bought a 1,400 sq ft cape cod style We have 5 people who love to take long showers but I have a 50 gallon gas hot water tank now and me and my fiancé shower for work and we can’t even take our time before the hot water is running out Nobody runs anything prior or showers Need a tankless hot water tank for continuous hot water if we wanted to shower for 24 minutes we could without a problem we wanted the Water heater circulation pump what unit would you recommend? Would you recommend doing 2 instead of 1? We have 2 full baths, dishwasher we run every other day, washer ran every other day doing 3 loads, whole house AC, 5-6 showers a day I don’t know if you need that information so I just gave it Thank you

    • Hello Cindy, thank you for your message. We’re sorry you’ve been getting an error; we’re trying to fix that part of the code now. First of all, you do have quite large simultaneous hot water needs. You would need a gas tankless heater, a big one. The biggest residential units are 11 GPM 199,000 BTU units; something like 11 GPM Rinnai RU199iN Sensei Super in our article about the best gas-powered tankless heaters is the best option in your situation (check the #1 recommendation there).

      With such high hot water requirements, you are correct to address the question if you need more than 1 tankless water heater unit. The best way here is to get the big 11 GPM Rinnai and see if it covers all your hot water needs. Most likely, it will be enough. If you would still require more hot water, you can always buy a 2nd smaller electric heater (something in 15-30 kW range). Hope this helps.

  35. We live in San Francisco and are thinking of installing solar panels and would like to switch to an electric tankless water heater. We are a family of 4 and have 3 bathrooms. The kids are still young so they don’t shower in the mornings but will do so at some point. What size water heater would you recommend? Do you have any idea how much electricity it would use per month? We run the dishwasher at most once per day and generally use cold water for the laundry. We are trying to get estimates for solar panels and we need to be able to estimate the kWH per month.

    • Hello Francesca, it would be amazing to run an electric tankless water heater on solar. For a family of 4 and 3 bathrooms and high inlet temperature, you would require a 25-30 kW tankless heater. You can check how much electricity does this size tankless water heater use per month here; with 25 kW, you are looking at 400-500 kWh per month, on average.

      Now, the problem here is that a 25 kW unit will, when running at 100% output, draw 25 kW of power. Running it for 1 hour at 100% will require 25 kWH which is quite a lot if you are drawing that power from a battery. However, you will likely not run it at 100% for a whole hour.

      Namely, you have to add about 500 kW per month for electricity needs and figure out if your solar system can generate that big amount of acute electricity draw. Hope this helps.

  36. We live in Annapolis MD and would like to replace out 40 gallon electric water heater with a tankless. We have two bathrooms and house is two story about 1500 sq ft. There are two of us, no kids, just the occasional overnight guest. We use hot water for showers, washing machine, dishwasher (but not daily), and faucets. Wondering what size electric tankless would be appropriate for us. We also wonder how quickly hot water flows from these units. It takes several minutes for hot water to arrive in sinks and showers with our current tank. Thanks for your help.

    • Hello Ellen, with a 2 family household, you are right to choose an electric tankless heater vs gas tankless heater. Given that you are not expecting to have several simultaneous hot water faucets and that ground water temperature in Annapolis MD is about 52˚F, you can look at 15 kW or 20 kW unit. Stiebel Eltron makes the best ones; you can consult this article about the best electric tankless water heaters for some recommendations.

      The 15 kW unit is the most optimum size. However, it’s best to have a bit of additional capacity just in case, so a 20 kW unit would be the safe bet. Hope this helps.

  37. We are finishing our basement and will have 2 bathrooms, laundry and a kitchen with dishwasher. We have the same upstairs (2 bath, laundry and kitchen).
    We have a 50 gal water heater but are thinking we need another. Would a tankless work and what size would be best?

    • Hello Angelina, tankless will definitely work. It’s a much better choice than a 50-gallon heater; a lot of people are replacing tank with tankless water heaters.
      Size-wise, you would need something in between 7 GPM to 10 GPM tankless water heater to replace a 50-gallon water heater (or to duplicate its effect). It really comes down to where you live and what is the inlet temperature in that area. In the south, you would need 7 GP, and 10 GPM in the north, just to give you some idea. Hope this helps.

  38. Hello,
    Ground water temp is 48. Family of 3 (1 adult and 2 teens) in 4 BR / 2-1/2 BA house. We avg 2 showers/night, sometimes simultaneously and are usually running the dishwasher 9-11pm. Sometimes also the washing machine, usually on warm. My guess is that we could get by with a smaller unit, but I’m concerned about resale. Since it’s a 4 BR house, I anticipate that the next family that buys it will have a larger family.

    • Hello Beth, this is a very smart insight. Given your hot water needs, a smaller unit will do, yes. However, if you sell your house to a family of 4 or 5, they might have to upgrade the unit; lowering the resale value. In such a case, installing a little bigger unit won’t hurt.

  39. Hello-

    I live in Texas. I am looking at installing a tankless gas water heater outside.
    Aside from the people in the family, does the square feet of the house matter any?

    That said. It would be for a 5 family unit.

    Thank you!

    • Hello Nubia, the square footage doesn’t really matter when sizing a tankless water heater. What matters is the simultaneous hot water use and water temperature (Texas has a favorable 67°F inlet water temperature). Due to this high inlet water temperature and presumable average hot water needs for 5 people, you are looking at 9 GPM gas units. You can check the best of these 9 GPM gas tankless heaters here; Rinnai makes the best units. Hope this helps.

  40. Hi! We currently have a 6500+ sqf home 5 baths with 6 people across 3 floors. We have two 50G inline hot water heaters. Our water source is private well. There is no natural gas to the property, only propane and electric. Is a tankless option even feasible for us?
    Thank you!

    • Hello Tara, tankless is certainly an option. Given a very high simultaneous hot water needs, you would do best to go with a propane tankless water heater because they can produce much more hot water output than electric units. The biggest residential propane tankless heater are 11 GPM units (199,000 BTU). In your case, you would probably need more than 1 tankless water heater. A big 11 GPM and a smaller 6-9 GPM unit would make sense here. Hope this helps a bit.

  41. Living in Southern Ohio, assuming 52* inlet, we’re a family of 5.5, 1,200sq ft home, 2 full bathrooms, 1 Kitchen sink and a Washer. We’re all electric, what would you recommend? Two tanks or one big one. Which ones if two ?

    • Hello Nikki, given the specs, you would need a 30+ kW electric tankless heater. Basically, the bigger the better. The best big electric tankless heater is arguably Stiebel Eltron Stiebel Tempra 36 Plus with 36kW power input (you can check it under #1 place in the list of best electric tankless heaters here; make sure to choose the 36kW model). Alternatively, you could go with two 20kW units or 1 20kW unit and 1 15kW unit. Stiebel Eltron is the No. 1 brand for electric tankless heaters. Hope this helps.

  42. I am thinking about buying a small tankless gas unit to serve one bathroom. There will be a shower, tub and two sinks. Everything in our house is run by gas except lights. I want something I can put under the house.

  43. Hi there,

    I’m in Chicago and currently have a 80 gallon gas water heater and we are looking to converting to a tankless system. There are 4 of us, home is 3400 sq ft and potentially could have 2 showers and dishwasher or washing machine going . What size tankless do we need?

    • Hello Austin, for such a big home 2 simultaneous showers, a dishwasher, washing machine you would need a big gas tankless water heater. Chicago has a low input water temperature – about 42 degrees – so getting a 10 GPM or even the biggest 11 GPM gas unit makes sense. You can check these big gas tankless heaters here; Rinnai produces the whole range of them up to 11 GPM (199,000 BTU). These are the biggest allowable residential tankless heaters. Hope this helps.

  44. Thanks for putting up all this valuable information. We have a family of 3 and live in Toronto, Canada in a 1000 sq.ft. home. We have had different contractors recommend different size units. All highly recommend Rinnai as first choice and some, Navien as a second alternative, with NPE-180S2, NPE-210S2, NPE-240S2, recommended by three different contractors and we are now in information overload. 🙂 One recommended a RU160iN and the other said we need the RU199iN because of our cold winters. He also said that although he is qualified to install either, he prefers Rinnai, because Navien has had issues with pressure vessel failures. We need enough water to support showers and occasionally a bath, as well as washing dishes (no dishwasher at this point.) We primarily use cold water for laundry, except for washing towels about once every two weeks. Any comment you can make on the veracity of the recommendations would be much appreciated.

    • Hello Cynthia, gathering as much information from all the different contractors is a smart move. Brand-wise, yes, Rinnai units are usually the best as we have pointed out here in a post about the best tankless water heaters. Those Navien pressure vessel failures are quite unwanted, obviously.

      Now, size-wise, Toronto does have a very low temperature of input water. However, 3 person household, 1000 sq ft and no multiple simultaneous showers will help us decide between RU160iN and RU1999iN. Namely, RU199iN is the most powerful residential gas tankless water heater; with 199,000 BTU/hr heating power. These are usually used for 2,000+ sq ft houses and 5 or more person household. Just having this info, even RU160iN can be a bit big but the contractors do have better on-site info. If the choice is between the 160 BTU and 199 BTU model, definitely go for 160 BTU model.

      P.S.: The argument that winters are cold is a bit deceptive. Inlet water temperature is usually underground water and the air temperature (even if it’s very cold) doesn’t decrease the underground inlet water temperature. Those temps are quite stable throughout the year.

      I hope this helps you get a bit more insight and it will be easier to make the choice now. 🙂

      • Thank you for this detailed explanation and recommendations. It will help me discuss this further with the contractors and make a decision. Our Canadian government has an incentive program in place called Canada Greener Homes grant but it does not cover tankless water heaters, only heat pump water heaters. Through the program, I will be allowed a grant for a heat pump and have decided to go with Bosch IDS 2.0 and a complementary furnace. The contractor was not keen on the heat pump water heaters and recommended the tankless instead. Do you have any feedback you could give me on the heat pump water heaters? Coincidentally, yesterday we had a problem with our gas 50 gallon water heater, which is a rental and the serviceman was able to resolve it quickly. It is an old unit (2005) and nearing the end of its projected life. He seemed to think we would be better served just going with a new gas powered water heater and that for a family of 3, we didn’t need a tankless unit as the upfront cost was high in comparison. Our government is trying to move us away from carbon generating products, thus the incentive only pertains to heat pump water heaters. The contractor I am using for the the furnace and heat pump, although a certified Bosch installer, did not think their heat pump water heater was a good choice because of the cold winters and the ability of the heat pump to generate enough heat. From what I understand, it is a hybrid and uses surrounding air to heat the water. Do you have an opinion of this kind of product in our environment?

        • Hi Cynthia, I completely get why the contractor is not so keen on the heat pump water heater. These are air-to-water heat pumps – they pump available heat from outdoor air and use that to heat up water. Now, if you don’t have cold winters, the outdoor air still has some heat to offer (a refrigerant-cycle system is used to extract heat even at low temperatures). If, like in Canada, you have cold winters, the heat pump will still be able to extract some heat from freezing outdoor air but it will need a lot of electricity to do that. That’s why the most commonly cited disadvantage of heat pumps is that they become less effective (this is measured by COP or Coefficient Of Performance) at very low temperatures; if the temperature go too low, it becomes practically ineffective. That’s why you will commonly see a heat pump + furnace system, right? The furnace is there to provide heat when the outdoor temperature is too low for heat pump to operate effectively.

          Heat pumps do only ‘burn’ electricity, not gas. That’s why they might be perceived as more ‘green’ than any gas-powered tankless water heaters, hence the government’s incentive program incentivises the use of heat pumps instead of gas water heaters (presumably).

          Here’s how you can think about gas tankless water heater vs heat pump water heater + furnace combo:
          – With the combo, you will most likely use the heat pump for 9-10 months per year. In the winter 2-3 months, the temperatures will be low, and the furnace will kick in to handle your hot water needs.
          – With gas tankless water heater, you won’t have problems getting hot water, but you will burn gas in order to get it.

          If you would be living in Texas, for example, heat pump would be an obvious choice (albeit electric tankless heater might suffice there as well). Even in New York, it would make sense. In Toronto, with very low winter temperatures, it’s quite a dilemma. It’s really hard to say which one would be best, you would have to calculate the costs and compare those, and so on. Obviously, contractors are usually very familiar with tankless gas water heaters, they work all the time, it’s tried-and-tested thing; the environmental effect is most probably a priority there. Using heat pumps for heating water is a bit of a novelty; it’s not unusual for a contractor to go with their default tried-and-tested thing, recommending the tankless option.

          As for the gas power water heater vs tankless option, your serviceman is quite right given your situation. Tankless water heaters are a 20-year investment in your home; if you own that home, it makes sense to go tankless. They are quite costly initially but will lower your water heating costs in the next 20 years. If you have a rental, however, you might be paying full initial cost for a tankless unit but will probably not be renting for the next 20 years in order to reap the full benefits of your investment.

          With the gas powered water heater, you start with a much lower investment but you will have a bit higher water heating costs. If you are renting your home, this makes more sense financially.

          I hope all of this makes a bit of sense. 🙂

          • Thanks for all the details you provided. It sure makes my decision easier. I agree that the contractors being familiar with tankless is a big part of why they recommend them over traditional and heat pump water heaters. Considering the high cost of electricity it seems to make sense to go tankless. Thank you so much for giving me material to discuss with our contractor who will be installing the furnace and heat pump. It was he who said that in his training Rinnai (and Navien) recommended the larger unit for Canada but after reading your reply and doing more research online, I am going to go back to him and ask him to price the RU160iN. The difference between his RU199iN and the other contractor who recommended the RU160iN, is over $800 and being on a fixed income, I have to watch every penny. Thanks again for answering my questions. You provide an invaluable service to us homeowners.

  45. Larry

    We live in Indianapolis, IN and just two of us. We have a natural gas, forty gallon tank and rarely use two sources of hot water simultaneously. What would you recommend, 7 or 9 GPM? Thank you.

    • Hello Larry, for 1 shower at a time, 7 GPM will be more than enough. Especially since Indianapolis has a relatively high inlet water temperature.

  46. Hi! We live in Pennsylvania and are building a primary bedroom addition on a second floor. Our current 50 gallon water heater is only 5 years old so I think that replacing/upgrading it because of an additional bathroom is silly. I would like to put a tankless water heater upstairs for just the shower and 2 sinks. Does this make sense? What size would I need? Thanks!

  47. I live is southern Idaho. I have two 40 gallon water heaters in series. My home is 3300 square feet with 4 showers. Would I need two tankless water heaters or just one large one.

  48. Living in Tennessee, have a family of 4, have dishwasher, washer, two bathrooms and kitchen. what size tank do I need?

    • Hi there, Tennessee has about an average inlet water temperature (about 57˚F). With two bathrooms, dishwasher, washer, and so on, it seems like regular 4-family hot water requirements; a 9 GPM gas tankless heater should be fine, 10 GPM is the safe best. Hope this helps.

  49. Hi there. I have a family of 5 with dishwasher, washer, 4 full baths, 1 half bath. I have a 3600 square foot house and It is also very long. I live in Fort Myers Florida, and I DO NOT have gas or a gas/option. Should I get multiple Electric water heaters, or would a single 18KW-ish water heater with an electric recirculation pump be a better option? OR, am I better off getting a large capacity (50 gallons ish) electric water heater in this situation?

    • Hi James, that’s quite a lot of simultaneous hot water needs. In most cases, a gas unit would be a must. But since this is Florida with a high 72-degree inlet water temperature, you will probably be able to use an electric unit. 18 kW, however, would be a bit small. You should probably go with the bigger 29 kW or even 36 kW unit. Here are some ideas on the best electric tankless heaters; the #1 Stiebel Eltron has big around 30 kW models that could be used for a 5 family home in Florida. Hope this helps.

  50. Hey. I live in central NJ near the coast in a condo about 1000sqft. It’s me and one person. We use the dishwasher on occasion, the washer weekly, and one shower regularly. I currently have a 40G electric water heater but I’m looking to go tankless. Any suggestion? I’m not too savvy with this. Thanks for your time.

    • Hi Matthew, tankless usually has better overall efficiency, so that’s a good change. You would probably need one of the smaller electric tankless water heaters since your simultaneous hot water needs are not all that high. Something like 15 kW or 20 kW Stiebel Eltron would be adequate here.

      We have a post about best electric tankless heaters here; you can get a bit more insight into these Stiebel Eltron units there. They cost about $600-$700, are quite easy to install (especially compared to gas ones) but you will need the upgraded 240V circuit to handle all that wattage. Hope this helps.

  51. Hello,

    We live in Ohio and have 3 people in our 1200sq house. Our current water heater says 240 volt/50 gallon.

    2 questions:
    Would I need to update the electric going to to a 27 or 36 kw unit or could I use the same breaker? Also, would a 5-6 GPM(27kw) work or would I need to go with 7+ (36kw)?

    • Hi Kyle, almost all storage water heaters (30-gallon, 50-gallon, even 80-gallon) use the same 4,500-watt heating element. If you have a 240V circuit, this heating element would draw 4,500W / 240V = 18.75 amps. The breaker can be loaded only up to 80% of its max. capacity, so the minimum breaker you have should handle 18.75A/0.8 = 23.44 amps. This usually means that you have a 30A breaker (correct me if this is not the case).
      Now, the 27kW or 36kW electric tankless heaters draw quite a lot of power. You will probably keep the 240V circuit, but the amp draw for these units would be 27000W / 240V = 112.5A and 36000W / 240V = 150A, respectively. If we apply that 80% rule, you are looking at the minimum amp breaker size of 140.63A and 187.5A, respectively. So, your current 30A breaker won’t cut it. These are very powerful currents, on par with electric vehicle chargers, so you need plenty of additional amp breakers.
      About size: Ohio has an about average 55°F inlet water temperature. For a family of 3, the 27 kW should be quite enough. Hope this helps.

  52. Hi. My husband and I are empty nesters, living in a 1250sf/3bedroom/2bath house in central NC. Grown children visit irregularly. We are both still working – he from home and I commute – and hot water needs differ daily. Sometimes, several fixtures run simultaneously. In the future, a family of four could live here and also simultaneously run several fixtures. Which would be a reasonably-sized tankless WH for present and future needs, 9 or 10?

    • Hi Kathy, well, we are talking about 2 person household now and 4 person household in the future. Given that this is central NC with an about average inlet water temperature, your suggestion of 9 GPM or 10 GPM is very reasonable. For starters, it might be more prudent to go with a 9 GPM unit. It will be more than enough for two people, ample for three people, and reasonably adequate for four people if you manage the showering schedule well. Hope this helps.

  53. hi
    leaving in New Orleans.
    4500sq ft house.
    4 people
    4 showers
    mostly 2 same time shower, sometime 3 but vert rarely.
    dishwasher, washer machine.
    2 sites for tankless water heater.

    what size do you recommend ? PS showers are mostly on same side of house, opposite laundry/kitchen

  54. This has been a really informative article
    we are big family living in a big hose ( 20000 squire feet)
    13 people
    14 bathrooms
    2 kitchens
    inlet water temp 70
    what will the right electrical system with circulation pump
    currently we have 2 X 80 gallons

    • Hi Ahmed, that’s quite a big house. Alright, most standard water heaters are powered by a 4,500W heaters, running on 220V. So, you are looking at an electric system that can handle 9,000W at 220V. The max. 40.9 amp draw. Accounting for 80% breaker rule, you would need amp breakers with at least 51.1A capacity. In practice, you go with two 30 amp breakers. Hope this helps.

  55. I’m confused about GPM. I’m looking at a Rinnaie High Efficiencey Natural Gas Tankless that says it’s a 9.4 GPM, but when I look at its Energy Guide PDF, it says the Capacity (max flow rate) is only 4.9 GPM. I don’t understand the difference. Which figure do I go by? Central SC, 2 adults in the home, might shower at the same time, is this tankless water heater sufficient? Thank you so much!

    • Hi Robin, this confusion is quite understandable. Namely, the hot water flow (measured in GPM) depends on two key things: A) inlet water temperature, and B) water temperature increase. When manufacturers like Rinnai name their units, they usually predispose very high inlet water temperature and very low water temperature increase to get that max. GPM flow as high as possible.

      The more objective metric to check the gas tankless water heater size are the BTUs. Residential heaters are allowed to have up to 199,000 BTU heat-producing element. That means that the 199,000 BTU unit is the biggest; these units are usually labeled as 11 GPM.

      For central SC and 2 adults, 2 simultaneous showers, the 9.4 GPM (or 4.9 GPM according to the EnergyGuide) should be more than enough. Hope this helps.

      • SOOO helpful. Thank you! Do you think 199,000 BTU is too much? I’m also looking at Rinnai RL75EN which is 180,000 BTU. I know it’s not efficient to get one that’s larger than you need.

        • Hi Robin, for 2 simultaneous shower (max. usage), the 160,000 BTU model is very likely sufficient. The 199,000 BTU is definitely too big; it’s the biggest there is.

          • Is this sufficient: Rinnai High Efficiency 7.5-GPM 180000-BTU Indoor Natural Gas Tankless Water Heater? I think it’s a Rinnai V75IN but I’m not certain of the model number. Is it too much? It’s much less expensive than the model I was looking at.
            I can also get a great price on a V65IN 150000 BTU. Which would be better for our needs?

          • I can get a great price on either of these:

            Rinnai High Efficiency 7.5-GPM 180000-BTU Indoor Natural Gas Tankless Water Heater Model #V75IN

            Rinnai High Efficiency 6.5-GPM 150000-BTU Indoor Natural Gas Tankless Water Heater Model #V65IN

            Is the 180000 BTU too much? Is the 150000 BTU enough? Would these work?

          • Hi Robin, the problem with sizing happens only when you have an undersized unit. If you have a bit oversized unit, the only con is that it costs a bit more. So, in this case, and to be safe, the 180,000 BTU would be a good choice.

  56. Hi,

    I have a 1300 sq. ft. ranch house in Chicago with two bathrooms. Possible maximum simultaneous usage is expected to be two showers and either a laundry washer or a dishwasher. I like my showers to be on the hot side.

    I want to install a tankless electric water heater. Is 27kw the right size? If not, what size do you recommend? Which brand, if any, do you recommend?



    P.S. Thanks for all of the great info and insights.

    • Hi Joel, thank you for all the information. Two (hot) showers and a laundry washer does required quite a big tankless water heater. It this would be a regular load, it would be best to go with a bigger gas unit. However, if you opt for electric unit, it should be quite big (your suggestion – 27kW – is quite on point, 29kW or even 36kW would safer for 2 simultaneous hot showers).

      Brand-wise, you can’t go wrong with Stiebel Eltron for electric unit. We have a list of the best electric tankless hot water heaters here, and you can see that Stiebel Eltron in positioned on #1 place for a good reason. It’s basically a go-to choice for electric units. Hope this helps.


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