What Size Heat Pump Do I Need? Heat Pump Size Calculator

Sizing a heat pump seems like a daunting task. How do I calculate what size heat pump do I need? How many BTU heat pump do I need? This is usually an estimate that you need an HVAC expert for.

We are going to simplify how to calculate what size heat pump do you need. That will enable everybody to roughly estimate the size of the heat pump (be it a mini-split heat pump or ground-source heat pump). We will do that in 3 key steps:

  1. First, we’ll look at how HVAC experts size heat pumps (using 8 factors from Manual J; the method was established by Air Conditioning Contractors of America).
  2. Then we’ll simplify these 8 factors with a useful rule of thumb (boiling down 8 complex rules into 1 simple general rule of thumb so that everybody can roughly estimate how big a heat pump should they get). Based on that, we designed the Heat Pump Size Calculator (check further below; you just input square footage and ceiling height, and it will give you roughly BTU you need). We also included a table of how big your mini-split heat pump should be given certain square footage.
  3. To demonstrate how Heat Pump Size Calculator works, we’ll solve 2 examples; ie. calculating what size heat pumps do you need for a 2,500 square foot house.

In the end, you should be able to roughly estimate (and calculate) how big a mini-split heat pump or how big a ground-source heat pump you need for your house.

Note: Do keep in mind that these are only rough estimations that can help you get a feeling of what size heat pump you require. An HVAC expert is required on-site to map out your home, determine heating/cooling needs, and so on for a specific situation you have.

Let’s look at how HVAC experts size a heat pump:

How HVAC Experts Calculate Heat Pump Size (Using Manual J)

Every HVAC expert calculates what size heat pump you need using the same set of principles. When you need heat pump sizing, they all know to look at the Manual J (the alpha and omega book of HVAC sizing) and follow the 8 rules.

These 8 rules include all the main and secondary factors when it comes to sizing any heat pump. They work both for sizing air-source heat pumps (these are mini-split heat pumps) and for sizing ground-source heat pumps.

Here are the 8 rules or factors from Manual J that HVAC experts should follow when sizing a heat pump:

  1. Determine the local climate (+ how many days a year you need heating/cooling). Obviously, if you live in Chicago, you will need a bigger heat pump than if you live in Miami, Florida. In general, colder climates require higher heating output (measured in BTUs or kW).
  2. Total square footage; one of the most important determining factors for sizing a heat pump. Consider the room distribution and general home’s layout as well.
  3. Windows; how many they are, where are they located?
  4. Occurrences of air infiltration; where it is and air infiltration quantification.
  5. Insulation quality; how well insulated is the house, does it meet the region’s efficiency rating?
  6. People; how many people live in the house?
  7. Temperature preferences; what is the ideal temperature of the home for homeowners?
  8. Heat generating appliances; which appliances generate additional heat (oven, fridge, washing machine, etc.)? Sum all of them up and quantify the overall effect on the indoor temperature.

Now, it’s rather complex to determine the effect of all of these factors. These 8 rules were meticulously put together by Air Conditioning Contractors of America and it’s a standard part of Manual J.

Don’t be taken aback by the complexity of everything you need to check to size a heat pump; even HVAC experts who have been in the field for 10+ years use certain simplifications.

Let’s boil these 8 factors down into 1 simple rule of thumb:

How To Approach Heat Pump Sizing On Your Own? (1 Rule Of Thumb)

Some of the factors in Manual J cancel each other out. Example: You can have a badly insulated home but don’t have many windows, and have several 1,000W+ home appliances (oven, washing machine, etc.).

The key determinant of how big a heat pump you need is how big your house is. The bigger the house, the bigger the heat pump you need, right?

Considering all the factors, we can roughly boil them down into a single rule of thumb. This rule of thumb comes very useful when you want to adequately determine the size of the heat pump you need.

Here’s the 1 rule of thumb:

30 BTU of heating output per 1 sq ft of living space.

This heat pump sizing rule is fairly simple to use. It roughly incorporates the averages from the rules found in Manual J. This rule is akin to the EPA’s rule of thumb for sizing air conditioners and it also relates to the heating BTU calculator.

For every sq ft of living space, you need about 30 BTU of heating output. That means, for example, that for a 1,000 sq ft home, you would require a 30,000 BTU heat pump (that’s a 2.5-ton heat pump).

We can use this simple rule to create the Heat Pump Size Calculator:

Heat Pump Size Calculator (Just Input Square Footage)

Another key parameter is the ceiling height. 30 BTU per sq ft rule of thumb is for standard 8 ft high ceiling. If you have higher ceilings, you will need a more powerful heat pump, and vice versa. Here is the calculator:


Using this calculator, everybody can roughly estimate what size heat pump they need. This is primarily a mini-split heat pump size calculator but can be used to roughly estimate the size of ground-source or even water-source heat pumps.

The heat pump size calculator outputs the resulting size of a heat pump in BTUs (British Thermal Units). You can simply convert that into:

  • Tons (in the US, the capacity of heat pumps is usually measured in tons). Use 12,000 BTU = 1 ton conversion, or make use of the BTU to tons converter here.
  • kilowatts or kW (in Europe, Asia, and rest of the world, the capacity of heat pumps is usually expressed in kW). Use 3412 BTU = 1 kW conversion, or make use of the BTU to kW converter here.

Note – how to make the estimate even more precise: If you live in the cold north (Canada, Illinois, Minnesota), it makes sense to add up to 40% to the total output heat pump capacity calculated by the Heat Pump Size Calculator. If you live in the hot south (Florida, Texas, South California), you can reduce the total output heat pump capacity BTUs by as much as 30%.

Using the calculator, we can create a table that specifies how big a heat pump you need depending on how big your house is (ie. square footage):

Heat Pump Sizing Chart By Square Footage

Home Size: Heat Pump Size (In BTUs): Heat Pump Size (In Tons):
300 sq ft 9,000 BTU 0.75 tons
500 sq ft 15,000 BTU 1.25 tons
750 sq ft 22,500 BTU 1.88 tons
1,000 sq ft 30,000 BTU 2.5 tons
1,500 sq ft 45,000 BTU 3.75 tons
2,000 sq ft 60,000 BTU 5.0 tons
2,500 sq ft 75,000 BTU 6.25 tons
3,000 sq ft 90,000 BTU 7.5 tons

You can see for the heat pump size table that, for example, a 2,000 sq ft house requires around 60,000 BTU or a 5-ton heat pump.

Let’s solve two examples to illustrate how you can calculate what size heat pump you need manually and via the calculator:

What Size Heat Pump Do I Need For A 2,500 Square Foot House? (Example 1)

Let’s say you have a bigger 2,500 sq ft home and you want to buy a heat pump for it. How do you adequately pinpoint how big a heat pump you need?

Well, you should call an HVAC expert, and he or she will use the 8 factors in Manual J to calculate the required size of the heat pump. For you to get a feeling of how many BTU heat pump you should be considering, you can use the simple 30 BTU per 1 sq ft rule to estimate the size of the heat pump for a 2,500 sq ft home.

Let’s do some manual calculating:

For 1 sq ft, you would need 30 BTU of heating/cooling output.

How many ton heat pump (or BTUs) you need for 2,500 sq ft?

Heat Pump Size (2,500 sq ft) = 2,500 sq ft * 30 BTU per sq ft = 75,000 BTU

You would need about 75,000 BTUs. If you convert that into tons, that’s a 6.25-ton heat pump. If you convert that into kW, that’s a 22 kW heat pump.

In short, for a 2,500 sq ft home, you would require a 6.25-ton heat pump.

Let’s see if the Heat Pump Size Calculator also gives us the 75,000 BTU result for a 2,500 sq ft home:

What Size Heat Pump Do I Need For A 2500 Square Foot House

The calculator confirms that our manual calculation was correct.

What Size Heat Pump Do I Need For A 1,500 Square Foot House? (Example 2)

In this example, we have a 1,500 sq ft home and would like to buy a mini-split heat pump for it. Obviously, the first question is what size mini-split heat pump do you need? Once you determine that, you can check out our article about the best mini-split heat pumps currently on the market here.

An HVAC expert on-site will determine the size of the mini-split heat pump exactly but we can estimate what the result will be by applying the 30 BTU per sq ft rule of thumb and manually calculating what size heat pump you need for a 1,500 sq ft home.

Here’s the calculation:

Mini-Split Heat Pump Size (1,500 sq ft) = 1,500 sq ft * 30 BTU per sq ft = 45,000 BTU

For a 1,500 sq ft home, you would need about 45,000 BTU heat pump. Let’s convert that to tons and kWs; that’s 3.75 tons (about 4 tons) and about 13 kW.

In short, you would need about a 4-ton mini-split heat pump for a 1,500 sq ft home.

We can confirm this manual calculation using the mini-split heat pump sizing calculator:

What Size Heat Pump Do I Need For A 1500 Square Foot House

As you can see, the heat pump BTU calculator produces the same result: 45,000 BTU.

Final Words

Always remember that for adequate sizing, you would need an HVAC expert that does the on-site sizing. The heat sizing here can serve as a rough reference.

Hope all of this helps a bit.

40 thoughts on “What Size Heat Pump Do I Need? Heat Pump Size Calculator”

  1. I have a Daikin contractor sizing my home of 1400+ sq. ft. requiring a 12,000 BTU mini ducted heat pump. I am concerned with the suggested size. The contractor after a second house visit has suggested a larger size heat pump at 15,000 BTU. My home is a standard ranch style home with three bedrooms, 1.5 bathrooms din/kit. and liv. . The layout is fairly standard my climate is the same as any of your New England states (Nova Scotia Canada). My contractor is a certified Daikin installer with an exceptional reputation. Who is right and who is wrong ?

    • Hello Harold, a mini-split heat pump is usually used as an auxiliary heating source and primary air conditioning source. Do you have a primary heating source? Something like a furnace or an existing ducted HVAC system? Based on the heating output of that heating source and the size of your home, the Daikin installer will try to choose the most optimum heat pump size. The Daikin installer’s first suggestion was 12,000 BTU and then he/she reevaluated the sizing and came up with 15,000 BTU? The size upgrade for such a home makes more sense. It would be best to trust a hands-on installer who can evaluate the heating/cooling needs on-site.

  2. I live in a 2,400 square foot two-story home with an existing gas furnace (7 years old) and air conditioning unit (21 years old). I’m interested in exploring whether it’s worth it to replace the A/C with a 75,000 BTU heat pump (according to your calculator) which would provide air conditioning as well as heat up to a certain point at which the gas furnace would take over in more extreme temperatures. I live in the Toronto, Canada area which would be similar to Chicago. I’ve already installed an electric heat pump water heater and I’m very happy with the savings so far. I’m also arranging for a Home Depot HVAC representative to estimate the cost. Also, which brand would you recommend for the best efficiency? Thanks

    • Hello Phil, you have the right outlook. The 21-year-old AC is probably not efficient by today’s standards. Quite possibly, it has a SEER rating (cooling) below 14 and an HSPF rating (heating) below 8. The new AC will probably have a SEER rating of 20 or more and an HSPF rating of about 10. That will likely save you about 40% of AC and heating costs. As far as the brand is concerned, it might be best to ask a Home Depot representative. Mitsubishi and Gree make great heat pumps, for example, with a 20+ SEER rating (very energy efficient). The best DIY heat pumps are made by MrCool, for example. They have a 10 HSPF rating and 20+ SEER rating; you can read more about MrCool DIY heat pumps here. Hope this helps a bit.

      • Glad to see you mentioning GREE (Mr Cool, too).

        We live in an outer Chicagoland community (6700-700HDD) and heat/cool our 100 year old 1100sqft home with a single 12000BTU GREE Sapphire, which is as high efficiency (SEER 30.5, HSPF 14) as any mini-split on the market of that size. We are extensively remodeling our house, but have not yet done most of the insulation upgrades. It is still at about a 1980s minimum insulation standard. While our 12000BTU mini-split alone cannot be expected to meet the full heating demand during our rare sub-zero hours of the year, those times are tiny (literally just a few hours) compared to the months when having the highest efficiency mini-split reduces our energy usage and cost. We can use inexpensive electric resistance heaters to increase output during those few hours of the year. I suppose we could even open our electric oven door if we really wanted to double our heating capacity (which we never do). Our very high efficiency, low-temperature (-22°F) mini-split puts out its rated 12000BTUs at 5°F and the vast majority of the winter it far exceeds that capacity. We also have a small glass front woodburner where we occasionally burn wood scraps and junk mail ;O) It not only can add warmth to the house if needed, but also serves as a pre-heater for the air that is feeding our heat pump water heater. We have shut down our 96% efficiency gas furnace and have disconnected from the gas grid. We save 20% on our electricity by getting it from a local Community Solar farm. So, our total energy cost, for everything, including all the charging of our modest electric car, comes to significantly less than $1000/yr. Try to beat that with fossil fuels!

        I am saying all this to give an example of what is possible for our existing cold climate homes, which will need to be electrified in the next few years.. My recommendation, as someone who has been designing highly energy efficient homes since the 1970s, is to size your heat pump for efficiency and economy, not for meeting the local 99% design temperature. The more efficient mini-splits are smaller. It may require installing more than one of them, but that also has the side benefit of providing a backup when one needs repair. Do not “design for the traffic jam” those few coldest hours of the year. Choose your heat pump for the 90% or so of the time when temperatures are not nearly that extreme. Then, let electric resistance heat make up the balance, during those rare extreme times when the heat pump efficiency is lowest and is not going to save significant energy/cost anyway. Heat pumps are not furnaces and heating systems that use them need to be designed with different priorities. Oversized heat pumps also cycle on and off way too often, losing further efficiency and their ability to properly dehumidify in summer. It also makes them a lot noisier.

        Another factor in Illinois and some other areas, is that our gas utility has a huge basic rate, before supplying us with any gas. It now approaching nearly $300 per year. If you can disconnect from their gas grid, you immediately have nearly a $300 head start over when you were using gas. You can still leave your old gas furnace in place, for if it is ever needed. Just stop paying the gas company for giving you nothing. BTW….when we told them to disconnect us, they didn’t believe we were on the up-and-up. Instead of removing our meter, they immediately sent out a crew to install a new one to (we think) make sure we weren’t cheating. We still get periodic letters asking if we want to start paying them again.

        Lastly, when designing a simple economical mini-split system, especially for an existing older home, it is critical to remember that warm air rises and cool air sinks, and that outdoor air that leaks in during winter can be very heavy, and very warm air near the ceiling can just get sucked outside through light fixtures, leaky windows, etc. So, constant easy air movement is your friend. Properly sized mini-splits (meaning not too big) do an excellent job of warming not blasting, and moving air almost continually, but they still are not capable of bringing heat down from upstairs, and cool air up from downstairs. Some have tried to remedy this by placing mini-splits (or just indoor heads) on each floor, but that way usually only one is needed for cooling and only one is need for heating. It can cost twice as much but only works maybe 10% better. What I recommend instead is to (at least at first) use a single mini-split and a vertical duct with a high efficiency fan, to “destratify” (even out) the temperature between floors. Ours single mini-split is upstairs, in our office loft, over our cathedral ceiling living room. An 8″ duct and fan move about 400CFM during the heating season, for a net cost of around 7¢/day, about $10 per season (the electricity used by the fan is effectively converted to electric resistance heating). It runs down through closets, so was a very easy installation. During summer the cool air flows down and spreads passively.

        I hope this has shed some additional light on the differences between sizing mini-splits and furnaces.

        Laren Corie
        Renewable Energy Educator
        and Designer of Passive Solar
        and Energy Efficient Homes
        since 1975 (retired to finally live
        my own fossil fuel free dreams ;O)

        • Where did you purchase your duct and fan? I’d like to do the same in a log cabin with a loft. It is so hot up there they open the window all winter. I’d like to put a stop to that!

        • Excellent write-up! Descriptive, clear , easy to understand. Heat pump companies should use this in thier advertisements (paying you of course)

  3. Hi, I am wondering if air source heat pumps are a possibility in a commercial office setting? The office I work in is approximately 1514m2 (16,297ft2). Is a large heat pump an option in this sized building? Or would multiple units be required? Any info would be great! Thanks

    • Hello Fergus, that’s quite a lot of office space. Depending on how well the whole office is insulated, you would require about 500,000 BTU of heating output. Of course, you can use a bunch of air source heat pumps to generate that heat. You can check our article about some of the best air source heat pumps for cold weather here. As you can see, most of these units are 2-ton units; they generate 24,000 BTU of heating output. For a 1,514m2 (16,297ft2) office space, you would about 20 of these. It’s possible; the real question is if the central heating system wouldn’t be a better solution. Hope this insight helps.

  4. I am planning to use concrete filled ICF forms that have R22 rated and have 2400 sq ft main floor and 1200 sq ft daylight basement (one side only), with 9.5 ft ceilings, in zone 4. Will this reduce my needed BTU calculations?

    • Hello Roger, the single most important factor that decreases the required BTU load is good insulation. The better insulation, the lower the BTU requirements. We have talked about the R-value of insulating materials here. If you check the concrete R-value, it’s quite low, however. It is rather difficult to theoretically determine what an effect the concrete-filled ICF forms will have, to be honest.

      • A friend in Southern Alberta has a house built with insulated concrete blocks. One winter family came for a sleep over and dinner, etc. He had to open all the doors and windows because the building retained so much heat. ICB construction is amazing. Perfect for heat pump installation. Great in the summer too.

  5. Brand New Construction
    St. George UT
    3600 square foot home
    Single Story
    Lots of window but energy efficient ones – Top of the line Marvin

    Cost and Suitability?

    • Hello Mike, the new constructions with superb installation can have an extraordinarily low BTU per sq ft requirements. Even in Utah, you’re might be looking at 30 BTU per sq ft; that would mean you need less than 10 tons. In this case, and for the project this size, it makes sense to schedule an on-site evaluation.

      • I have a huge 2 story home im going to have built soon and it’s about 3700 square feet total. My neighbor has pretty much the same size home and has a 4 ton at I believe a 13 or 14 seer and he says he’s comfortable. But I feel I should have a 4 or 5 ton at a much higher seer. What do you think

        • Hello Allen, theoretically you would need about a 6-ton unit. However, when you figure out all the other factors, including insulation and so on, you will likely see a lower tonnage requirement. The most exact way to estimate the tonnage, however, is exactly as you did; if the neighbor has the same size home and a 4-ton unit is sufficient, go for that. The SEER rating doesn’t influence the cooling/heating output. It is a measure of energy efficiency. Hope this helps.

  6. Hello Learnmatics,
    I have a new home built in Nova Scotia Canada, the 852 Sq/Ft 8′ ceiling walkout basement is constructed with R-23 Nudura ICF. Main floor is 9′ ceiling at 1747 Sq/Ft with R-29 insulated walls. i am looking at mini splits heat pumps. your toughts?


    • Hello Patrick, those R-23 and R-29 insulations are very useful and will decrease the heating output you need to warm your home. If we take a general approach for well-insulated building and use the ’30 BTU per sq ft’ rule of thumb, you would need a 77,970 BTU (about 6.5 tons) heat pump for your combined 2599 sq ft home. Now, you do have higher than average ceiling height on the main floor; this, and the fact that this is Nova Scotia we’re talking about, you might be looking for an 8 ton or even 10-ton unit. The best thing for a more accurate estimate would be to call an HVAC expert and he or she will use the Manual J directions to estimate the heating load you need. Knowing the insulation R values will be of great help here.

  7. I live in ohio and have a 1998 1,700 sqf manufactured home with 9 foot cieling’s. My home has alot of windows and a slideing glass door. My home is located on a hill with some wind block. The furnace is a down flow heat pump and it went out this is the original i think because every thing is faded and cant read nothing my question is what is a good complete unit to replace this out dated unit my house gets cold in the winter and the old unit was running me around $500 in the winter and $195 in the summer thank you

    • Hello Wes, replacing a furnace with a heat pump is generally a good idea, if the winter temperatures are not extremely low. With a mini split heat, you will likely lower the cooling and heating costs quite a bit. The trouble comes at extremely low temperatures (coefficient of performance (COP) for heat pumps can fall from 3.0 to less than 1.0). That’s why, in cold climates, a furnace backup still makes sense. Hope this helps a bit.

  8. 2700 sq ft home, 9 feet ceiling downstairs in kitchen, living room, and dining room, cathedral ceiling family room 14×25, with vaulted foyer 16×16, 2stories. Upstairs 4 bedrooms, 8 feet ceilings with 2 baths. Home built to NJ PSE&G energy efficient compliant for the 1996 year, R-19 walls, R-30 attic, plus low-E windows. Currently has a 5-ton AC and 125,000 gas-furnace 90%+ which were installed in 1996 and need replacement. Would appreciate your advice. Should I replace the current A/C and furnace with a dual heat pump and furnace or continue with a new A/C and furnace? Is there going to be a large cost increase to replace with a dual heat pump and are there any advantages worth the added cost? What would you do? Thanks,

    • Hello Larry, this is quite a common dilemma. The dual heat pump and furnace systems are preferred because the heat pump does all of the cooling and most of the heating (the furnace comes on just at very low winter temperatures). Heat pumps are incredibly energy efficient (they have a coefficient of performance (COP) of 3.0 or even more). At lower temperatures, the COP can drop below 1.0, and only then it’s more efficient to use the furnace. New AC and furnace doesn’t have that advantage; the cooling costs will be the same as with the dual system but the heating costs with separate AC and furnace will be much higher.
      Of course, a dual heat pump and furnace can cost more and, given your current setup, it will be costlier to install them. Nonetheless, the lower heating costs will pay for the extra costs of the system.
      In these cases, I would always go with the dual system. The biggest drawback is the initial investment; if you have that, the solution is quite clear. Hope this makes sense.

  9. I am looking at getting a heat pump with 4 cooling units. I have been reading websites on sizing a heat pump. Should a heat pump be calculated on the size of the house or the size of the space to be heated / cooled?

    My home is approximately 2,000 sf. But, the areas I am looking to heat cool is 1,142 sf. (Living room / dining room 625 SF, a master bedroom 234 SF and 2 smaller bedrooms 1bout 140 sf each.). Ceilings are 8 ft. not conditioning closets, bathrooms, hallways, vestibules, etc. So what do I need?

    • Hello David, that is a good question. We always use the space to be heated/cooled (amount of air). However, we usually talk about square footage presuming the standard 8 ft ceiling height. This ceiling height gives us the amount of air from square footage since the volume of air is calculated as Length × Width × Height.

      In your case, you are not looking at 2,000 sw ft; you are looking at those 1,142 sq ft. You now have a known square footage and 8 ft ceiling height, that’s great. For a rough estimate, it’s useful to calculate 30 BTU per sq ft of living space (8 ft ceiling height). So you are looking at 1,142 × 30 BTU/sq ft = 34,260 BTU or about 3 ton heat pump. Given your layout, you could go with 18,000 BTU air handler in the living room / dining room, 9,000 BTU air handler in master bedroom and 2 smallest 6,000 BTU units in each of the bedroom. If the layout allows, it would be better to use one 9,000 BTU unit for both smaller bedroom.

      Hope this estimate helps. For an in-depth sizing that figures in location, insulation and so on, HVAC experts use Manual J calculation. You could call your local HVAC company and get a guy to calculate that for you on-site.

  10. Hi,

    I’m looking to get a mini spilt ductless heat pump for a 450 sq ft foot print cabin with a 300 sq ft loft and the cabin is open beam construction ceiling at about 16 feet. So, total sq ft including the loft is about 750 sq ft, but the loft and the bottom level (450 Sq ft) are open to each other.

    So, at 450 sq ft and 16 foot ceilings I get 27,000, but 750 sq ft with 8 foot ceilings I get 22,250

    Any help calculating here would be great.

    • Hello Michael, your calculations are on point. Basically, given the premises we specify in the article, you are looking at 49,500 BTU or about a 4-ton mini split. Those 16 ft ceilings do increase the heating needs a bit, yes. It is likely that you will be able to heat the cabin and loft with a 3-ton mini split as well (36,000 BTU).
      The advantage the loft and cabin are open to each other offers you is this: You can use a 1-zone mini split for the whole place. Usually, if you have 2 separate spaces, you would need a 2-zone mini split because the airflow from just 1 air handler cannot adequately propagate (along with the warm air) to the other room. In your case, you can just pick 1 big mini split, something like this MrCool 36k unit that you can DIY installation. Hope this helps.

  11. If I have a 1300 square foot upper unit of a townhouse. Our town houses have bungalows under them as well.
    Anyhow, I I have a plan to have a heat pump paired with my furnace what size capacity would I need? Is it still 30btu per square foot when you pair it with your furnace?

    • Hello Fern, if you pair a heat pump with a furnace, you can use 30 BTU per square foot or lower. The idea is that even though your heat pump will not have sufficient heating output, you can still switch the furnace on and produce that additional heating output. So, you are looking to have a combined heating output (heat pump + furnace) of at least about 30 BTU per sq ft or 39,000 BTU total for a 1,300 sq ft space (8 ft ceiling height). Hope this helps.

  12. I’m planning on replacing our gas furnace with a heat pump in the near future. I’ll of course go through a professional, but I’m trying to estimate roughly what size I’d need so that I’m going into the process with a ballpark estimate.

    Our house is 1100sq foot, in the Bay Area, mild climate, 1950s build, recently fairly well insulated but not nearly as well as new construction could be. However, at some point we are going to add 500sq ft with a garage/basement conversion and another 300sq ft kitchen addition, but we’ll be converting from gas before we do that.

    We usually have the heat on for six months in the year, and from our gas bill I estimate current we use about 30-35 Therms/month of gas for heating in December and January, and 120 Therms total for the year.

    Our current gas furnace is 20+ year old, 80,000BTU, 80% efficiency, and vastly oversized (previous owner put it in but didn’t insulate – we added insulation and halved gas usage). Our smart thermostat shows our furnace runs for about 40-45 hours per month in December and January, so a total (hours x 80,000/31) of 100-120k BTUs per day, ie about 30-36 Therms per month and close to my estimate from our gas bill.

    So I calculate we need a heating system to provide up to 100k BTUs of heat per day – and up to maybe 180k BTU after our remodel.

    However this seems low to me. Running the heat pump for 20-30minutes/hour means a 10k-20k BTU heat pump would be sufficient? This is much lower than the 30-50k BTU range from the rule of thumb. If the oversized furnace is affecting efficiency by much, it might even be lower.

    So, am I going about this wrong? I don’t want to oversize it, but also don’t want to find we’ve installed something that doesn’t meet future needs.

    We do like it cool and usually have the thermostat set to max 70F, but might (will probably) end up bumping it higher as we get older.

    • Hello Paul, very smart to get at least a sense of how many BTUs/tons you need before the HVAC professional arrives. Given that you are in the Bay Area, you can take the 30 BTU per sq ft rule of thumb. Now, you currently have a 1,100 sq ft house; that would be 33,000 BTU or about a 3-ton heating output. When the garage and kitchen are done, you are looking at 1,900 sq ft of 57,000 BTU or about 5 tons. The 180k BTU numbers do seem a bit excessive. Even if you would be living in Canada, that would be too much.
      Now, the HVAC professional will take into account the installation R-values. That is the right thing to consider; however, it does seem that anything over 70,000 BTU would be a bit too much. Very interested to see the actual sizing by the professional.

  13. I live in a 763 ft condo I will be replacing a Rheem 1.5 ton heat pump (r-22). According to your calculations I would need a 2-ton heat pump (r-410) is this correct? Was my current heat pump undersized?

    • Hi Dan, this is a theoretical calculation that does not include the insulation R-values. It’s a good estimation tool but the best information about adequate size of a heat pump you get is if you already have a unit installed (as you do). Is your heat pump providing adequate heating in the winter and adequate cooling in the summer? This is all you need to know. If yes, you have the right size heat pump for your condo. Hope this helps.

    • Hi Don, this really depends on the average ceiling height. If the average ceiling height would be 12 ft due to the cathedral ceiling going as high as 14 ft or even 16 ft, you should add about 50% to the calculation for the 8 ft ceiling. 24×36 is 864 sq ft in total; you are looking at about 40,000 BTU of heating capacity. That’s a 3.5 ton heat pump (42,000 BTU) or maybe even 4 ton heat pump (48,000 BTU) since Ottawa can be cold in the winter and the cottages usually have subpar insulation. Hope this helps.

  14. Can you provide any info on bTU/kwh demand needed for converting radiant floor heating system with a NTI on demand boiler to a heat pump as the primary source of energy for the radiant floor and domestic water. Napa California, 3100 Sq Ft. Aluminum covered Warmboard with pex tubing floors, open beam ceilings, R-22 walls, R-32 ceiling, R-19- floor.

    • Hi David, that seems to be quite a complex system. Sadly, we don’t have enough knowledge to help here, hope you find someone to help you out.

  15. I’m trying to get quotes for ASHP but the installers just want to know how old your house is and how big. Why don’t they just ask how much gas you used last year to find out what the demand will be. If my 65% efficient 40 year old boiler used 8300kW last year what size ASHP do I need? In the worst month Feb used 1500kW of gas. So I calculate that could be replaced by a COP4 ASHP using 362W every hour during Feb. So even a 2kW ASHP seems way over the top …no?

    1500kW over 28 days = 2232W/h
    providing 1450W/h @ 65% efficiency
    equivalent to 363W/h at COP4

    • Hi John, what you suggested is a fantastic idea. HVAC people start with the size of the house and built year because the first job is to size the heat pump adequately. The idea is that the biggest the house, the bigger the heat pump. When the house was built is a good indication of insulation, and thus the heat loss. This is just what we are trained to do, but it’s all theoretical.
      What you suggest is practical. If you know how much energy you needed to heat up a house with a previous system, you can take that usage and calculate the size of the new system.

  16. I have a basic question about the square footage calculation for sizing a heat pump. I have a typical rambler style house with a fully furnished basement. The main level and basement are each 1,200 square feet. For purposes of calculating heat pump size, Should I use 1,200 square feet, or 2,400?

    • Hi Jerry, if you need heating/cooling only on the main level, that would be 1,200 sq ft. If you want the whole house to be conditioned (including basement levels), that would be 2,400 sq ft.


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