AC Tonnage Calculator: Insert Sq Ft, Get Tons (+ Chart)

Before buying an air conditioner, you have to make the AC tonnage calculation. Not sizing an AC unit properly can cause $1,000s in wasted AC costs or future electricity costs.

According to the DOE,

“…an air conditioner generally needs 20 BTU for each square foot of living space.”

We need to convert BTU to tonnage. 1 ton equals 12,000 BTU. That means that, on average, we will need 0.0016 tons per square foot.

To adequately estimate the AC tonnage you need, you can use the AC tonnage calculator per square foot here (as always, you can also use the comment section below and we will try to help you out; we have currently over 100 comments there):

 

For example, a standard 1,500 sq ft house would need a 2.5-ton air conditioner. To help us out, here is a quick table showing how many tons (and BTU) air conditioners we need for certain square footage:

Tonnage Table

Area (Square Feet): BTU Tonnage
600 sq ft 12,000 BTU 1 Ton
900 sq ft 18,000 BTU 1.5 Tons
1,200 sq ft 24,000 BTU 2 Tons
1,500 sq ft 30,000 BTU 2.5 Tons
1,800 sq ft 36,000 BTU 3 Tons
2,100 sq ft 42,000 BTU 3.5 Tons
2,400 sq ft 48,000 BTU 4 Tons
2,700 sq ft 54,000 BTU 4.5 Tons
3,000 sq ft 60,000 BTU 5 Tons
3,300 sq ft 66,000 BTU 5.5 Tons
3,600 sq ft 72,000 BTU 6 Tons

Example 1: 1.5 ton AC covers how many square feet?

Now you can answer this question. A 1.5-ton AC unit covers 900 square feet of living space.

Example 2: How many square feet does a 3-ton air conditioner cool?

How many square feet does a 3-ton air conditioner cover is quite easy to answer as well. 3-ton is equal to 36,000 BTU. If you apply the 20 BTU per sq ft rule of thumb, you can see that a 3-ton air conditioner cools about 1,800 square feet spaces. You can check the best central air conditioners with prices of 2, 3, 4, 5, 6 ton AC units here.

Example 3: 1 ton AC covers how many square feet?

1 ton AC covers 600 square feet. You can check which of these 12,000 BTU mini splits are the best to use here.

Here is the most frequently asked question about tonnage, BTU, and area (square footage):

How Many Tons Of AC Per Square Foot? (Tonnage Per Square Foot)

Simple answer: You need 0.0016 tons of AC per square foot.

In short, if you’re looking to cool down a 600 sq ft area, you would need:

AC Capacity = 600 sq ft * 0.0016 tons/sq ft = 1 Ton

In general, for every 600 sq ft, you need 1 ton of AC. This is a general estimate.

You can use the ‘tonnage per square foot’ formula to calculate how many tons of AC you need. Here is the formula:

AC Capacity (Tonnage) = (INSERT AREA IN SQ FT) * 0.0016 tons/sq ft

You can insert the area and calculate how many tons of AC you need per certain square footage.

When you calculate the tonnage, you can check out several 1-4 ton mini-split air conditioners here:

Hope this helps. If you have any questions, you can pose them in the comments below.

112 thoughts on “AC Tonnage Calculator: Insert Sq Ft, Get Tons (+ Chart)”

  1. 12/19/2020
    I live in the Dallas, TX area
    My A/C unit is 3.5 tons
    We have 18″ and 9″ return ducts to the upstairs unit.
    My house was built by Ryland, square footage is 2661 and insulated well.
    Although there are hot & cold spots in the house, typically in rooms with large windows, our unit does not run all the time in winter or summer and has lasted 11 years so far.

    Based on the tables & calculations I can find online, my unit should be somewhere between 4.5 tons and almost 6 tons.

    I would expect an under-spec’d system to run all the time just trying to keep up. But ours does not. We had an A/C service come out recently and he suggested that we add more return ducts.

    I am trying to grasp what the symptoms are when the unit is under-sized. I would have expected the unit to burn out long before now if it was working to too hard.

    What am I missing?

    Reply
    • Hello Greg, you have sound reasoning. There are two potential symptoms of under-sized units: not sufficient cooling, and a shorter lifespan of the AC unit. Your cooling is sufficient and the modern ACs are made to last 20 years even if you run them on 100% all the time (regular maintenance is required). Given your situation, you will probably have very little symptoms of these sorts. In short, you’re not missing anything. Hope this helps.

      Reply
    • If you have one 3.5 ton unit for a two-story production built home of 2661 sqft I’d have a HVAC contractor or mechanical engineer run it through the manuals for size and air flow. Well worth the money to be comfortable and efficient.

      Reply
    • Sizing an HVAC system by square footage is a very crude general rule of thumb. It sounds like the contractors built an efficient house and sized the hvac system appropriate for your home.

      Every building is unique and each one requires load calculations to properly size the hvac system to match it. There are many factors that go into a load calculation. The doors, windows, insulation, air tightness, direction it faces, climate zone, and many other factors all effect the heat loss and heat gains. For example, you could take 2 identical built houses and face one of them a different direction and it will change load calculations and possibly the system size by a 1/2 ton or more.

      A brief overview of system sizing starts with load calculations following ACCA Manual J. This will calculate the heat losses and gains of each room in the house and how many BTUs/hr of cooling and heating are needed to meet the demands/loads. After load calculations then on to ACCA Manual S for equipment selection. Equipment manufacturers rate thier equipment to have a certain capacity at a certain design condition (I’m not going to go to in depth on this). Example a 3 ton unit may have a capacity of 38,000 BTUs @ 75F, 36,000 BTUs @ 80F, 34,000 BTUs @ 85. If you live in Texas and have temps as high 105F that effects the rated capacity of the unit and has to be factored in. Now that equipment has been selected based on its rated capacity, it’s time for ACCA Manual D, duct design. Good duct design is probably the hardest part to get right. The equipment that was selected will provide a certain amount of BTUs and move certain amount of air and it’s the job of the ductwork to distribute those that air to where it’s needed at different amounts per room all while doing so with as little restrictions as possible and as quietly and unnoticed as possible. You don’t want everytime your system turns on your return grill starts whistling, or one room in your house is always to cold while another room is always to hot. I could get way more in depth talking about duct design and CFMs, static pressure, air velocity, equivalent lengths, but I think I’ve went deep enough.

      A mis-sized HVAC system, whether it’s undersized or oversized (you can have to much cooling causing mold and other issues) causes an uncomfortable home, shortened equipment life, increased component failure rate, high power bills, etc. Morale of the story, any contractor that just sizes your hvac system by square footage is a red flag to call someone else.

      Reply
  2. I bought a 15 year old two story home in Houston. My appraisal shows the house is 4300 sqft (2400 on first floor and 1900 on second floor). There’s high open ceiling from entrance all the way through the living room. Property faces west, so it gets a lot of evening sun in the front. The house has tall high ceiling glass windows in the back, but the morning sun in the back is partly shaded by high a ceiling patio. The house seems to have good insulation (but it is 15 years old). The house has the original system installed by the builder with two 3 ton units (Evaporator coils were changed in 2013). I am about to replace this system. Is this tonnage adequate for this house? Should I go to a 4 ton/3.5 ton combo or perhaps a 5 ton/4 ton combo? At what point do we run into the problems of an oversized AC?

    Reply
    • Practically, the tonnage is sufficient if your home was sufficiently cool in the summer. Theoretically, 1 sq ft requires, on average, 20 BTU. 4300 sq ft translates to 86,000 BTU, or about 7.2 tons. Add Houston climate, high ceilings, and subtract good isolation and you’re looking at about 8 tons.

      Oversized ACs are more expensive to buy and install; that’s the only major drawback. Hope this helps.

      Reply
    • The “good insulation” is subjective… This is a really tricky climate zone and depending on how your insulation is installed will play a major roll in the success of your mechanical system. I would check to see if there is an air seal at the attic floor under the “good insulation”. This is needed to properly separate the conditioned space from the non conditioned space. The code only requires builders to put R-38 on the attic floor but does not require them to air seal the attic floor . Fiberglass doesn’t act as an air barrier. First Defense is a company in Houston that specializes in solving the actual problem before spending the money replacing mechanicals and ducts, only to find you’re having the same problem .

      Reply
    • Hello Joseph, according to EPA’s recommendation, you would need 20 BTU per 1 sq ft. That means about 240,000 BTU in your case. 12,000 BTU equals 1 ton. The safe bet would be to buy a 4-ton unit (48,000 BTU in total).

      Reply
  3. I have a commercial space rented as a barbershop with aprox 6-7 workers at a time and about 6-10 customers in the shop at a time during peak hours. The space currently has a 6ton ac unit. The space is only 1200 sq ft and a floor to ceiling about 12ft. Will a 3.5-4 ton unit be enough to properly cool my shop?

    Reply
    • Hello Isa, 4 ton (48,000 BTU) unit should be theoretically enough to cool down spaces up to 2,400 sq ft. In practice, the Energy Star guide recommends that you add 600 BTU per person. Let’s say you have 17 people in the shop, that would be 17*600 BTU = 10,200 BTU. So, you still have about 38,000 BTU left; that’s more than enough to adequately cool down your shop.

      Reply
  4. Must a/c condensers be placed w/I 2 feet of house, or can they be more remote?
    I would like them placed away from the house, but do not know if that is possible without problems.

    Reply
  5. Could you help me with Line-set or AC refrigerant line sizes ??
    3 ton AC unit waiting for electric run of 30 amp 220volt , but some knuckle head ran a 20amp yellow romex and what seems to be about a 3/4 line set, could be smaller maybe a 5/8 line set
    on 2nd deck patio , 2 floor to be AC’d is about 1800 sq ft
    I think the line set is to small
    The electric is definitely to small
    Nameplate rating is 25.7 amps

    Reply
    • Hello Micheal, sure thing. Well, if you need a 30 amp, 25.7 amps won’t cut it. It’s hard to talk about the specific situation but a 3 ton AC usually has a 3/4 line set. Hope this helps.

      Reply
  6. I LIVE IN THE CHICAGO AREA AND AM GOING TO REPLACE MY AC I NOW HAVE A 3 1/2 TON COMPRESSOR THE SALESMAN SAYS THAT ALL WE NEED IS 2 1/2 TON I FEEL THAT IT MIGHT BE TO SMALL FOR OUR 1800 SQUARE FOOT HOUSE WE ON A SLAB WHAT IS THE CORRECT SIZE TO INSALL LOOKING AT 16 SEER

    Reply
    • Hello Ray, for the Chicago area, 2 1/2 ton unit (30,000 BTU) just might be enough. 3 1/2 ton unit (42,000 BTU) might be overkill for 1,800 sq ft in Chicago. In Florida, for example, that would be adequate.

      Reply
    • I live in Florida and have a 3.5 ton a/c for 1925 sq ft. Can I add two runs to cool lanai that is 232 sq ft or do I need a new small unit for lanai?

      Reply
  7. Hi,

    I am sizing units for a commercial space of approximately 2500 sq feet with 14 foot ceilings. It will be a store so I anticipate no more than 20 people present at a time. But, the previous system was a 10 ton unit connected to three ducts. A new A/C contractor suggested I instead install three 4 ton units, each running off one of the ducts (They do cover the space in zones). He claims that will be more cost-efficient and avoid needing something called an “economize.” Does this make sense? Thanks

    Reply
    • Hello Lydia, the combined cooling output of three 4 ton units will be 12 tons; that’s sufficient cooling output for your situation. 4-ton units can be more energy-efficient than 10-ton units; you can get a lot of 4-ton units with 20+ SEER rating (here are some examples of 4-ton central air conditioners with SEER and costs).

      “Economize” is not an expression we in HVAC are familiar with. The best thing would be to ask what is the SEER rating on 4-ton and 10-ton units. If the difference in SEER rating is about 2 or greater, go with that unit. Example: If a 10-ton unit has 18 SEER, and 4-ton units have 20 SEER, go with 4-ton units. Hope this helps.

      Reply
      • An economizer uses outside air for cooling when the outside temp is below a certain point, typically about 60F, instead of refrigerated cooling. That way you’re only using the blower, not the compressor. A lot of techs don’t like them and they can be cranky, but they’re also a huge energy saver.

        Reply
  8. Hi,

    I am looking to add an A/C unit to a warehouse space, 4700 sq ft with 20 ft ceilings, 1 over head door to be opened 4-5 times a day for a minute or so, 3 employees, in a cold climate (Minnesota) so it will only be running 3-4 months out of the year. What size tonnage unit shoud I be looking at? Thank You.

    Reply
    • Hello Jeff, that’s a huge warehouse with 20 ft ceilings. To calculate the AC tonnage required, you can follow the ’20 BTU per 1 sq ft (8 ft ceiling)’ rule of thumb. So 4700 sq ft * 20 BTU = 235.000 BTU or about 20 tons; but that’s if you would have 8 ft ceilings. You have a 20 ft ceiling; you need to add an additional 150% to that calculation. This would yield a total tonnage of about 50 tons; which is quite a lot. We’re talking about 10 5-ton units for example.

      These calculations, however, might not be as precise when we’re talking about the tonnage required to cool down a large warehouse. We would recommend talking to an HVAC expert on-site. You can call your local HVAC companies or fill in this form and you’ll get some information and free quotes for the warehouse. Sorry, that’s the best we can do; estimating the total tonnage requirements in warehouses is not something we have a lot of experience with.

      Reply
  9. I am in Ohio and have a 1,300 sq. ft. commercial space that is divided up into 6 rooms off a hallway, 1 small bathroom and an entrance area about 8’x15′. There are usually 5-7 people in the space at a time. Currently it has a 3.5 ton roof top ac unit located on the far rear corner. It has about a 25′ run of 12″ diameter flex duct connecting it to the main duct. When temps are above 80 degrees & 50% humidity the unit can not keep the office below 75 degrees and constantly runs. I was quoted to replace it with a 4 ton ac unit centrally mounted on the roof connecting just above the main duct line. Based on the square footage of the space I am concerned that 4 tons is oversized and won’t efficiently remove the humidity. Is this an accurate concern or is more tonnage better?

    Reply
    • Hello John, you have quite a complicated situation here. Calculating the exact tonnage is quite difficult; the best thing to do is to use the 3.5-ton rooftop unit as the benchmark. If that unit doesn’t lower the temperature sufficiently, you need a bigger one; and a 4-ton unit has about 17% higher cooling output than a 3.5-ton unit. There is no concern about overkill here or the consequential short cycling due to 50% humidity. As far as we can see, the 4-ton unit is adequate for this situation.

      Reply
  10. I bought a manufactured home that has 2293 sq ft the company I bought it from installed a 3 ton central unit. Is this unit enough to cool my home? Haven’t moved into the home yet. Please advise me what to do.

    Reply
    • Hello Teresa, it’s hard to say without the additional information, but let’s say we use a rule of thumb ’20 BTU per sq ft’. For a 2293 sq ft home, you would need roughly 2293*20 BTU = 45,860 BTU; this is about 3.8 tons. If you leave in a colder climate (New York, for example), the 3-ton central unit might suffice. If you live in a hot climate (Florida, for example), a 4-ton unit would be a safer option.

      Here is a suggestion: move into the new home and see how well the 3-ton central unit performs. If it doesn’t adequately lower the temperature and humidity, you can always buy an additional window or portable unit as an auxiliary cooling unit.

      Reply
  11. Our house is 1232 sq. feet on a slab with high ceilings in every room going from 8 feet to 12 in some rooms and 16 in others (perhaps the average would be 13 feet?). Our previous AC is 2 ton. It cooled the house ok except on those rare super hot periods. We are in Southeast Iowa. The company for the new AC has ordered a 2.5 ton. Will this be overkill? Do we need 2.5? I do not want the house to become more humid.

    Reply
    • Hello Irene, air conditioners both cool down air and dehumidify it. With a bigger unit, it will become less humid, no worries about that. 2 ton AC unit is, in theory, undersized. For 13 ft ceilings, you would require at least 25 BTU per sq ft. At 1232 sq ft, that would be 30,800 BTU or about 2.5 tons. Even in practice, 2.5-ton seems like a very adequate sizing. Hope this helps.

      Reply
  12. I live in Houston, Texas. The house is 2346 sf, the wall is 9 ft, the sun is everage, the weather is is hot. Based on the two calculations above, the AC should between 4 and 5 tons. The builder installed 5 tons. During the day time, it take 20 minutes to cool the house. At night time, it takes 5 minutes which is a short cycle.
    I think the AC is oversized, do you think so ? It should be better undersized than oversized

    Reply
    • Hello David, for the summer heatwave, the 5-ton unit is a good idea. Short cycling is an indication of an oversized AC unit (with rotary compressors); you can run it at the lowest speed to prevent that from happening. Right now, it might feel like you have an oversized AC but when the heatwave kicks in, you are ready for it. It’s always a problem with the bigger AC systems with a high tonnage cooling capacity.

      Reply
  13. We live in Austin, TX. The first floor is 1,800 square feet with the living room and kitchen making up 1,000 feet of that space. Both those rooms are the farthest from the handler/return and have 12’ ceilings. The unit is a new 3 ton 18 seer smart system (compressor talks to the handler). My assumption is it is undersized. The kitchen and living room are ~5° warmer than the stat and do not reach desired temp. My thought is to supplement with two mini ducts in the living room. Thoughts?

    Reply
    • Hello Michael, the basic rule of thumb by DOE is ’20 BTU per sq ft of living space with 8 ft ceiling’. Your 12 ft ceilings are 50% higher; so let’s say 30 BTU per sq ft. You get 1,800 sq ft * 30 BTU per sq ft = 54,000 BTU or 4.5 tons (this is the minimal figure; you have to add 4,000 BTU for kitchen and about 20% because Austin, TX is hot). In short, the 3 ton AC system is undersized and not by a small margin. Practically you see this: 5°F warmer than thermostat temperature with, presumably, 3 ton running on 100% cooling output all the time.

      Your solution with two mini ducts is great. You’re looking at two 12,000 BTU or even 18,000 BTU mini splits. If you need some help, you can check some of the best 12,000 BTU ductless mini splits here. Hope this helps.

      Reply
  14. I need to replace my 27 year old HVAC system. My current air conditioning system is a 3.5 ton SEER 12. My question is if I go with a 21 SEER can I go down to a 3 ton system? The HVAC tech told me to go with a 4 ton system. Our house is about 900 sq ft per floor with 2 floors above ground plus the basement for a total of 2,700 sq ft. We have not had any problems heating or cooling our house with the older system.

    Reply
    • Hello Mike, you will still need a 3.5 ton HVAC system. SEER is a measure of energy efficiency, not cooling/heating output. The cooling output should stay the same but if you upgrade from 12 SEER to 21 SEER, you will reduce your cooling/heating bills by more than 40%. You could go with a 4 ton but if the 3.5 ton was sufficient for the past 27 years, there is no need to increase the cooling output.

      Reply
  15. hello
    I would like to get some recommendation as to what tonnage AC unit we should get for a floor with 10 rooms, bathroom, hallway, altogether about 3300 sqf, about 9 feet height, all around very exposed to the sun, NYC climate. There can be even 30-40 people residing here at the same time. It can be either one big unit or two smaller ones at the opposite ends of the hallway. What would you suggest?
    Many thanks

    Reply
    • Hello Eszter, 3,300 sq ft with 12.5% above-average ceiling height would, applying 20 BTU per sq ft rule, be about 6.2 tons. Add the sun exposure and so many people residing in there, and you’re looking at a 7 to 8-ton unit. You should be looking at two 4 ton multi-zone mini-split units. For 10 rooms, you will need at least 5 air handlers to evenly distribute the cool air.

      Reply
  16. I live in florida and have a two story townhouse. The unit is a dual zone for upstairs and downstairs. We are roughly 1500-1600 square feet. What size unit would I need?

    Reply
    • Hello Evan, if you apply the 20 BTU per sq ft rule, you would require about 30,000 BTU or 2.5 ton units. Given this is hot Florida, 3 tons should be considered. Dual zone with each zone generating 1.5 tons or 18,000 BTU seems like adequate AC sizing.

      Reply
  17. I live in Savannah, Ga. My house is 3 bedroom 2 ba, 2 car garage with bonus room over garage and 2299 sqft. I have 3 rooms with 14 ft ceilings. The hvac is
    3.5 ton,10 seer and we have had this system since Dec 2006. Our master bedroom is on the opposite side of the house from the hvac unit. This room has always been very hot in the summer and very cold in the winter. We are now upgrading to a 4 ton unit, do you think that’s to big or an upsize.

    Reply
    • Hello Bernard, if you have a 10 SEER unit, upgrading to an 18+ SEER unit is definitely a smart financial choice. 14 ft ceiling is way above average 8 ft ceiling height. Upgrading from 3.5 ton to 4 ton might not be sufficient. As you’ve said, the master bedroom wasn’t only hot, it was very hot. 5 ton unit might be a more adequate sizing. To answer your question, the 4 ton unit is not too big.

      Reply
  18. First, let me say how gracious you are with information.
    I live in deep south Louisiana, heat indices have been 100+ on a regular basis this summer. Additionally, it’s super humid nearly 100% of the time. My home is 1720 sq.ft.
    The front is completely shaded and faces Northwest; the back is about 60% shady with the only direct sun on the back [with lots of windows] during the hottest part of the day.
    Currently there are two units installed in this house, both Carrier, totaling 4.5 tons. One of them is in the attic; the other in a hallway closet. There is no ceiling in that closet and it is open to the attic. I’ve been told it must be open because the heater uses natural gas. I deplore that!
    After 27 years of service, the Control Panel on one of the units has gone out.
    One HVAC company says it would cost $800-$1000 for a new control panel, not including installation.
    Considering the age of the units and my distaste for the no-ceiling situation, I believe the best decision is to replace both units with One Unit. The salesman informs I need a 5 ton unit. A Carrier 5 ton, 17 SEER with dual stage cooling is his recommendation. His bid for the entire job is $18,500.
    Naturally, I will get a couple more bids from other companies.
    I’m asking for your opinion, given the above information, what would you recommend for my situation?

    Reply
    • Hello Carmen, thank you for your question. The hands-on HVAC experts will know best. Just follow a few directions, such as you know that 4.5 tons is enough, that’s the capacity you’re looking for. In most cases, a higher SEER unit is a better investment. You’re already doing the smart thing – getting several bids from HVAC companies – this is the key thing to do if you don’t want to get swindled. If you need more quotes, you can get up to 4 quotes for free here. Hope this helps a bit.

      Reply
  19. I recently had a rialto home built. It’s 2 stories, 8ft ceilings with the living room open to the second floor. My master bedroom, media room.and master bath are not as cool. Setting the temp to 70 at night, the upstairs bedroom is 60 degrees and downstairs bedroom is 74 and airflow seems to be an issue. Our system is 2 zones, upstairs and downstairs and is a carrier, high efficiency 16 seer. It’s a 4 ton but this chart shows I should have a 5 ton for my 3100 sqft home. Hvac keeps coming back adjusting dampers but don’t seem to think the siE of the unit is the issue? Am I wrong? I carrier, the builder nor the hvac company will answer my question on the size of the unit being appropriate. Thoughts?

    I feel like If I let this go the ac unit will have to work harder and not last as long and I’ll be stuck with the bill once warranty runs out.

    Reply
    • Hello Adriana, this seems like an inadequate airflow issue indeed. You can rearrange the current setting by putting the bigger of the 2-zone units upstairs (the one with higher airflow). That would distribute the cooling more evenly. In such a case, it makes sense that you find somebody who can help you hands-on, it’s a bit of a tricky situation.

      Reply
  20. Hi there,
    I live in Arizona and house is 2-years old, 1,500 sq ft – A/C runs hard from for six months especially from June through October. My house gets super warm if we don’t keep it set at 74’ – what size will keep our house cooler and not stop the AC from continually running. ACs go out a lot out here.

    Thank you so much.

    Reply
    • Hello L.B., your AC unit seems to be undersized by 0.5 or 1 ton. It would be best if you would look for a new AC with up to 1 ton additional cooling capacity. Hope this helps.

      Reply
  21. I bought a 1900 sq ft bi level house in NJ outside of the NYC area. The lower level is cooled by a ductless mini split. The top level, which is where I need the central ac, is approximately 1400 -1500 sq ft with 8 ft ceilings. Two people will be living in the house. There is a sliding glass door that opens onto the back deck. House faces east. There is currently a 3.5 ton system which I think may be oversized. What size ac would be best, 2.5 or 3 tons? Or am I off base here?

    Reply
    • Hello Nancy, you’re asking exactly the right question. For the upper part, the 3.5-ton unit seems like overkill. Given its NJ, 8 ft ceiling, east facing, and so on, you would need about 20 BTU per sq ft. For a 1,500 sq ft area, that’s about 30,000 BTU or 2.5 tons. You’re not off base at all; the 2.5 ton AC would be a much better choice than the big 3.5-ton unit.

      Reply
  22. My national home builder put a 2 ton unit in my 1,680 sq ft home and during the summer, we have difficulty cooling 2 rooms of the house. I just had an 11 month inspection and the inspector said the unit is 2 tons and should be at least 2.5 – 3.0 tons for 1,680. The ceiling throughout are 8′. The warranty group tells me he is using old standards and they have engineering reports that claim 2 tons is sufficient. In light of the fact that i have 2 rooms 3-4 degrees warmer than the rest of the house, I think the home builder is trying not to upgrade the system appropriately. Can you comment of what size the system should be?

    Reply
    • Hello Robert, you’re correct; the 2-ton unit is undersized. The easy rule of thumb for AC is ’20 BTU per sq ft’; if you have 8 ft ceiling height which you do. 1,680 sq ft * 20 BTU per sq ft = 33,600 BTU = 2.8 ton. So, even this simple rule tells you you would need a 2.8-ton unit; something between 2.5 and 3.0 tons, as the inspector figured out. It’s difficult to admit a professional mistake, maybe that’s the reason why they are trying to convince you that everything is ok. Hope you resolve the issues as smoothly as possible.

      Reply
  23. So I’m building a home in South Texas ( RGV, coastal area) with living sq ft at 2000 with 9 ft ceilings around the edges and comes up to 10 and about 16ft in the living area cathedral style. Builder is consistent stating that one 5 ton unit is enough to cool the home without burning itself out and more tonnage will cause mold. Even though during the planning process they discussed needing 2 units and even built the 2nd AC pad. As a previous comment I’m afraid he’s using old statistics. Can you help? Thanks.

    Reply
    • Hello Anthony, you have a kind of vibrant ceiling height, but let’s say that the average ceiling height is about 12 ft. That means that – using the DOE standard – you should be looking at at least 30 BTU per sq ft. With a 2000 sq ft home, that’s 60,000 BTU or exactly 5 tons. Now, South Texas is hot, so you should be having higher-than-national-average cooling needs. That may be a good reason to look for 2 units with a net capacity larger than 5 tons. Nonetheless, the installers had an on-sight look and has more detailed information. If you do install the 5-ton unit and realize it’s too small, you can always add a 1-ton or a 2-ton unit to the areas that are not adequately air conditioned by the 5-ton unit. As of the old statistics, AC tonnage calculation is not an exact science and you can get different results using roughly the same standards. Older HVAC installers usually have a ‘feeling’ for sizing AC units that may not correspond to the current standards. Hope this helps in understanding the two different suggestions.

      Reply
        • Hello Eric, 5 ton unit for 2,600 sq ft seems like an adequate estimate. This is just to get an idea. In most cases, it is best to have an on-site check by a local HVAC company. They will use Manual J sizing which accounts for a number of other factors.

          Reply
  24. We just had a house built but feel they put the wrong condenser in. We have a dual zoned home between the upstairs and the down stairs. 9ft ceiling down and 8ft ceiling up. 3150sq ft and they have a 3.5 ton condenser. Trance Model 4TWR4042G, is this too small or is different when zoned like this? The builder says it’s fine but……. Thought?

    Reply
    • Hello Mike, the 3.5-ton unit is 42,000 BTU. Given the DOE’s ’20 BTU per sq ft’, you should be looking at about 3,150 sq ft * 20 BTU per sq ft = 63,000 BTU. That’s a 5.25-ton unit. Now, you don’t really be very alarmed about this. HVAC installers, in general, know how to calculate the AC tonnage for a specific house. The climate, insulation, windows, sunshine, and so on have an impact on cooling requirements as well, beyond the rule of thumb. Chances are that the 3.5 unit is adequately sized; however, you can’t really neglect the possibility that it’s undersized. Theoretical calculations are one thing. Practically, however, you can see the real-life cooling needs when the summer heatwave kicks in.

      Reply
  25. My house in Las Vegas is a two story with a total square footage of 1575 sq. ft. About half of the house is floor to ceiling high and the other half has the 2nd floor. Can you suggest how many tons of condenser unit do I need if I have to replace the old existing unit? I don’t know how many tons the existing. Thank you

    Reply
    • Hello Loreto, the best thing to do here is to check the tonnage of the existing AC unit. It’s written on the unit; either on the outdoor or the indoor unit. We can also try to theoretically calculate this: so let’s say that about 800 sq ft has 8 ft ceiling and the other 800 sq ft has about 17 ft ceiling height? Since it’s Las Vegas, where tends to be quite hot, you would need about a 2-ton unit for the first 800 sq ft and about a 4-ton unit for the second 800 sq ft. All in all, the net tonnage you’re looking at is close to 6.

      Reply
  26. I am looking to replace my ac. I live in Fort worth, TX (north tx), my home is 20 yrs old 2394 sq foot ranch home with and currently has a 5 ton original ac. I had 2 estimates for a new ac and one tech said based on my duct work looks like I have an oversized ac. We do have issues in the summer and seems the ac is always running. He sounds convinced but I’m afraid to downsize it and it not cool the house. The other guy estimate for exactly what I have currently and when asked him he said it would be 400 to 600.00 to do a calculation. Please help I’m afraid to spend the money for a smaller unit and it not be correct.

    Reply
    • Hello Anita, if you have a 5 AC unit and it has to run all the time in the summer, you have an adequately sized AC. If you experience short cycling (AC turning on and off frequently), that’s a sign of an oversized AC. Without those signs, and if you need the AC to run all the time, it seems that 5 ton AC is quite adequate, and getting a smaller unit would present a risk you don’t really have to take.

      Reply
  27. I need to replace an old central air unit. This is an office space on 1250 sq ft in NYC. The waiting area is 400 sq ft. with heavy people traffic. The old unit was 5 ton. In my view 3 ton should be enough. I would like your suggestions.

    Reply
    • Hello Julio, it depends on the people traffic and insulation. If the 5 ton was massively oversized, the 3 ton should be a good compromise. You might want to call an HVAC expect to adequately size the unit; the difference in cost between 3 and 4-ton unit, for example, runs in several $1,000s as we have written in an article about the costs of central air conditioners here.

      Reply
  28. Hello Dear, how many tons Ac I need for the below room:
    Length: 14 FT
    Width: 12 FT
    Ceiling Height: 10 FT
    Door Size: 3.5 FT
    Climate: Very Hot
    Sun Exposure: Very Sunny.

    Reply
    • Hello Rahid, the basic rule of thumb is 20 BTU per sq ft for 8 ft ceilings. 14×12 room has 168 sq ft, and you have 10 ft ceilings instead of 8 ft ceilings; you have to add 25% to that; so you’re looking at 25 BTU per sq ft for 10 ft ceilings. That would be 4200 BTU or about 1/3 ton. Now, you have a very hot climate and high sun exposure. You would do much better with a 2/3 or 8,000 BTU unit. Hope this helps.

      Reply
  29. Hello! My house is 1539sq ft with cathedral ceilings from 11ft to 8ft tall. I want to replace my ac unit but I’m not sure which size my house needs. It is currently 17 years old and has 3.5tons. I’ve had two estimates- one person suggests 3tons due to a major mold issue that could have been cause by the Florida humidity and the air conditioning unit being too big. The other suggests that I replace the unit with the exact size I have now (3.5). I’m confused as to who’s side to take. What is the standard for a 1539sqft home?

    Reply
    • Hello Gloria, the smaller 3-ton recommendation is due to high humidity. The contractor probably concluded that your current AC is too big and that’s why it’s short cycling; that would result in adequate cooling but inadequate humidity reduction. One option here is just to get an additional dehumidifier to fight the humidity levels.

      Here’s the sizing for 1,539 sq ft: You use the rule of thumb (20 BTU per sq ft of living space; 8 ft ceiling), increase it due to cathedral ceilings (average ceiling high is presumably 9.5 ft; an increase of 19%) to about 24 BTU. That would result in 36,936 BTU or 3.1 tons. Now, Florida is hot; in most cases, contractors in Florida will increase the AC capacity to 3.5 tons. That seems like a reasonable AC tonnage.

      With the current 3.5 ton unit, do you get adequate cooling quite easily? If yes, the 3-ton unit might give you a sufficient cooling load and may (may!) decrease humidity if the high humidity is a result of an oversized AC short cycling. That, however, doesn’t seem likely. A combination of 3 ton AC unit and dehumidifier would make sense. If the 3.5-ton unit can barely give you sufficient cooling then it makes no sense to downsize it to 3-ton unit. I hope all of this is at least a bit helpful.

      Reply
  30. Hello,
    I am building new construction. Its a 1400Sq ft upstairs on top of full basement with concrete walls. Upstairs ceiling height is 8FT with cathedral ceiling in a small ((250sqft) living room. Ceiling height in basement is 9ft. Building in the North Georgia hills (zone 7) with average sun exposure. Wanting a split system for upstairs/downstairs with separate thermostats. What tonnage would you recommend for this house? Thanks much.

    Reply
    • Hello Charlie, here it would make sense to call a contractor for an on-site Manual J tonnage estimation. The simple formula you can use is 20 BTU per sq ft, which would yield you a 28,000 BTU. That’s about 2.5 tons. The Manual J estimation will probably be between 2 and 3 tons, but you really want to do that because the difference between 2 and 3 tons is quite large. Hope this helps at least a bit.

      Reply
  31. My wife & I purchased an 1800 square foot house in Northern Florida – lots of humidity & heat in the Summer. 60% of the house has 25 foot ceilings & 40% of the house has 11 foot ceilings. We have a single compressor with two thermostats (upstairs and downstairs) – currently have a 3 ton Seer 15 looking at replacing it with a 3.5 ton Seer 14 and replacing evaporator at the same time as the original system has been problematic… any suggestions?

    Reply
    • Hello Bill, you have a quite complex situation here. The most basic rule of thumb you can use is this: 20 BTU per sq ft with 8 ft ceiling. If you have 25 ft ceiling, you’re looking at triple that, about 60 BTU per sq ft. 60% of the 1800 sq ft house is 1080 sq ft; if you multiply it with 60 BTU, you get 64,800 BTU or 5.4 tons. You also have to account for additional 720 sq ft 11 ft ceiling part; that will yield you additional 1.8 tons or so.

      According to this rule of thumb, you would need a 6-7 ton unit. 3.5 ton unit might be too small. With a bit more complex house (high ceilings are difficult to calculate), it would be best if you consult with an on-site HVAC expert. They use the precise Manual J method for estimating tonnage. An expert will give you an adequate tonnage estimate; you can avoid a whole lot of troubles an inadequate tonnage estimation could cause. Hope this helps.

      Reply
  32. Soon i will be needing a new furnace & A/C unit installed. I live in SE Michigan. We built our house in 2004 and the units are originals. Currently we have a 3 ton A/C unit and frankly never really had any issues with it. Our home is a 3200 sq Split Level. High ceilings on the 1st floor and 8ft on the second floor. Our house faces south and do get a lot of sun coming in on the front of the house. Also, we really don’t have tall trees in the sub to help with shade. What are your thoughts re A/C size and recommendations on brand of furnace / AC I should look at? BTW – we plan to be in this house 10-12 more years.

    Reply
    • Hello Steve, for furnace and AC sizing, the most thorough theoretical method is to use the Manual J sizing. However, practically, it’s best to look at your current HVAC tonnage. If the 3-ton unit is sufficient, you don’t really have to go theoretical; you already know this is the size you need. For 10-12 years, you can literally go with any brand. The best brands usually have a 20 or even 25-year lifespan but if you need it for 10-12 years, you can go with cheaper Goodman or Rheem units. Hope this helps.

      Reply
  33. Buying a new home, 7000 sq ft in three floors here in NJ. Specs state three HVAC units. 2 of 3 tons and 1 of 3.5 Tons. Based on article and calculation it seems I may be missing and extra unit. Based on some of the comments, it feels it maybe enough. What would be best way to determine this?

    Reply
    • Hello Patrick, for such big homes, the 20 BTU per sq ft rule of hand is not all that accurate. You’re looking closer to 15 BTU per sq ft, especially if you have good insulation. The specialists use Manual J estimation – the HVAC standard estimation – and that’s the most accurate way to estimate the cooling loads. You should follow the advice of the specs.

      Reply
  34. Hello, my name is Clayton and I live in Abilene,TX. I’m looking at replacing my heat pump unit outside and blower unit in the attic for my home due to the old one is a 2002 model and I’d like a more energy efficient system. My old system never shuts off during the summer months at all except for maybe an hour or so around 3am causing for high electric bills. My home is 1780sqft 3/2 split bedroom single story with 10′ ceilings in 90% of the home except the kitchen which has an 8′ ceiling height. I have had two different HVAC companies come out to give quotes and each one had a different size unit they wanted to install. One said to put a 4ton unit in and the other said to go with a 3.5ton unit, both with a 16 SEER rating. I understand from reading the previous comments about short cycle which can lead to not getting all the humidity out of the home. Having been giving two different size units to install, I’m at a loss of which would be best for the home. Also, the front of the house faces south and the home isn’t shaded much at all. Thank you for all the advice and recommendations you have been giving out.

    Reply
    • Hello Clayton, it’s a good idea to upgrade to 16 SEER unit. The difference between 3.5-ton and 4 ton unit is not that big; you’re talking about a 12.5% capacity difference. If the old heat pump provided sufficient cooling/heating, go with that tonnage. Given the size and location of your house, the 3.5-4 ton estimate seems to be adequate. Now, a 0.5-ton oversized unit will not cause short cycling. It’s a small increment here and you can always run it at a lower fan speed setting. The 4-ton unit is a safe bet. The 3.5-ton unit might be even more optimized + it has a lower upfront cost. At the end of the day, it’s your decision but you pretty much can’t go wrong with either a 3.5 or 4-ton unit. Hope this helps at least a little bit.

      Reply
    • Hello Marz, as a general rule, for each person, you should add 600 BTU. Now, if you apply the 20 BTU per sq ft rule (8 ft ceiling), you get 108,340 BTU for 5,417 sq ft. Probably you have higher than 8 ft ceilings, so you are looking at 150k or 200k BTU just from that. If you add 600 BTU per person and 175 people, that’s an additional 105k BTU. So, realistically, you are looking at 300k BTU or 25 ton AC unit for a full comedy club. With the space this size, you should consult an HVAC professional that will come to check the place out and calculate the AC tonnage using Manual J directions. Hope this helps.

      Reply
  35. Hi everyone I have a home that is 4 years old. It was built by Taylor Morrison. The house has 3 ton below and 2.5 ton above. The square feet upstairs is 1540 and the 1st floor is 2448 for a total of 3988. I have one room that bumps up a garage attic that is consistently warmer in the summer months and cooler in the colder months. I have had multiple AC contractors come out to try to balance my heating and AC needs for my upstairs. It hasn’t solved the problem of my room that is consistently hotter or colder. Does this mean that they shorted me in the tonnage upstairs? The HVAC guys said my insulation is pretty good for a production home. So what gives ?

    Reply
    • Hello Alan, it seems that the problem might be tonnage or, more likely, airflow. Usually, AC contractors suggest using a bigger AC. That’s the easiest way to solve thing; albeit not very cost-efficient. Given that this is an upstairs garage attic, the more likely possibility is that you have sufficient AC size but its effect is not adequately felt in the garage attic because the airflow doesn’t reach the attic as it should. In the case of mini splits, the nearest air handler is too far away from the attic. In case of central AC unit, the pressure in the ducts in the attic is too low.

      Now, how to solve the thing without buying a bigger air conditioner? The simplest way is to use a fan that propels the air from colder rooms to the warmer attic. This facilitates the airflow needed to cool the attic. Just use one of the simple floor fans, for example. Hope this helps.

      Reply
  36. I’m replacing a 25 year old unit with a Carrier 16 seer unit. My home is in Dallas, 1765 sqft,8’ ceilings, is a mid-century home and has no insulation in the outside walls, but the attic insulation is good.
    My AC guy wants to put in a 3 ton, but was supposed to install a 4 ton. I’m asking him to change it out, but they are back ordered.

    He also said he could just replace the coils in the 3 ton to be 4 ton…is this true/normal?

    He also said he could use a Trane condenser with a Carrier unit??

    Please advise. I don’t want s mix matched unit!

    Reply
    • Hello Katy, it’s fairly unusual to mix and match. Just replacing the coils doesn’t increase the tonnage; you would also have to upgrade the compressor, TVX valves, and so on. It seems like you may have a sub-optimal HVAC guy.
      For a 1765 sq ft home in Dallas, with subpar insulation, the 3-ton unit may not be enough. A 4-ton unit is a much better choice. If you need some help, you can fill in this form and get quotes from 4 HVAC guys in Dallas. Hope this helps a bit.

      Reply
  37. I live in Houston, Texas my home is 2,700 sq ft (2 story) No shade, my builder is trying to put a (1 only) 3 ton 16 seer unit into my home is that enough for my size of home? What will happen if they install too small of a unit?

    Reply
    • Hello Robin, 3 ton for 2,700 sq ft in Texas, no shade? Seems like you are going to sweat a lot 🙂 If you use the simple 20 BTU per sq ft (8 ft ceiling height) rule of thumb, you should be looking at 54,000 BTU or a 4.5-ton unit. Now, add that this is Texas and no shade. Unless you have like R-49 insulation all around the house, the 3 ton unit seems to be quite undersized.

      You could add cooling power by using portable or window AC but those are usually contingency plans. It might be best to order a professional cooling load calculation here; just to get to a more exact tonnage.

      Reply
  38. Hi,

    First, I appreciate this page and all the replies you’ve given. I have a 950sq bi-level townhouse in Chicago. According to the above calculator, I need a 1.76 ton unit. Would I be okay going with a 1.5 ton or should I round up to 2 ton? I am concerned with humidity/mold, so would it be a safer bet to go with a 1.5 ton unit?

    Reply
    • Hello Matt, thank you. That’s a good question. An optimist would say go for a 1.5-ton unit but, pragmatically speaking, a 2-ton unit will be a much safer choice. You can always run a 2-ton unit on a low or medium setting, but you can’t run a 1.5-ton unit on a superhigh 1.5+ ton setting; this is generally the logic when in AC tonnage dilemmas. Humidity-wise and mold-wise, you can do the same with a 2-ton unit. These units can decrease the airflow (CFMs) in order for more air moisture to condensate on the indoor coils (slower airflow = better dehumidification). Hope this helps.

      Reply
  39. It’s time for a new AC system!
    I have a 25 year old ranch 2080 sq.ft. all 8′ ceilings. Finished basement @1180 sq. ft. Good insulation, good windows and doors of average size. Full sun all day. Located in Northern Kentucky. 2-1/2 ton AC standard common ducting for square footage. Keep thermostat at 72.
    Anywhere in mid 80’s F or below it keeps house comfortable but does run a lot. When it gets hot the unit runs continuously but can never make set-point. System recovers in the evening as outside temp cools down.
    1) I believe calculations require 3.5 ton to cover main level, would you concur?
    2) Not sure how to calculate basement due to it naturally staying cooler so the differential from set-point is less. Your insight here would be appreciated.

    Reply
    • Hello John, without a prior A/C installed, the theoretical calculations is the best we can do to estimate the cooling loads. In practice, however, you have the advantage: You already have a 2.5-ton unit that is not exactly sufficient all the time.
      Given this info, the 3.5-ton unit seems to be quite an adequate estimate. You might go with a 3-ton unit but with the summers becoming hotter in recent years, the 3.5-ton unit is a safer long-term choice.

      Basements, as you have pointed out, are really difficult to calculate. Namely, you do have lower temperature there but higher humidity levels. In theory, you could go below 20 BTU per sq ft rule of thumb for basement; something in terms of 10-15 BTU per sq ft. But there is a humidity factor there that’s really hard to estimate. Having an HVAC professional come take a look and make some measurements would be best for the most adequate tonnage estimation. Hope this helps a bit.

      Reply
  40. I have a 55 year old single story 2650 sqtf. currently 2 evap coolers. Looking into to go to refrigerated AC (split units). we had 4-5 quotes, some 14 (up to) seer and some 16 seer (up to).
    Some say 2 units (3 ton), some say 2 units (3.5), other say (1 unit 3 ton and 1 unit 4 ton). The house has one side bigger than the other.
    Kitchen side smaller than bedrooms side.
    Small side has all 8 ft ceilings.
    Larger side has some 8′ some 9′ and one room with a pitch from 8′ to 10′

    Any recommendations or comments appreciated.

    Reply
    • Hello Joe, all the tonnage quotes are in the 6-7 ton range. Given this is a 55-year-old house, is probably has below-average insulation and, taking that into account, the tonnage makes sense. 2 units make sense as well. Usually, higher SEER rating is better; it’s a bigger financial blow initially but the cooling savings in 10-20 years will usually pay for the difference between 14 SEER vs 16 SEER rating. Do you perhaps have dollar quotes? Just to check if 16 SEER is not overpriced.

      Reply
  41. although my 3200 sqft 2 story home came with low SEER 5 ton unit from 1999 home is at 5,700 ft and one local installer told me that a 4 ton unit would be fine for that size newer home at this altitude and that sizing charts are based on sea level and higher elevations calculate at lower tonnage. Is that correct?

    Reply
    • Hello Stephen, the SEER rating is a metric that determines how energy efficient your unit is; it says nothing about the capacity (AC tonnage). If the 5-ton AC has absolutely no problems lowering the indoor temperature to 72 degrees in 100+ degree heat, a smaller 4-ton unit might be an option. Hope this makes sense.

      Reply
  42. Hi, I am in Utah at 5000′ altitude. My house is 2900 sq ft, plus basement of 2400 feet but we do not need to cool that. It never gets above 70 in summer. Has a west facing living room with a lot of glass in living room, but triple paine with a filler, Ergon I think. Have 5 ton 12 SEER from 2003 that should to be changed out. Currently the system is starting to freeze up. In summers we can reach 100 with normally low humidity, like 10-15%.
    My AC guy says that a 4 ton with either 13 or 16 SEER is adequate because I only have 16 vents, two were not sized properly during a remodel in 2002. He seems to be insistent that because of the vents 4 ton works and will actually act like a 4.2 ton. He thinks the 5 ton won’t help that much. Our master bedroom gets much colder then rest of house and I try and run thermostat at 74 degrees.
    any suggestions?

    Reply
    • Hello Les, those are some interesting suggestions. I have never heard that a 4-ton could ‘magically’ produce extra 0.2 tons. Joking aside, if you just use the basic 20 BTU per sq ft rule of thumb, you get 4.83 tons. Now, theoretical estimates are one thing; practically testing a unit will obviously give you a much better result. You have been testing a 5-ton unit from 2003.
      If you feel that it is very easy for your current unit to cool down your house, you can think about getting the smaller 4-ton one. However, if you are pretty much happy with the current AC tonnage, you should go with the 5 ton. It is also important to factor in that these AC units last for about 20 years (as did your 12 SEER unit) and summers don’t seem to get any colder. In short, 5 ton is a safe bet. 4 ton can be a bit risky, despite 16 vents.

      Reply
      • Thanks for the input. I appreciate it. I won’t see the end of another 20 year cycle, so that is not important to me. The AC guy said that because I have a larger tonnage unit, that is why some of my rooms are colder than others. I keep all vents open 100% at his suggestion. Is that possible? Thanks again

        Reply
        • Hi Les, your AC guy seems to think that you have an oversized AC unit and that’s why it short cycles; that would explain why some rooms are colder than others (uneven temperature). Here’s why this happens: You set a temperature to 72 degrees on the thermostat. The AC now has a mission: Just get the temp to 72 degrees. Once you get to that temperature, the AC will shut off, it has achieved its mission.

          Now, the ‘some rooms are colder or hotter than others’ problem happens if the AC unit is too big. If it is too big, it will quickly reach 72 degrees in one room, and shut off. Other rooms, however, will be hotter (let’s say 75 degrees). If you go with a smaller unit, they 72 degree temp will be reached slower but all the rooms will benefit from having the AC run for longer and will also be able to reach 72 degrees.

          In short, if the AC guy thinks that the unit is oversized + you have experienced problems when temperature in one room is lower than in the others, installing a smaller unit makes quite a lot of sense. It seems that you have a good AC guy 🙂 Hope this helps.

          Reply
  43. Hi and Thank You for the great website!
    I am in Louisiana. My elderly mom lives in a roughly 700+sf home; 1-bedroom, 1-bath, 8ft ceilings. The central A/C & heat unit is a 2-ton Rheem, approx 5yrs old.

    The issue is my mom has become very sensitive to cold, especially combined w/wind. The central unit’s blower is pre-set from the mfg on High speed. There are seemingly easy-to-follow instructions to change the blower to Low speed; there is no Medium. I myself find it uncomfortable to be in the path of a vent when the unit cycles On. If processing paperwork, it will blow around unless weighted down or I move.

    My concerns lowering the blower speed are the humidity/mold issue mentioned in earlier posts and the unit running longer to achieve the same amount of cooling(unit lifespan & higher utility bills). Partially closing the vents helps some but I assume that actually strains the unit somewhat? Again, the unit must run longer to achieve the same amt of cooling.

    Both the Black/High speed wire and Red/Low speed wire(the cap is to be moved from the Red to the Black wire; Red replacing Black) are 240v. There is a capped Yellow wire for 208v. I do not understand the purpose of changing to a lower voltage? Also, I assume other changes must be made for that; not just a simple wire swap?

    I do not want to create environmental issues inside my mom’s house or possibly damage the unit.
    Sincere Appreciation in advance for any assistance and information you can provide in layman’s terms!

    Sincere Gratitude, Jeff

    Reply
    • Hello Jeff, it’s quite heartwarming to read how well you take care of your elderly mom. Now, if we take the 20 BTU per sq ft rule, you would only need 14,000 BTU or 1.2 tons to cool the whole house. At High speed – current pre-setting – the Rheem unit delivers 2 tons of cooling. At Low speed, it will usually generate much closer to 1.2 tons needed without the powerful papers-flying-around airflow. Yes, it may take a bit longer to cool the place down but if you set the thermostat temp to let’s say 75 degrees, it will keep that temperature.

      Humidity-wise, the AC running at low speed will take care of humidity very well. With lower airflow, the condensation rate on the indoor evaporator coils is increased since the contact between the cold coils and the air is longer.

      The simplest solution here would be just to change the setting to Low speed. Usually, these units have switches, control panels, or remote controls where you can do that. This switching wires thing is a but unusual, especially on a 5 year old unit. Check if you can switch to Low speed easily (without those wire changes).

      Difference between 240V and 208V is power and thus cooling output. If the unit requires 240V to produce 2 ton cooling output, it will produce 1.73 tons of cooling power at 208V (since you decrease the voltage by about 13% and the amps are the same). This is just a guess, but it may be that th 240V is for High speed (2 ton output) and 208V is for Low speed (1.73 ton output).

      I hope at least some of this might be helpful and alleviate the problems your mom has with the HVAC system.

      Reply
      • THANK YOU Learn Metrics!!
        That information REALLY helps. Unfortunately, the wire switching is the only method available. There are basic nutshell instructions for doing it printed on a sheet on the outside of the upper exterior panel. The sheet shows the wiring & schematic diagrams w/those instructions in the lower left corner. I’ve read some comparable units have jumpers similar to older computer IDE hard drives but the diagram shows wire switching only.

        If I understand correctly, you believe the 240V Low speed[red wire] will produce a slower blower speed than the 208V[yellow wire]. That would make the 208V setting a “medium” between the 240V High and Low settings. I am hesitant to try the 208V setting first because I know so little about electricity and have no idea if other changes are necessary in order to avoid damage to the unit [if that is even an issue].

        While I realize the unit will require to run longer for the same amount of cooling, I am hoping the slower blower speed/rpms will offset an increase in utility cost.

        I do need to find out how to clean the indoor evaporator coils without damaging them. I checked them using a mirror & flashlight and they are very dirty. I don’t think a couple cans of compressed air will be robust enough. I assume the accumulation makes the unit work harder to achieve the same amount of cooling, resulting in increased wear & tear and higher utility bills.

        I can’t Thank You enough for the swift response and excellent information!
        With sincere appreciation, Jeff

        Reply
        • Hi Jeff, thank you for such a thorough reply. I have to admit; this wire switching sounds a bit like from the stone age. Hopefully, you can resolve it by using the manual. And yes, evaporator coils have to be cleaned. If there are dirty, the heat exchange will be impeded as well as the dehumidification rate. If the coils are very dirty, they might freeze over because of the inadequate heat exchange; cleaning them properly is the first task here. Wish you all the best.

          Reply
  44. We have a 2054 sq ft double wide mobile home with a 3:12 pitch shingle roof and 2:12 pitch cathedral ceilings in the Houston area. We currently have a 4 ton 13 SEER 2008 condenser. It is running constantly from about 12 noon to 7pm, with thermostat set at 77 and temp inside 85. We are looking to upgrade it but the companies we have consulted are only recommending another 4 ton based on our sq ft. Shouldn’t they consider the additional cubic feet created by the cathedral ceilings? With looking at the 2005 HUD recommendations for non energy star mobile homes, and our location, we should have a 5 ton. How do I calculate the required tonnage by cubic ft and where do I find the docu?entation for the calculation?

    Reply
    • Hi Anna, given that a 4-ton unit leaves you with an 85-degree indoor temperature, you definitely need more cooling power. One possibility is that the companies are only checking square footage (disregarding the ceiling height). Example: 20 BTU per sq ft rule only applies if you have an 8 ft ceiling height. If you have a 12 ft ceiling height, that’s an additional 50% of the air you need to cool/dehumidify and the rule of thumb would be closer to 30 BTU per sq ft (50% more than 20 BTU per sq ft).
      Mobile homes are very difficult for calculating the cooling BTUs for since the insulation R-values are lower. But all that is theoretical. You have a a much more valuable practical experience with the 4 ton unit and the 4 ton unit is not enough. A new 4 ton unit will produce the same amount of cooling as the current one. That’s why it makes sense to get a bigger – 5 ton or 6 ton – unit.
      The documentation for how to do these calculations (in detail) is found in Manual J guide. Every HVAC guy is aware of this guide; it’s quite strange they suggest the same 4 ton tonnage that is too small, according to the practical experience.
      You might try to fill this form here; it will connect you with HVAC contractors in your area and you can discus the sizing with them. They are vetted guys that will know Manual J guide by heart. Note that you have a cathedral ceilings and that 4 ton unit currently in use does not provide sufficient cooling output. Hope you get a good on-hands HVAC guy that will help you out properly.

      Reply

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