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Slant fin heat loss app correlation to cooling

So I have about a 2200 square foot house. I did the heat loss calculations. The results were about 36000 btus.designed for 70 degrees internally and external temperature 0 degrees.

I had an hvac guy come showed him my heat loss calculation but he is saying I need at least a 4 ton AC unit. Told me my previous 3 ton unit is too small.

Seems like based off my heat loss numbers that is oversized being as the heat differential will be even less than the design for heating.

Am I correct in that I can use the calculator in the same fashion or am I missing something.


  • Hot_water_fan
    Hot_water_fan Member Posts: 1,278
    edited March 28

    A few things are happening:

    1. Your HVAC guy is winging it. They didn't do a cooling calc and even if they did, it would come back at 4 tons because that's what they want to install. They picked 4 tons out of thin air - conveniently for them, it's more expensive. I'd find another if you want to be comfortable - bigger is often worse!
    2. Heating calcs are different from cooling calcs - so in that respect they're right. You need to account for humidity and solar gain in a cooling calc, which are less important in a heating calc. Obviously the delta T will be less in your situation, which you picked up on - but other loads will contribute some.
    3. If you have a 3 ton AC already, unless it runs nonstop on the hottest afternoon of the year and cannot bring the indoor temp down to 72 or thereabouts, you're not undersized. This is a quick way to establish the ceiling on your cooling loss. You can then decide if you want to put in the cooling load calc effort to determine if you can install a 2.5 ton or smaller unit. If you install a variable capacity unit, you have more margin for error.

  • Resipsa88
    Resipsa88 Member Posts: 5

    Thanks for the help. I put a three ton AC in about 4 years ago, but then my decided she wanted to expand the house so we added about 30% to the house. So I am just trying to determine if I am undersized now as the HVAC guy suggested. It is a variable system. Do you know where I can find the formula for cooling calculation that includes humidity and solar gain.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,632
    edited March 29

    Load calc for heat loss does not calculate the solar gain or the latent heat needed to condense the humidity in the hot air in the house to the lower temperature. (that is why there is condensation dripping off the AC coil).   Since the 3 ton did the home comfortably you may only need 2.5 or even less for that same space.  The addition throws a little monkey wrench into the gears.   That addition will lose and gain heat differently than the old part of the house.   What is the percentage of glass to wall area?  Did you use the same R value of insulation in the addition as the old section? (probably not). Is the infiltration of the new addition the same as the old part of the house?  I don't think so!  

    A properly calculated manual J heat load may reveal that you may only need 3 tons for the new and old together. But a manual D will tell you if connecting that same trunk duct with some 6" flex duct will be a balanced system or if the new section will heat and cool differently. Balancing may be a problem.  The heat loss and heat gain are not a linear swap.  Some areas may not need as much heat as they do cooling and vice versa. Consider this; a sun porch with 85% glass to wall will need more cooling on the south side of the home than the identical room on the north side of the home.  But the heating load will be identical. That same room when compared to the rest of the home will need a much larger heating and cooling load than any other room in the home where the windows only take up 25% of the wall.

    Mr. Ed

    EDIT. PS

    Here is a whole house Manual J printed FORM J1. You are supposed to use one form for each room to get the ductwork CFM you need for using a Manual D.  A properly calculated Manual D is needed to get a truly balanced system. This is what I would recommend. 


    In the case of this example, they used the FORM J 1 for the entire house because the duct system was already in place and was balanced with the old system. You should at least do a FORM J1 for the old section and another FORM J1 for the new section. Then you can see if the duct system is adequate or if you need to use different zones with separate thermostats.

    You may also consider keeping your 4 year old system in place and adding a separate Ductless mini split for the addition, or maybe just a small inverter Mini with an indoor section that can be ducted.  It is amazing what is available now, that was not available just 10 short years ago.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Resipsa88
    Resipsa88 Member Posts: 5

    I knew the heat loss calculation was not an exact translation. But heat loss does have a much larger delta T comparitively since i live in New York Metro area.

    I was actually having them redo the duct work to balance the the variable zones by square footage, but as you pointed out that may not be a direct correlation because sun exposure. The new extension actually was added to the north east side, so luckily it will have less sun exposure.,

    I agree with you. I actually was in the fortunate/unfortunate position due to termites that I needed to open up all the walls in my house, allowing me to reinsulate and even put exterior insulation. The new construction will have a bit more insulation in the walls due to it being constructed out of 2x6.

    Thank you for the link I will actually put my mechanical engineering skills to use.😂 I

    As you said if i am going to add it will be a mini system, as I am already adding them to the basement, but I would prefer to not have the added expense. Lets see how I do on my calculations.

  • Resipsa88
    Resipsa88 Member Posts: 5

    Ed, any chance their is another link, or directions to get to the link you posted. It says access denied. Right now I was using the 1997 ASHRAE hand book to do my calculations

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,278

    Also, consider if balance is actually important to you. Maybe a room is a degree or two warmer - how much will you pay to fix that?

    From the contractor's perspective - they don't get paid for a cooling calc and they won't get a call back if the system is too big, so the incentives are working against you.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,632

    Try Google search: ACCA FORM J1


    Maybe this link to download the spread sheet. acca.org› HigherLogic › System › DownloadDocumentFile.ashx

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Resipsa88
    Resipsa88 Member Posts: 5

    So I ran the calculations from the ACCA (although modified it slightly for a bit cooler internal control than recommended) and came up with slightly less than 39,500 btus. I have a 3 ton (36000) variable system. Seems like it would be a waste of money to upgrade to a 4 ton unit when I am only undersized by about 4,000 btus.

    EBEBRATT-Ed Member Posts: 13,440

    You really need to calculate the heat gain and heat loss there are a lot of factors that go into it.

    That being said you HVAC guy is off base.

    Around here (I am in MA) for the average house the cooling load is about 1/2 the heating load. But that is a rule of thumb.

    Heating and cooling jobs are too expensive to size equipment off rules of thumb