My question is simple. Is heat load calculation the same for sizing air conditioners as it is for sizing radiators? I.e if an AC guy gives you the heat loads which lead him to select particular air conditioners can you use those BTU figures for sizing radiators or designing your hot water radiator system (determining flow rates and head etc?)

  • 1
    Now you’ve got three answers - yes, no, and maybe. Any of them could be right, depending on what you actually have from the AC guy. Can you clarify whether you have a complete manual J calculation, or just the heat loads for the cooling system? In any case, you cannot simply use the cooling system BTU/hr to size a new heating system. – Mark May 3 '18 at 11:53
  • Well the AC guy did use Manual J, but I presently have the AC system size. I guess I can revise my question to simply: is Manual J for a +N degree temperature difference the same as for -N in climate zone 4A – rhl May 4 '18 at 9:57

No. Heating and cooling loads are dependent on two things: (1) the house dimensions, insulation, windows, etc., and (2) the climate in which it is located. Some areas have very cold winters and moderate summers, so the heating load will be high but the cooling load quite low. In another area with moderate winters and hot summers, the opposite will be true. Both have to be calculated independently.


Yes. If your contractor calculated a Manual J for your house, then they calculated the cooling and heating load required to cool and heat that specific house in your climate and based on features of your house (volume, insulation, orientation, windows and their SHGC/U-factor, etc). If done correctly, the numbers they provided for heating are valid for any heating system you install. You need to review the calculations though. They should have a report and you want to ensure it is for your house and matches the features of your house.
Good luck.


Maybe. Depending on how the contractor came to the conclusion that they did, and what information they provided you. You may, or may not have enough information to size your heating system.

As mentioned, if the contractor did a Manual J, and provided you with a full report. Then you should have enough information. However, if the contractor simply estimated the loads based on similar jobs, or didn't give you all the numbers. Then you might not have enough information to size the heating system.

Based on the information you've provided, the answer is... Maybe.


An ACCA Manual J heat load calculation will tell you how many BTUs a house is loosing or gaining with a particular outside temperature and a desired inside temperature. For instance if your design outdoor temp is 30 degrees in the winter and you want your home to be 70 degrees, that is a 40 degree split. They then measure up all surfaces that can conduct heat to the outside such as floors, ceilings, windows and doors. Throw in a few other factors, run it through the Manual J tables and you get the amount of BTUs lost or gained at a particular split. So no matter what type of heating you use you will need that many BTUs to keep your home at that temperature. There are then additional manuals to determine the size equipment needed. For forced air furnaces, if you were to figure you were loosing 80,000 BTUs and you wanted an 80% efficiency furnace you would get a 100,000 btu furnace and assuming the calculations were exactly right and the split was always 40 degrees your furnace would run 24 hours per day. Normally your split would be lower than that and Manual J allows for wiggle room so in reality your furnace won’t run 24 hours a day.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.