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How to calculate heating load? (DIY Homeowner)

I see suggestions to calculate the heating load for your house when sizing a new boiler in threads like here and here.
Could someone define this or link to an explanation?


  • Jamie HallJamie Hall Member Posts: 12,358
    It isn't rocket science. It is a bit tedious. Go to Slant/Fin's website, under Homeowner, and wander around a bit and you will find their heat loss calculator. It asks you a number of questions about each room in your house, but provides decent and usable suggestions for the less obvious values. It doesn't cover all the bases -- if you have a superinsulated house, for instance, it has some problems -- but it works and does all the necessary math for you.

    You need that information for either forced air or hot water heat (you do not need it for steam -- boiler size for steam is calculated in a different way). The reason you need it is that the boiler must be sized to match the expected maximum load, but no more. This is one of those situations when enough is enough, and more isn't going to help and will cost you money.

    As for definition -- I presume of "heat loss" -- that is the number of BTUs per hour (BTUh) that the house will lose to the great outdoors. Nothing fancier than that. And the number you want is the number of BTUs per hour on the coldest nastiest day you expect to get.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • DocfletcherDocfletcher Member Posts: 475
    Looks like Slant Fin's calculator is now an app download both android or Apple.
  • GilmorrieGilmorrie Member Posts: 113
    I've never tried the mobile phone app. But when I last used the version for full-fledged PCs, I thought running the program on a tiny screen might be challenging.
  • GordyGordy Member Posts: 9,523
    I will add that using the proper “design day” is a must, and not using the coldest day you have, or the geographic area has seen.
    pecmsgdelta T
  • elifelif Member Posts: 2
    Thanks for the feedback folks. I have downloaded the app and will use it to measure the house.
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