Click here to Find a Contractor in your area.
In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
Need to contact us? Visit

Formulas for heat-loss calculations

Can anyone confirm any of these formulas for me?

BTU losses = sq. ft. X (1/R) X temperature difference in F

Losses to ceilings are 15% higher due to radiation losses?

BTU losses via infiltration = room volume Cu. Ft. X .018 X temperature difference in F X A

A= .2 for well sealed rooms, ranging to .5 for poorly sealed rooms



  • JStarJStar Member Posts: 2,668
    Heat loss

    They might be right...for one specific type of home. Which home? Who knows?

    Better do a real Manual J.
    - Joe Starosielec
    [email protected]
  • ChrisChris Member Posts: 3,056
    Heat Loss for An Outside Panel

    Ti-To/R-Value = Btu/hr Sqft

    Ti = Temp Indoor

    To= Temp Outdoor

    R-Value = Self Explanitory
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Zeffern_CochraneZeffern_Cochrane Member Posts: 12

    Thanks Chris that yields the same result as my first formula so thats one down probably a few more to go.

    Jstar your response intrigues me. Another post in a different sub category suggested that the heat loss requirements in a home heated with radiant floor heat would be less then some outer home with a different kind of heating system (Forced Air I presume). I have a heat loss calculation done by a hydronic heating supplier that came up about 15,000 higher then what I calculated on one of those web calculators. I have tried several and with just simple square box designs came up with a different answer on each one. That has led me to trying to develop a spreadsheet to come up with numbers that I can have confidence in.

    One of the things I stumbled across was this factor of infiltration and apparently this has a huge factor in the final results. I believe that the calculations done by the supplier are on the high side of things, and why not if you can sell a 200,000 boiler to someone who needs a 150,000 boiler all the more profits to you.

    Can anyone suggest resources that may lead me to discover the true effects of infiltration on heat loss. Were building the house our selfs so we are achieving a very good vapour barrier but I am sure that there will always be a lot of losses through tiny cracks in doors and window gaskets ect.

  • ChrisChris Member Posts: 3,056
    Suggest You Purchase

    Modern Hydronics 3rd Edition avail here in the store. Lays it out quite nicely. Infiltration is the main reason why a radiant loss is less then a fin tube or air loss. The more the convection the more the infiltration loss. Pretty basic stuff.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • NRT_RobNRT_Rob Member Posts: 1,009
    not really true.

    more PRESSURE differential more in/exfiltration. not more "convection". If you dump 500 cfm in a bedroom with no way out, you will drive exfiltration. ditto for a room with a return and no way for air to get to it.

    taller buildings with stack effects is more in/exfiltration.

    more holes of course = more infiltration.

    radiant loads can be lower because duct losses can be lower. you might drive less infiltration but it's not inherent to the application of forced air to have higher infiltration.

    zeff, your formula are correct but your inputs are still where the magic is. GIGO my friend. you have some heavy reading ahead of you on that front to get good numbers.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Zeffern_CochraneZeffern_Cochrane Member Posts: 12
    edited April 2013

    Thanks Chris & Rob good advice I am sure maybe I am getting to hung up on this BTU stuff and should just go with what the supply house guy says. In the end I will need someones stamp of approval for the building inspector so it does not matter what I think it will be up to who ever can certify that the system meets design temperature conditions. I looked at the book section, seems like it covers quite the range of all the things that I want to know. And probably a lot that I don't need or shouldn't worry about for a one house build. Though I have been contemplating a couple of those books from Dan that were way less expensive and probably more appropriate to the kinds of things I really should know. Books like the installers guide and pumping away.


    One more thing Rob can you send me a larger picture of that one in your signature it looks really nice I would like to look at it more closely.
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!