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Community and Q&A

Heat Loss Calculator

Steve95065 | Posted in Mechanicals on

I have just purchased a house in Santa Cruz, CA. It is about 2400 sq ft, two story and was built in 1975. It has Hydronic baseboard heat. The construction is pretty typical for that era. I have scheduled a blower door test to see what my starting place is. I plan extensive remodel and energy upgrade over the next year.

I have prepared a detailed floor plan and find 96 feet of Slantfin baseboards. Using Slantfin specs, I get about 550 to 600 btu per foot of baseboard which adds up to at most, 60,000 byu’s. There is a 200K BTU boiler in the basement. Does this make sense?

I would like to do a thorough heat loss calculation but I can only find very expensive software or one free program (from Slantfin) that will only run on my iPhone. I find it very tedious to enter this much data into an iPhone.

After doing all the bath and kitchen remodeling, replacing the windows and upgrading the crawlspace and attic sealing and insulation I plan on putting in a new condensing, modulating boiler so I would like to size it properly.


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  1. GBA Editor
    Martin Holladay | | #1

    The output of a length of baseboard radiation varies depending on the design of the unit, the boiler water temperature, and the flow rate. A typical spec for rating radiation is 1 gpm flow rate at 180 degrees F.

    Note that the rating is in Btuh/foot, not BTU/foot. Your specs for common types of Slant Fin radiation are about right; 580 Btuh/foot is a good number to use.

    Plumbers and heating contractors have been using rules of thumb to size boilers and baseboard radiation for decades, so there is no guarantee that the radiation installed in your 1975 home was appropriate. Moreover, if you are planning "extensive remodeling and energy upgrade" work, then the heat loss in 1975 is irrelevant.

    What matters is the heat loss in 2013, after your energy upgrades are complete.

    I'm afraid that someone needs to perform a Manual J calculation to size your heating system. If you can't do it, I suggest that you hire an energy rater or energy consultant to do the calculations for you.

    If you are lucky, you might even find a heating contractor capable of performing a Manual J calculation. If a contractor claims they can do it, ask them to show you the paperwork once the calculation is done.

  2. user-945061 | | #2

    I frequently find houses with modulating equipment where the design day heat loss is < the lowest stage. A lot of modulating equipment has a first stage of ~ 40,00 BTU/hr. With an upgraded house in a mild climate your Man J will probably be less.

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