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BTU requirement per Sq feet

CarlCarl Member Posts: 1
I was referred to this site by a friend of mine who couldn't answer this question: How many BTUs are needed to heat ? square feet of housing (in cooling terms it was stated that 12,000 BTUs per 500 square feet of house).

I have an old boiler (~1964 weil mcClaine) and a new addition (built 20 years ago and heated with radiant heat panels). I want to put a new forced baseboard loop into the addition but don't know if my boiler can handle it.

I appreciate any input.


  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Rules of thumb... x btu/hr/sq.ft. of floor seem to be [finally] going by the wayside in favor of full heat loss/gain calculations (almost exclusively computer-program based).

    Amount, direction, type of window glass and frames; levels of insulation; amount of air infiltration; fireplaces; outside design temperature are just a FEW of the variables that come into play.

    Even the older (and generally much simpler) heat loss instructions published in pre-computer-era heating texts will provide a more realistic number than any btu/hr/sqft rule of thumb. Even the REALLY old Mill's Rule (while really geared to standing-iron steam system) would at least provide some form of reasonable comparison between spaces of different constructions/exposures...

    Never forget the "garbage in = garbage out" rule when doing heat loss calculation!
  • BoilerproBoilerpro Member Posts: 1,231
    To give you an idea...

    of how accurate rules of thumb are, I'll take your 1 ton per 500 Sq ft for cooling example. My home is a 2800 sq ft two story frame with 700 sq ft of windows, 9ft + ceiling down, 8 ft + up. I cool it here in northern IL with 1.5 tons of cooling. When running a full engineering cooling load calculation, it came up with slightly over 2 tons. Using the rule of thumb would yield a cooling load of nearly 6 tons of cooling, rougly 3 1/2 times what is known to work and roughtly 2 1/2 times what a good cooling load calc would yield. The oversized equipment would be more expensive to run, humidity would be a constant problem, and equipment would fail more often because of excessive cycling.

    For heating, I hear around here numbers of 25 to 30 btus/sq ft for newer homes and 35 to 40 for older. My older home only needs 18.5 btu/sq ft and runs with a slightly oversized 67,000 btu/hr output boiler.
    Rules of thumb are for lazy contractors or ones that don't want to get up to date and give the customer the best value for thier dollar. IMHO

  • Mike BMike B Member Posts: 155
    Dont use em

    Boilerpro is right. Get a proper heat loss calculation done. Your existing boiler may already be too BIG(!). I've riped out 120,000 btu systems and repoaced em with 60's. No problem.

  • Mark EathertonMark Eatherton Member Posts: 2,542
    Rules of thumb

    are in most cases grossly over sized. I think they were developed by the manufacturers. Do a real loss calculation. There's free software available to do it right on this site. No excuses.

    I've heard through the grapevine that the new German building code standard requires an average load of less than 7 btu/sq ft/hour. That makes us Americans look even more wasteful than before.

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