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Rule of thumb for btus

BriH Member Posts: 3
Can anyone give me the a rule of thumb btu. I've been told LxWx40 and I've been told LxWxHx20.

I was also curious about CFM for room and BTU for mini splits. I appreciate any help!




  • Tim McElwain
    Tim McElwain Member Posts: 4,608
    Not sure if this is what you are looking for:

    Length x width x height = cubic feet National Fuel Gas Code allows 50 cubic feet of air for every 1,000 BTU's. This requirement is sometimes referred to as the 1/20 rule, because 1,000 BTU per Hour divided by 50 cubic feet or, for each cubic foot of room volume, you can install 20 BTU/Hr.

    The BOCA code is instead of 50 cubic feet 40 cubic feet. Find out what local code officials allow.

    Hope that is what you are looking for.
  • jonny88
    jonny88 Member Posts: 1,139
    rule of thumb

    I have heard lxwxhx4 will give btu for room.havent tried it but I know someone who swears by it.Give it a try and let me know how accurate it is
  • Mike Murray
    Mike Murray Member Posts: 22
    answer to "rule of thumb:"

    Hmmm, I havent been on this forum for quite some time but I have to agree with Tim.  Basicaly, calculate it.  No "rule of thumb" at all.  But if you want a "rule of thumb" then it depends on your location, since a rule of thumb from where I live may vary greatly to where YOU live.

    If you know the design temperature for your location then it's fairly simple to figure each and every heating system in your area but to calculate for another place that has a much lower maximum heating demand, or a much higher one, then the figures might change dramatically.  Actually, quite significantly.

    The "design temperature" (the friggin' coldest it's expected to get where you live) is going to be the deciding factor in the equation that determines how many btu's, or kilojoules, or what have you to replace that heat loss.

    All the other variables such as square footage of wall space, window area, door area, etc, will factor in and make their own impact on heat loss.

    SO, basically,  if your situation requires say, 50,000 btu's of input for YOUR location, but my location requires 90,000 btu's to make up for the "design load" heat loss, then there is no "rule of thumb".
  • Rich_49
    Rich_49 Member Posts: 2,766
    All thumbs

    Here's a rule of thumb for ya , the only rule that matters . GET A PROPER HEAT LOSS DONE .  Here is what rules of will get you , IN TROUBLE .  Here are some I have heard .   North of the Mass Pike 40 BTU sq ft , South of the Pike 30 BTU sq ft . How bout this real live horror , 150 sq ft room with a window that is 10 feet wide and 5 feet high on 1 exposed wall  next to a room that is 340 sq ft has the same size window and 1 exposed wall .  You would never guess that the BTU per sq ft requirement were so different on those 2 .

      Measure the house in detail and punch in the numbers , also learn how to do it when the power is out by hand or with a calculator . There is no rule of thumb , only the right way . Read several posts here and see what the consensus first question is on any problem having to do with heat .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,929
    edited February 2014
    Design Temperature

    Design temperature is NOT "the friggin' coldest it's expected to get in your area". It's the coldest average temperature for your area. The key word is "average" which means that when you're doing your heatloss calculations to design square feet of radiator sizing and water temperatures, there may be some historically cold days where the owners need to throw on a sweater.

    But to Mike's defense, who wants uncomfortable customers. I usually over design my systems with hotter water temperatures or larger radiators. Our design temperature here in the East Bay is 36F (yeah, read it and weep), but I design for 32F. It means more gas usage, but fewer nagging customers.

    Only after you calculate heat losses for your jobs over time and in your area will you get a feel for rule-of-thumb numbers.

    Just my 2 cents.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab