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schorched air question

Roy_6 Member Posts: 8
Thanks Mike, thats the perfect answer ;0)


  • Roy_6
    Roy_6 Member Posts: 8
    schorched air question

    Good morning Gentlemen,

    Without performing a full heat loss/gain calculation for a 2nd floor 800 sq' appartment in a renovated century house,is there a rule of thumb/ballpark answer to this question.

    Assuming that there is a properly designed duct system installed, how many CFM is required for say a 50000 Btu heat loss? how many CFM is required for say 2 tonnes of cooling for the same appartment.

    Thanks Guys, any input would be most appreciated.
  • THP_6
    THP_6 Member Posts: 31

    For the AC 800 cfm. For the heat 1000 cfm.
  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291
    800 sq ft and 50K heat loss???

    That's 62 btu's per sq ft!!! Do something about that before you install anything. Is that a calculated heat loss or someone's "rule of thumb"?
  • Roy_6
    Roy_6 Member Posts: 8
    thanks for the input

    I have been asked to troubleshoot an install that I provided a small boiler for. Put the boiler in 2 years ago...duct system etc supplied and installed by others because AC required...I don't do AC. Now after 2 years operation client says AC did not work well this summer. I pointed out that the AC was installed by others, client says "oh but the heat did not work either"; funny how during the winter when heating was operating I did not get a call then!
    Anyway; when the boiler was installed the home was still under renovation so did not know the final layout or size of the apartment so I picked the heat loss out of the air. As a point of customer relations I am going to see the job because it is my boiler and to help the client. Being a hydronic installer, don't know too much about forced air so that was the reason for the "rule of thumb" CFM.
    I suspect that as the renovations were ongoing at the house the coil may be plugged or something.

    As always, thanks for the help, you can always get help at the wall.
  • Roy_6
    Roy_6 Member Posts: 8

    Just in the shower and thought of what I was realy trying to ask.
    Is there a general rule for heating? for each 10000Btu you would need this much CFM?
    The 400 CFM per tonne of cooling I had heard about.

    Once again thanks for the input.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Assuming 150°F register air supply temperature and 70° return air temperature here's the calculation:

    (BTU hr load * 15.4) / (60 * 19.2) = cfm

    15.4 is the number of cubic feet of air per pound @ 150°F

    60 is the number of minutes in an hour.

    19.2 is the number of BTUs available per pound of air given the above temperature conditions.

    For other supply/return temperature combinations you must recalculate cubic feet per pound and BTUs available for the supply/return temperature difference. Tables for both are readily available.


    The above formula can be simplified (ONLY FOR THOSE ASSUMPTIONS) as:

    BTU load per hour * 0.0136 = cfm

    This is about 13.6 cfm per 1,000 btu/hr of load or 136 cfm per 10,000 btu/hr load. Again, DO NOT FORGET THAT ASSUMPTIONS HAVE BEEN MADE. Seeming small changes will result in quite large differences in CFM.
  • Mitch_4
    Mitch_4 Member Posts: 955
    heat rise


    Your example shows the heat rise for the unit at 80°F. that is 20 year old info. Most modern units are 35-65°F heat rise.

    Check the manufacturers rating plate for the recommended heat rise for the forced air system you are using, and plug that number into Mike's formula.

  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Did stress that those numbers were for ONE situation (they're also based on dry air at standard atmospheric pressure).

    Again, tables are readily available although you might have to convert weight per cubic foot to cubic feet per pound and/or BTUs absorbed by 1 cubic foot per ° to BTUs absorbed by one pound, but both are simple conversions. Am not finding any on-line dry air tables except in metric measures.

    Say the heat rise is 60°F (supply air at 130°; return at 70°):

    10,000 btu * 14.9 / 60 * 14.58 = cfm

    simplifies to:

    10,000 btu * .0168 = 168 cfm

This discussion has been closed.