Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Pipe sizing based on BTU’s

Mosherd1 Member Posts: 70
When looking at charts and rules of thumb, often it will say a certain sized pipe can carry so many btu’s, but I’m assuming that is based on 180* water, so if looking at sizing pipe to feed a radiant manifold but your supply water temp might be only be 110*  do you need a larger pipe to get the same btu carrying capacity?


  • mattmia2
    mattmia2 Member Posts: 9,646
    It is based on the delta t. Usually it is 20 Fahrenheit degrees. You have to work out the actual delta t across your emitter and flow to figure out how much heat will actually be transferred. It could be 20 degrees f from 120f to 100f, it doesn't matter the absolute temps, just the change.
    EBEBRATT-Ed Member Posts: 15,525
    You need to know the TD or temperature drop of your system.

    1 gallon /min (gpm) of water circulated with a

    10 degree temp difference 5,000 btu/hr
    20 degree temp difference between supply and return will transfer 10,000 btu/hour=baseboard system
    30 degree temp difference 15,000 btu/hour
    40 degree temp difference 20,000 btu/hour

    Do in this order:

    Heat Loss of building or space
    install enough tubing to offset the heat loss at the design average water temp you are using.
    Figure how many gpm you need to circulate for btu load.

    So a radiant system might be designed for 120 supply and 90 return for a 30 degree TD 15,000byu/gpm

    So if the heat load was say 60000 btus you need to circulate 60000/15000 or 4 gpm

    The 4 gpm would be split into say 4 circuits of 1 gpm each

    so your tubing would have to carry 1 gpm (each circuit) and enough tubing to output 15000 btus at an average water temp of 105 degrees which is midway between 120 & 90

    The pipe from the boiler carrying the entire load (4) gpm would be 3/4 copper

    Pipes size for gpm:

    1/2'-1.5 gpm
    3/4-4.5 gpm
    1"---8 gpm
    1 1/4"-14 gpm
    1 1/2-27 gpm
    2"---45 gpm

    This doesn't apply to pex