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Two condensing gas boilers totalingm 700,000 BTU can a 2” copper header carry that BTU load

bob eck
bob eck Member Posts: 930
Contractor is looking at installing 2 condensing gas boilers totaling 700,000 BTU
Supply and return to each boiler is 1 1/2”
There are 10 - 3/4” branches on supply and return.
Can he use a 2” copper header must he use 2 1/2” copper header. From what I saw 2” copper carries 450,000 BTU at a 20 degree delta T
Am I correct on this?
Does anyone know if 2” Aquatherm pipe header carry more BTU than copper tubing.


  • hot_rod
    hot_rod Member Posts: 22,121
    Short answer is no in both cases. 1-1/2" is a bit small for 350,000 at 20∆. 2" is small for the total load.

    Is the 700,000 input or actual boiler output? A 350K input boiler running 85% is 297 actual output, that is what you need to size pipe and pump to.

    The boiler manufacturers usually show you a few flow rate options. If you can run 25, 30, 35 ∆ the piping size would go down with lower flow rates.

    Same with the 700K is that the building load or combined input of the two boilers. What type of distribution? can citron a larger ∆?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    EBEBRATT-Ed Member Posts: 15,517
    It depends on the delta T your running .
    @20 deg. td you need to pump 1 gpm for every 10,000 btu of boiler output

    so 700,000= 70 gpm 2 1/2 " pipe header 2" to each boiler

    30 degree td=1gpm/every 15000 btu boiler output. you could use 2" pipe header but your maxed out flow is 46 gpm total
    1 1/2" to each boiler

    40 degree td= 1gpm/20,000 btu boiler output= 35 gpm total flow 2" header, 1 1/2 to each boiler

    The pressure drop through condensing boiler sometimes gets a little crazy when the flow changes so watch that.

    also at a 20 deg TD say 180 supply/160 return base board average temp would be 170

    30 degree td 180 supply/150 return baseboard average temp would be 165

    40 degree td 180 supply 140 return baseboard 160 deg

    Pumps, baseboard, flow, boiler, pipe size. It all has to match up to work right

    Aquatherm would have the same flow not enough difference to matter. It probably has less flow resistance than steel.

    What temp are you running? I have used some aquatherm. If used on heating you need a ton of supports it flexes a lot. Check the ayuatherm book supports can be as close as 2-3 feet depending on the size