boiler sizing

Jim_109

when sizing a new boiler, do you size the boiler according to the home's heat loss or the BTU requirements of the indirect hot water heater? do you take the larger of the two numbers? THANKS!

ALH_4

well...

It depends. If the DHW load is large enough, it may require more heat than a properly sized space heating boiler can supply. However in that case, additional boiler(s) or a separate DHW system are probably required to avoid oversizing the heating boiler.

Normally the boiler is sized for the heat load and the DHW tank is sized with some reserve capacity. Even a small boiler has more heat output than nearly any residential water heater. It takes approximately 8.33 btu to heat one gallon of water one degree Fahrenheit.

Brad White_9

I second what Andrew said.

All the right reasons. Most small houses I have seen of late have higher indirect DHW losses than heat loads anyway. (Example: Typical 1500-2000 SF renovated older house with a 45-60 MBH heating load. DHW tank needs about 80 unless you can wait for it to charge.) Newer, tighter houses of the same floor area might be in the 30-40 MBH heat loss range.

All but a few boilers, mod-cons at least, are in the 80-95 MBH input range anyway, so you are covered.