sizing boiler for hw coil

Steve Eayrs

IF you have to have an on demand dhw coil..........and
IF you think the heat demand for the house can't be your second priority.......then yes you would need to size according to what you wrote. BUT.......
between controls that give dhw priority, and a storage tank for dhw purposes, (either indirect fired w/ coil in tank, or storage tank w/ internal coil in boiler, or external heat exchanger and a storage tank.), you don't have to size the boiler so big.
You can size the boiler for the biggest of the two loads, (which is quite often the dhw), and figure out the dhw demand closer, and reduce the boiler size even more by using a storage tank of some sort.
No need to oversize. It costs more to install, its a waste of fuel, etc.. but don't just guess how small you should go. There are enough factors involved that it would be worth your dollar to hire a pro to size it for you.

Steve

boberg

sizing a boiler for HW coil

I have read a few weeks ago that one should allow 40k btu addition to accomodate every gpm of a submersible coil when sizing a boiler. I.E if a coil has a rate of 3gpm the heating load must have an addition 120k btu to accomodate the hw demand . This seems a little high, especially for a residential boiler. Can someone inform me as to the right rule of thumb. Thank you sport fans