Indirect vs. boiler

Jim_109

when sizing a new oil fired boiler for a hot water heating system, do you size the boiler based on the home's heat loss or the indirect't needs?

It is a small home with an estimated 50,000 heat loss.

Brad White_9

Balancing Act

Always an excellent question.

"Normal" practice (whatever that is) says to size the boiler for the greater of the two. That will be safe and that will work.

The downside: Say that your indirect requires 80 to 100 MBH to drive it in a reasonable amount of time. Your boiler would be double the size it needs to be for the heating load...

Chances are though, that the smallest oil boiler you can find will be over-sized anyway, as far as your heat loss is concerned. You may find your smallest available oil boiler at 0.60 GPH input and maybe 70 MBH gross output. There may be even smaller ones, this is just a quick look in a catalog.

What I would do is calculate a compromize, accepting that my boiler will be over my heating load even on a design day.

I would select my indirect to work with my boiler (increase storage and allow more time to heat up). I would also put my heating circuit on an OD reset regimen via a 4-way mixing valve. This will help protect the boiler and prevent boiler high temperature spikes during DWH production from affecting the heating circuit.

Your heating flow can continue while the boiler output is entirely directed to heating your showers. Your only limitation is the flow rate relative to the boiler output at limit; you have to keep your return water above 135 degrees and that will determine your recovery rate.