How to size boiler in a house that also has a wood burning stove?

HeloDog

I’m getting estimates from HVAC companies to replace my propane boiler (135,000 BTUh). My one story house (2,300 sq ft) also has a wood burning stove. Heat for 3/4 of the house is provided by the wood burning stove - without any help from the radiant heating system in the floor. But the heat from the stove doesn’t quite get to a couple of the rooms that are furthest away it. And in those rooms, the thermostats will occasionally command heat from the boiler.

So should the replacement boiler be sized to heat the whole house (the radiant heating system is plumbed throughout the house) or sized for only the rooms I actually heat with it (about 1/4 of the house)?

I starting thinking about this because I could tell the boiler was short cycling the other day when only two zones were turned on and commanding heat. The burners were turning on and off every 7-10 minutes. It hit me that this might be what it does most of the time during the winter because most of the zones are getting plenty of heat from the stove.

SuperJ

I would size for the whole heat load, but make sure it's done accurately without a lot of extra fat. There will be times that it won't be feasible to keep the wood stove going.

You want to make sure you have the turndown, zoning and/or buffering to handle the smaller loads while the stove is running. Here is a calculator to help size a buffer tank to avoid short cycling https://www.lochinvaru.com/resource/calculator/buffer.html

135kbtu for a 2300sqft house seems pretty high for most places. I wouldn't be surprised if a 80kbtu 10:1 modcon could handle the load once you run the numbers. If it still can't I would suggest improving your envelope leakage and insulation where feasible. If you end up sizing the boiler extra tight, you can always add some electric heat rather than jumping up to the next size boiler (and compromising on the minimum fire rate). Minimum fire rate is an important spec to look at, lower is almost always better, but a buffer tank can make up for a lot.

HeloDog

Thanks SuperJ. That's good info.