Run air source heat pump WITH Central Boiler?

[Joacchim]

When I connected my Central Boiler to a heat exchanger in my forced air furnace plenum in 2012, I added a separate thermostat so I could run the wood source heat exchanger without using my previous (and still in place) primary heat source, an air source heat pump with oil backup. For the past couple of winters I've stopped using that separate t-stat, instead using my heat pump, which of course uses the heat from the C.B. whenever the furnace fan comes on. I did this thinking I would realize some savings on my electric bill, and some savings on my wood supply, but not perhaps as much as if I'd run just one or the other. I can't say that I have saved on either - didn't try to compare before/after (plus I think it'd be difficult to get a fair comparison).

So I'm asking, do you guys think this (a little savings on both wood and electricity) is a correct assumption? Or was I better off just using the wood-fired heat? I'm getting older and harvesting my own firewood is getting old, too

 

[chew72]

Well I can tell you exactly what my usage is and my set up sounds very similar to yours. My "primary" heat source is a heat pump with electric backup that's turned off. We typically heat with that above 0°C and heat with a Froling 30kw wood boiler when it's below 0°C . The boiler heats the 1000sqft main floor using 2 heat exchangers in the centrally ducted heat pump in fan mode. We don't heat the upstairs as we like it cooler in the bedrooms.

Since we have a smart meter on our house I can tell you exactly what our daily electricity usage is. The temperature has been between -5 and +7°C When using just the boiler to heat we use 18-21 KWh. (That's also the same if its -25°C out) In the last 2 weeks on the days we've used the heat pump our daily usage is 42-50 KWh so 21-32 KWh a day to heat in the shoulder season with the heat pump. That might give you an idea of the savings you'd get from burning wood.

I'm sure you saving when burning wood so long as the heat exchanger is in the supply duct and not the return. You can also just save the wood for the coldest parts of winter to get maximum efficiency out of the effort put in.

 

[Joacchim]

Well I can tell you exactly what my usage is and my set up sounds very similar to yours. My "primary" heat source is a heat pump with electric backup that's turned off. We typically heat with that above 0°C and heat with a Froling 30kw wood boiler when it's below 0°C . The boiler heats the 1000sqft main floor using 2 heat exchangers in the centrally ducted heat pump in fan mode. We don't heat the upstairs as we like it cooler in the bedrooms.

Since we have a smart meter on our house I can tell you exactly what our daily electricity usage is. The temperature has been between -5 and +7°C When using just the boiler to heat we use 18-21 KWh. (That's also the same if its -25°C out) In the last 2 weeks on the days we've used the heat pump our daily usage is 42-50 KWh so 21-32 KWh a day to heat in the shoulder season with the heat pump. That might give you an idea of the savings you'd get from burning wood.

I'm sure you saving when burning wood so long as the heat exchanger is in the supply duct and not the return. You can also just save the wood for the coldest parts of winter to get maximum efficiency out of the effort put in.

Thanks. I try to do as you do and use the heat pump alone when it's above freezing, but sometimes I'll still keep the woodburner going if it looks like temps are still going to dip below freezing in the next day or two. My heat exchanger is in the ductwork downstream of the heat pump - hoping that's what you mean by "in the supply duct". I know I'd save a lot more in electric energy cost if I could just burn wood (alone) when it's really cold, but I'm trying to save wood, and from what you're saying it sounds like what I'm doing isn't hurting that objective. I have solar panels so the electric cost isn't hurting that much, although winter production isn't that great, either. Thanks again.