Compatibility of woodstove and minisplit heatpumps?
Hi folks,
My wife and I will soon be moving into a 1950’s single-story cape in northern NH. It’s about 1000 sqft with an unfinished basement and an attic for insulation only. The kitchen and living room are an open floor plan with a central woodstove, and we want wood to be a central part of our heating into the future. Eventually we want to remove the existing oil boiler and hydronic baseboards. At that point, we will want another heat source which can supplement the stove while we are present (but away during the day), and cover the whole-house load when we leave for a few days in winter.
I like the concept of minisplit heatpumps (and PV is a potential in the future) – but I’ve been reading here about their high minimum set points (typically 60F?) and losing efficiency when they cycle on-off. It seems logical to put a heatpump in the main living area, but I wonder if the woodstove running would cause the heatpump to function inefficiently? What’s the thinking on the pairing of woodstoves and minisplits? Anything to look out for, or guide our thinking?
Thanks!
Doug
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Replies
>"I’ve been reading here about their high minimum set points (typically 60F?) and losing efficiency when they cycle on-off."
Both Mitsubishi and Fujitsu have modes that allow a 50F setpoint. With Fujitsu it's called "MINIMUM HEAT", with Mitsubishi you have to enter "SMART SET" mode to set it to a temp below 60F (but it won't go below 50F.)
The efficiency hit from cycling on/off is real, measurable, but not severe enough to matter. The efficiency at a 50F indoor setpoint will be higher at any given outdoor temp than when it's set to 70F indoors (the indoor temp where they are characterized in AHRI efficiency testing) due to the much lower delta-T it's pumping against.
When the wood stove is running high enough for the mini-split to be mostly cycling you really don't care about the efficiency hit, since it's using so little electricity. As long as the mini-split is reasonably sized for the room/zone load without the wood stove it's average efficiency will still be pretty good.
There are very different animals.
What is great about a wood stove is they put out a ton of BTU and the leaky wall and window provide plenty of air to replace what is going up the flu and the drafts nicely spread the heat around the rooms.
What is great about a mini split is that can add just a little heat when and where you need it. When you put it in a well sealed and insulated envelope you don’t need much heat so everything is great.
If almost all your heat is really going to come from the wood stove keeping and using the old oil burner as backup heat seems like a no brainer.
It seems to me the first place to focus your efforts is to air seal and insulating the place until the wood stove will not work unless the window is open. If you buy heating equipment before you reduce the load you will end up with oversized equipment that will not work right when you get around to doing the hard unsexy work.
Walta
I think in a cold climate there is no better combination than a wood stove and minisplit in an air tight, well insulated home.
A main component of your plan needs to be making everything very air tight, insulating your basement, bringing your attic to R-60 and also determining how much insulation is in your walls, and consider replacing siding so you can add 3" of foam to the walls.
It really can't get too cold for a wood stove, but all winter long there are times when a fire can 'cook you out,' exception maybe for a catalytic stove like the blaze king. The mini split fills that in-between weather perfectly. It also gives you a little more flexibility with the woodstove, if you are exhausted and decide to start the evening fire at 5PM, you can count on the mini split to kick on in the early morning rather than waking up cold.
Just some ballpark figures, after 1 winter with our mini split, upstate NY very similar climate to yours. We burned about 3/4 cord in a relatively inefficient wood stove. Mini split added apx $30 a month to our winter electric bill @ around .13 cents per kw, that was keeping it set at 68F because we have a 1 year old. Next winter I think we'll set it to 64 or 66 which should substantial reduce that cost. I let the mini split heat the house several times when we were below 0F and it really didn't skip a beat. I would have absolutely no concerns with the unit keeping the house from freezing while we were on vacation.
Today is another good example, 38F and rainy, costs almost nothing to run and the house is perfectly comfortable.
Also throw in the ability dehumidify and cool with house in the heat of summer.
>"Also throw in the ability dehumidify and cool with house in the heat of summer."
Northern NH has a real enough cooling season with many days with low sensible load and high latent load, when judicious use of "DRY" or "DEHUMIDIFY" mode on the mini split can make a real difference in comfort. The cooling mode flexibility of a mini-split does a much better job of managing it (and at higher efficiency) than window-shaker AC or most central air.
Thanks folks - these are very helpful responses!