Backup wood stove heat and stack effect
Hello,
I am building a 900 square foot, two story house on piers. I will have r-40 in the walls and elevated floor, and r-60 in the roof. I will air seal to PGH standards. The first floor is a small office and my utility room and is about 33% as big as the second floor. I live in Maine and need a back-up heat source. I will heat with a heat pump.
I want to put a small wood stove on the second floor in my living space so i can enjoy the occasional fire and have a back up heat source. I am hoping that, relying on the reverse stack effect, it would be sufficient to keep the first floor above freezing should old man winter rear his ugly head and the power go out. Is it reasonable to assume that with my air sealing and insulation, it would perform in this way?
ps. There is an open stairway joining the first and second floors.
Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Stephen, I'm not aware of a reverse stack effect, but one thing I learned in science classes is that "heat goes to where there isn't any." Though warm air is more buoyant than cooler air and so it tends to rise, if the lower level is cold it will get some heat from upstairs. With R-40 in the walls and roof, and good air-sealing, it probably wouldn't freeze anyway, though my mother-in-law's addition in central Maine, which is built similarly, without heat, did freeze in the super cold snap we had a few months ago. I don't remember whether you're off-grid but if you aren't, I'd add a piece of electric resistance baseboard heat just to be safe, especially for times when you aren't home. But I would be very surprised if your lower level dipped below freezing when you are running the wood stove.
Thanks Michael. I will be on the grid in Waldoboro. At 700 square feet with an open floor plan ,I am hoping to heat the second floor with one heat pump head (saving the wood stove for electricity outage backup). I will have to do a cost benefit analysis of installing a second head on the tiny first floor (200 square feet) or installing baseboard as you suggest. One factor that is kind of unique is that I will be catching water off of the roof and storing it in a 500 gallon tank on the first floor for my household use. This should significantly chill the space (at least in the cooler months), which means that if I want to use my downstairs office, I should provide heat. I also imagine that heating the first floor would raise the water temperature in my tank a bit so that the water heater wouldn't have to work as hard. Either way, at 500 gallons, the temperature of the water in the tank will go a long way in determining the temperature of the room. Having never lived in a super insulated, air tight house, I am trying to imagine how heat will behave in the space.
This is something that the standard model -- ie, Manual J -- doesn't account for. It assumes that each room is going to have a heating source, there's not a good model for heat flow between rooms.
That said, my experience is that modern, tight, well-insulated houses have very even temperature distribution. I honestly think your bigger problem is going to be getting the wood stove to modulate low enough to avoid overheating.
For what it's worth, I don't need backup heat in my house in Whitefield. Two minisplits kept the house comfortable during that extreme cold episode in early February, when it hit -17°F.
Putting in a wood stove means the expense of a chimney and, if your house is really tight, you have the often discussed make-up air problem.
If you need heat during power outages, a good generator is more useful, since along with your minisplit, it'll run your fridge, well pump and other stuff. Our Honda 7000 watt generator also runs our induction cooktop and, if we're careful with load, our heat pump water heater.