DonB
ArboristSite Member
To those of you who heat exclusively with indoor stoves: what humidity level do you try to maintain in your house?
...I suppose the blowers circulating all the air around the stove/insert bakes all the moisture out of the place...
That's a common, but incorrect assumption. Heating appliances have nothing to do with indoor humidity (which is actually relative humidity). The reason indoor humidity drops in the winter is the air outside is cold and cold air doesn't hold as much moisture. Thus even though the relative humidity of the outside air might be 75% at 30 degrees, the actual moisture (sometimes referred to as water grains) in the air is very low compared to 75% relative humidity on a 90 degree day. Since no house is 100% air tight, the dry outside air inevitably makes it inside. Then when heated to indoor temps, the relative humidity drops to 30% or less.
We've only lived in our house ~18 months and have fought high humidity since we moved in. Even last winter with the gas furnace running, we had to run our dehumidifier most of the winter to keep the relative humidity down to around 50%. The house sits on a slab, and they ran the furnace ducts through the slab. My hypothesis is the ducts are allowing moisture from the slab to get into the air flowing through them. This year with the woodstove running full time, our relative humidity has stayed around 40% so far.
Here is a chart that tells feels like temperature with humidity level.
...Interesting. Must be new construction. They can take quite a while to dry out. Seems like it should be just about done by now though...
Nope, house is ~40 years old. Where we live the soil is really damp and I imagine it keeps the slab under us pretty moist as well. Concrete makes an excellent sponge.
We have a furnace vent near a full length mirror in the bathroom and last winter if the furnace hadn't run in quite a while as soon as it kicked on the mirror would immediately fog up. After a minute or so it would clear, though, but it was indicative of how much moisture was being put in the air every time the furnace run.
As to humidity level of wood heat versus other heat, the air doesn't care what is heating it so the type of heat in and of itself makes no difference whatsoever. Locoweed's comment about outside air supply, though, makes sense. Most high efficiency furnaces draw combustion air from the outside, and even those that don't won't use as much air as what is going up the chimney from a wood stove. So if you are seeing lower humidity levels from using a wood stove, it about has to be from the increased level of outside air being exchanged into the house because of the chimney. I had never really looked at it that way before, but it clearly makes sense.
Yep, it sounds like you have a waterlogged slab. Being the ever curious one, I'd pull back some carpet and do plastic sheet test to see how long it takes to make a swamp.
I like your idea about increased air exchange with a wood stove but I'm sure there's more to it than that. I'll do some checking to see. Preliminarily, even though I only have some coals in it now, the air being pumped out by the stove is 100° @ 10% while the ambient room is 75° @ 41%. It's useless data without psychrometrics but it sure seems like there's some serious drying going on within the path around the firebox.
I don't know where it's going but three thermometers and two hygrometers are in agreement and in reasonable concurrence with the Honeywell VisionPro IAQ system located around the corner at the end of the hall.
I appreciate your expertise. My assumption has always been that stoves simply bake moisture out of the air but I've not given it much thought...
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