Indoor humidity vs. wood heat

[DonB]

To those of you who heat exclusively with indoor stoves: what humidity level do you try to maintain in your house?

 

[wdchuck]

40% is the goal, tough to get there, and just as tough to maintain it with our humidifier, goes through 7gallons per day. Someday I"ll tighten up this corn crib, and life should be better.

 

[mtfallsmikey]

I use a tower-type honeywell humidifier, located in the hall near the return air for my furnace. Tough to do that,even. Need a large console type to make a difference.

In extremes, we put a teapot on the range to generate steam..real efficient, huh?:looser:

 

[MNfarmer]

I usually keep mine set between 50 and 55 %. I have a Kenmore Quiet Comfort that seems to keep up pretty well. On real cold days when the furnace is working it'll use between 5 and 7 gallons a day.

 

[DonB]

I like it around 45%, same as summertime. Even though our coastal climate is always humid, it seems like a full-time job keeping the humidity up during heating season. I suppose the blowers circulating all the air around the stove/insert bakes all the moisture out of the place.

This week, even though it's been dang cold, the outdoor humidity has been 70-98%. Yesterday our indoor humidity reached 32% so I broke out the humidifier.

 

[LTREES]

I got the biggest that HD sells and set it at 55% that keeps the house nice. The med. sized ones just don't do the same job. It cost @ $170.00 ? I love it and keeps the whole house comfortable.

LT...

 

[Coldfront]

I have a inside forced air wood furnace in the basement. Last year I bought a Honeywell humidifier it has 2 tanks that hold about 3 gallons each I have to fill it every day so I'm using about 6 gallons per day and when it is really cold I can only get my indoor humidity to about 25% it helps but I wish I would have bought a bigger one with about at least a 12 gallon tank. Here is a chart that tells feels like temperature with humidity level. I would be happy if I could get it up to about 40%

 

[Locoweed]

I also aim for 40%. Living in a dry climate takes a bit of adjustment, but once you get used to it, the 40% seems ok to me. As I age, my skin is drier. We used to only aim for 30%

Keep the humidity level in mind when you read the thread about outside air for a stove, it helps keep you inside humidity up by not having it go up the chimney.

 

[habanero]

...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.

 

[DonB]

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.

I understand the physics of humidity and agree to a certain extent but find direct wood heat to be a slightly different animal. We've never had much problem when using gas or electric forced air, either here or in the much drier climates of Tucson and Denver. Normal household activities kept it tolerable. When I switched to a wood stove in Denver back in the early 80's, I noticed the house would rapidly become dry as a bone regardless of normal moisture added by cooking, bathing, laundry, etc. Even a couple of old, leaky rental houses we have down here (electric forced air furnaces) stay quite tolerable without supplemental humidification

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.

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.

Habanero. Nice handle. I've decided to try maintaining my "pet" habañero plant (in a 15 gallon pot) as a perennial by utilizing thermal mass, a shelter around it and a light for the real cold nights. It's looking good so far. The other night when it went down to 28°, the plant felt 54°.

 

[DonB]

Here is a chart that tells feels like temperature with humidity level.

No wonder it takes my jeans three days to dry out after working in the woods all day when it's 90/90

 

[bama]

I don't normally do much humidifying. I can always get the place warmer, but if I add too much humidity, the windows get very wet and frost up on the bottom. I don't like the static, but at least there are not bedbugs at 20-30%.

 

[habanero]

...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.

 

[DonB]

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.

 

[habanero]

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.

The concrete is sealed on top (at least where we've exposed it), it's just the ducts that pick up moisture.

On to your RH measurements. Are you saying you see 10% RH in the heated air and 41% RH in the ambient air? If so where's the moisture going? It can't be destroyed, it has to be going somewhere. Your RH at 100 gives a dew point of around 35 degrees. That dewpoint at 75 degree ambient gives an RH of 22%. Somewhere along the way you're either cracking water to give hydrogen and oxygen (and it's not combusting to give water back), or you have a nuclear reaction going on and the hydrogen and oxygen are forming heavier elements. Neither's likely. A better explanation is your measuring device isn't accurate in both ranges or isn't equilibrated.

 

[DonB]

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.

 

[Scootermsp]

House more humid with OWB

I noticed that with the OWB going this year, my RH is about 10% higher than with an oil furnace burning. I actually have my Dehumidifier running in my basement. I usually shut it down by late September.

 

[habanero]

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.

Well, if they're off, they're all off the same then... I'm going to have to do some reading on the subject of relative humidity versus temperature. As a chemist I'm usually dealing with absolute moisture, so relative humidity isn't something I've studied in depth.

The moral of the story is, though, that an isolated mass of air containing a given mass of moisture is still going to contain that same amount of moisture whether it's 20 degrees or 120 degrees. Even dismissing RH for a second, just the absolute % moisture is going to change, though, since you're changing the volume of the air dramatically over that range. That may well explain the readings you're seeing around your stove-at 100 degrees the air is going to be significantly less dense than at 75 degrees and that's going to change both the % moisture and RH.

This whole discussion highlights why RH is such a voodoo number. There so many variables at play it's just too complex.

 

[DonB]

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. It hasn't become enough of a mystery to bother with further education.

Today I have more of a fire going and the windows are closed. Stove output air is 138° @ 3%. Ambient is 76° @ 36%.

I checked all my instruments against the Honeywell unit. These are generally assumed to be dead accurate. Everything matches up. Of course, as you pointed out, this doesn't mean these other hygrometers are still accurate at higher temps but they do seem to be somewhat consistent.

 

[habanero]

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...

But see that's the thing, moisture can't just "bake out". It has to go somewhere. My inclination is it is going up your chimney and the air coming in to replace it is much drier and this leads to a net decrease in the moisture in the home air.