Dry heat myth?
Every now and then, I hear people refer to how nice the heat is from a wood stove in our climate (marine zone 4), because it produces such a “dry heat.” Whenever I hear that, I think to myself “combustion actually produces moisture, so I don’t think wood stoves are particularly dry.” I reason further, in my mind, that the only drying effect a wood stove may have on the interior humidity of a home would likely be a result of the tendency for wood stoves to grossly overheat one area of the home, which would drastically reduce the relative humidity in the air, thus averaging out to reduce the humidity in all of the home’s interior. However, I surmise, if ANY heat source was cranked way up, then they would all have a similair “dry heat” effect on the interior.
Am I right, or missing something more subtle to the building science surrounding heat sources? I feel like no matter the heat source- mini split, high efficiency gas furnace, radiant in-floor, baseboard electric, wood stove, whatever- that they all are just plain old heat, and none of them are “drier” than another, and all work correspondingly to dry the air only as an effect of the temperature increase reducing the relative humidity. Can someone please set put this myth to bed, or reveal my shocking ignorance?! 😉
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Burke, I have a 47,800 btu.hr wood stove that we use for primary heat in my home due to a crap hydronic floor heat installation with no insulation below the slab or on the perimeter.
A big consideration is the stack effect. Once the air in the flue/ chimney gets hot enough, the air naturally rises due to convection. If you live in a climate like me with dry, cold air, you are bringing this air into your envelope to make up for the combustion air needed for the stove
I have worked on boilers (commercial and industrial) where the stack effect of one unit needed barometric dampers and the other five did not. In utility boiler settings this is generally not an issue as they use balanced draft (induction [ID] and forced draft [FD] fans)
I have burned wood to heat my 2100 sq. ft house with single standing floor fan to push air over the stove for the last four years with little issue other than effort to load, split and stack wood. I should also not I have a "special" house that has a huge heat loss due to my un-insulated slab on grade home. We have had days/ nights where we are burning hard all day and don't adjust the the damper enough and end up with an 80 F home. We usually just turn the air down on the stove and let it coast down. I checked the flue today for creosote, and with a catalytic convertor and no cleaning since last burning season, I decided it was a waste of time. I pushed the sweep down and there was less than an 1/16th of an inch build up through out the stack that was visible. This was somewhat surprising to me as we regularly let the stove die down over night producing lower temps that might create creosote.
In my area, zone 4, central , near St. Louis (STL) I have tend to keep my relative humidity (RH) low by burning wood combined with the cold. I usually sit below 40% in the winter. Realize that with a flame burning unit, of any sort, you have to bring in make up air. If you are bringing in air that is very moist compared to your interior, you will see a rise in RH. I however have had to use humidifiers sometimes to raise humidity levels while I burn wood.
In past summers prior to my new mini splits, my RH numbers are regularly in the 60-80% range even with active cooling although it was grossly oversized, obviously an issue for dehumidifying, contributing to the higher summer humidity.
Burke,
You're not the first person to identify and ridicule the dry heat myth. You're right. Heat is heat.
The major factor determining indoor relative humidity (RH) levels during the winter is the home's infiltration rate. Leaky homes have lots of exfiltration and infiltration; as heated air leaves the house, it is replaced by cold outdoor air. Cold outdoor air is always dry, so the leakier the house, the dryer the indoor air.
If your indoor air is dry during the winter, the solution is to seal the air leaks.
If you have an old, leaky house, your air will be dry, whether you heat your home with a boiler, a furnace, or a wood stove.
The people may consider the heat from the wood stove more pleasant because it is radiant heat rather than forced air. When they describe it as dry, they are mis-characterizing why they find it pleasant. (They are probably running a humidifier while they praise this "dry heat.")
Typical discussion in my house around this time of year for the last 10 years has gone like this.
Spouse: It's dry in here, our pets are full of static, and my nose is so dry it hurts.
Myself: Cold air is dry air, and our house sucks in dry air because of the air leaks and our duct design.
Spouse: Well we're not going to spend $20k fixing all that stuff. Don't we have a humidifier?
Me: Yes honey we do, but it's noisy.
Spouse: Okay well just don't turn on the bath fan when you shower. I want to move. Don't you want to move? We've been here too long.
Too funny. haha.
Martin's answer is precisely correct. But it does leave out a few nuances that mean the myth has a grain of truth in it. The heat sources that are usually considered dry heat air forced air systems and wood stoves. In many setups, running those systems will increase the infiltration rate. So even though the heat itself is no different from any other heat source as far as the effect on humidity, the overall system can make the space drier.
In particular, many forced air systems are built with leaky ducts outside the building envelope, e.g., in the garage, attic, crawlspace. Air leaking in or out of the duct there directly or indirectly introduces outside air. Also, unbalanced duct systems can lead to increased pressure differences across the building envelope and drive more infiltration through the envelope leaks, whether the ducts themselves are leaky or not.
Wood stoves almost universally draw their combustion air from the conditioned space, and thus pull in more outside air to replace it. And they draw more air than the need for combustion, sending that extra air up the chimney drawn by stack effect. A good setup will minimize that excess air, but it's not hard to find combinations of stoves and their operators that lose a lot of air up the chimney.
If you want this effect, you can of course just turn on an exhaust fan, or crack open some windows, to get more outside air, regardless of what type of heating system you have.
In a climate where the temperature is just barely cool enough to require heating, and the humidity is high, it is possible to have a situation where it is difficult to maintain comfortably low humidity in the house without heating more than is necessary for comfort. In those situations, running a dehumidifier can make sense. The dehumidifier produces heat more efficiently than an electric heater, because you get the heat of vaporization of the water returned as sensible heat, and it gives you the dehumidification as a bonus. So that would be one example of a heat source that truly provides "dry heat".
Okay, sounds like we're all on the same page. Thanks for the responses!
Charlie,
I'm sure that you know that I agree with you. I just want to clarify that when a forced-air system or a wood stove increases air leakage through a home's thermal envelope, it is due to factors other than the heat.
If you have leaky ducts, seal the duct seams.
If someone mistakenly installed ducts outside the thermal envelope, move the ducts or fix the problem.
If your wood stove needs a combustion air duct to bring outdoor air to the firebox or the area near the stove, install the duct.
And, most important of all: If your home's thermal envelope is leaky, seal the leaks.
And stop blaming your heating system for drying out your indoor air!
Even a pretty large wood stove at maximum fire draws less than 25 cfm in combustion air, which is not a huge drying factor But it's still worth providing dedicated combustion air path in tight homes. Ducting the combustion air to a wood-stove's firebox (in models designed for it) reduces the amount and risk of backdraft spillage into the room air, and is usually preferable on that basis to a proximity vent.
With poorly designed/implemented duct systems air-handler driven infiltration can easily exceed 200 cfm in poorly in air leaky homes, which IS enough to become a real drying factor. Sealing both the ducts and the house can make a real difference.
Thanks for further clarification Charlie, Martin and Dana. Charlie, I was typing at the same time as you were, so didn't see your comments before my last post! Can you please more clearly explain the concept of "sensible heat?" I hear it often, but am not totally clear. (BTW- you are describing the conditions in our area perfectly: the need for some heat, but still very high uotdoor humidity in the winter, so we have a very hard time getting out indoor humidity below 50% with typical thermostat settings.)
At the risk of steering the discussion away from the so far reasoned and sensible explanations...
Wood fires are one of the few sources of heat that draw occupants to close proximity. Sitting in front of the fire, you and your pets end up experiencing "drying' because it is usually a lot hotter than you would put up with if you were, say, reading under a stream of air from a ductless mini-split. My cat can become almost too hot to touch. The Scottish term for what happens after prolonged exposure is "corned beef shins".
Burke, sensible heat just means ordinary heat. The heat that produces an increase in temperature that you can measure with a thermometer. The term is used when there are other more esoteric forms of energy in the mix, such as the heat that goes into vaporizing water or melting ice, to try to clarify that you are just talking about the regular heat.
It can also be used to make people think you are smart and sophisticated and that they shouldn't question your logic. If just talking about sensible heat doesn't work, start throwing in references to entropy, enthalpy, and exergy until they back down.
"you are describing the conditions in our area perfectly: the need for some heat, but still very high uotdoor humidity in the winter, so we have a very hard time getting out indoor humidity below 50% with typical thermostat settings."
While zone 4C locations have a lot of rain and fog in winter, the typical outdoor dew points in locations such as Seattle average about 35F in January varying between 30-40F over a typical week. (The 25th and 75th percentile are about 30F and 40F respectively.) Even by May the dew points don't get over 45F very often.
When you heat up 40F dew-point air to 70F the RH is about 35%, heating up 45F dew point air to 70F yields 40% RH. So it should quite easy to achieve average interior humidity levels well below 50% simply by adjusting the ventilation rate.
See the dew point graph near the bottom of this page:
https://weatherspark.com/averages/29735/Seattle-Washington-United-States
In foggy-dew coastal Aberdeen/Hoquiam the outdoor dew points are only 3-5F higher than in Seattle, still not a wintertime problem, but it could become an issue around half-past April or so. 50F dew point air @ 70F yields 49% RH.
https://weatherspark.com/averages/30508/Hoquiam-Washington-United-States
Thanks for the explanation Charlie. The exergy of your entropy was very enthalpic!
Dana- you're right- it does seems simple. But the way it plays out seems more challenging. I guess what I really meant was that it seems like we are having a hard time getting our indoor humidity down without feeling like we are overventilating, and wasting a lot of energy to get there. We are on Vancouver island, where most cities are right on the coast. Humidity generally always sits between 80 and 100% in the winter (and just above freezing) and between 60 to 90% in the summer. Bringing in that air and warming it up, especially as we get closer to freezing does make it easier to drop the humidity, but it also demands more of your heating system. Add to that a family that showers, baths, and cooks all their meals every day, and the indoor humidity levels just always still seem to sit at right around 50% inside. We could increase ventilation and use more energy to heat up more outdoor air, but I really just wanted to see if there was something I was missing to how different types of heating systems might actually have an impact on indoor humidity. The consensus holds, however, that they basically do not. Charlie's comment that since dehumidifiers also gain the benefit of the heat being released from the phase change of the water vapour condensing to liquid was quite interesting. Something I had not thought of, and maybe does warrant them getting a bit more consideration I than I had previously thought.
The depth of knowledge and the willingness to share it on this site never fails to make me smile!
Sometime I am going to run some numbers on a situation like a 45 F outdoor dew point, a 50 F outdoor temperature, and a 65 F indoor temperature, and compare the energy needed to reduce indoor humidity and provide heat by ventilation plus a mini-split vs. providing some of both with a dehumidifier. Of course it depends on the efficiency of the particular equipment, but it would be interesting to see how often the dehumidifier can beat the ventilation plus heat strategy. Of course you need some outdoor air ventilation in any case, but when the outdoor dew point is only a little below what you want inside, the amount of air flow needed can be large.
Charlie- That would be interesting to see. Please show your "math for dummies" if you do, as those calculations are beyond me, but something I would like to understand.
"Wood fires are one of the few sources of heat that draw occupants to close proximity. Sitting in front of the fire, you and your pets end up experiencing "drying' because it is usually a lot hotter than you would..."
Hey Malcolm, you're right.
My toes were cold and wet the other day, so I gladly sat down on the hard floor in front of the woodstove. Next thing you know, a cat is curling up in my lap. My feet were dry in no time (too hot even) but somehow it was not-so-easy to get back up again!
Lucas,
Glad to see you checking in. Your house, and the discussions you had around it's assemblies, acted as a real benchmark in my knowledge of energy efficient buildings.
Malcolm,
That's nice to hear. There are things I'd do differently another time around of course, but really I couldn't be happier with the way things turned out with the project. Passive solar + superinsulation = not a bad way to go. Even Mariko agrees. Cheers.