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Bryan weighs in on the idea that furnaces dry out the air than other HVAC technologies. He also explains what happens when you oversize a furnace and gives his opinion on the practice.
When we talk about “dryness,” we must distinguish between low relative humidity (%) and low absolute moisture content (in pounds or grains of moisture). Raising the temperature of the air doesn’t reduce the amount of moisture in the air, but it does reduce the total percentage of humidity in the air.
Relative humidity compares how much humidity there is in the air to how much there could be (100% or saturation). Warm air can hold more moisture than cold air, so cold air is “drier” than warm air in terms of total moisture content even when the relative humidity is high. For comparison, think about a shot glass that’s 85% full vs. a half-full 64oz Big Gulp; you’ll have a lot more liquid in the 64oz Big Gulp (hot air) even though it’s less full than the shot glass (cold air).
A furnace takes the cold air and merely warms it up. All the original moisture is still there, but the furnace has decreased the air’s relative humidity percentage; the warmer air can now hold more moisture before it becomes saturated.
The furnace doesn’t “burn” moisture out of the air and dry it directly, but the occupants of a building will feel their mucous membranes and noses start to dry up. Humidification strategies are sometimes needed for maximum comfort.
Forced-air systems are likely to bring in outside air, which is likely to introduce dry air to the indoors. (That is especially true if the furnace brings in some of its combustion air.) Forced-air systems also heat the air quite well, but it doesn’t necessarily heat the objects in the space.
Conversely, radiant heaters heat the surfaces and people in a space much more evenly, meaning that there will likely be a lower impact on the humidity.
When it comes to oversizing furnaces, the idea is that you could dry the air out more because you’re moving more air. However, you’re also likely to experience more leakage, which could increase the amount of dry air coming in as well and give you less control over the humidity.
Overall, we recommend using properly sized equipment or radiant heating strategies for the most control over the humidity in the space. Of course, that goes along with proper sealing the ductwork and utilizing sealed combustion to bring combustion air into the space.
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Hey brian here i'm going to talk specifically about the thought or the opinion that many people share, that gas furnaces cause the air to be drier in a space than other types of technologies, and specifically that oversizing, a gas furnace will result in drier air. Inside of a home, so let's talk first about this concept of dry, because there's two different meanings dry can mean lower relative humidity and dry could mean lower absolute moisture content in pounds or grains. It doesn't matter how much you heat air, you don't change. The amount of overall moisture content in pounds or grains, but what you do is you reduce the amount of percentage of moisture there is based on how much there could be, or the saturated point of that particular at that particular temperature.

So when we say relative humidity, that's really what we're saying we're saying: how much humidity is there relative to how much there could be based on the temperature and warmer air can hold more moisture than colder air cold air is always dry air in terms of absolute Moisture content very cold air can hold very little moisture. Very hot air can hold a lot of moisture, but that still doesn't answer the question as far as relative humidity. So you could have a case where you know very cold air and it's very high relative humidity, meaning it's very close to how full it could be. The way we often describe it is, you know, imagine a shot glass versus a 64 ounce.

Big gulp hot air is like the 64 ounce, big gulp. It can hold a lot of moisture. You could have a completely full shot, glass dump it into that large 64-ounce cup, and it would barely fill it and that's exactly what happens when we take a cold air mass and we heat it up with something like a furnace. We take air that has the same amount of moisture in it, but by heating it we've decreased the relative humidity percentage, not by changing the amount of moisture, but by changing the amount of the amount of moisture the air can hold so first kind of myth to Bust, a furnace does nothing to burn the moisture out of the air.

It doesn't directly dry the air. What it does is by heating the air. It drops the relative humidity percentage, and that does cause you to dry out. You know you feel your nose.

Your mucous membranes drying out due to that low relative humidity, and that's why, in a lot of very cold climates, when you heat the air, you do see very low relative humidity percentages, and you often want to get that up into the 20s 30s percentages. In order to be more comfortable inside the home, using something like a bypass dehumidifier or something like that, but let's talk about the theory that oversizing, a furnace or furnaces themselves are worse than other types of heating. One thing that is unique when you're using forced air when you're moving air through the structure is that it's more likely that you're going to bring in outside air with those sorts of strategies, and that's just because you're messing with the pressures inside the structure. Especially if you have a furnace, that's bringing in some of its combustion air from the outside, meaning it's bringing in some of the air for combustion from the outside that outdoor colder air.

That you're reheating is going to tend to be drier, especially if you're using some sort of humidification inside, but even just by its very nature. Indoor air tends to be more humid because you do things like laundry and take showers and baths and cook food and all that sort of thing, that kind of puts off some moisture inside the space. So when we're bringing in outdoor air, we are making it drier and forced air systems tend to do that. Another thing with forced air systems is we're heating, the air, but not necessarily the objects in the space as well.

So when you use something like an old radiant type heater, the radiant type heater didn't heat the air as much. It heated the object, so the furniture and the floors and the ceilings and the occupants and everything that's in the room and because of that it would bring up the temperature more evenly without potentially overheating the air in order to keep it comfortable. So you could potentially with a radiant type system, not actually warm the air quite as much and still be comfortable, and when you don't warm the air as much, you don't drop the relative humidity as much. So that's another possible argument in the case of oversizing.

You could potentially dry the air out more just because you're moving more air and when you're moving more air, it's more likely that you're going to leak air in from the outside, due to leaky ducts and that sort of thing just because you have more forces. There's more velocity, there's more turbulence and there's more negative and positive pressures, making it more likely that your duct system is going to leak. When you oversize, the system tend to run higher static pressures, which also is going to be more of a driver for outdoor air. To come into the space, so in order to keep the humidity in the home, where you want it to be, your best bet is to use a properly sized furnace or some sort of radiant heating strategy and make sure that the home is properly sealed that your Ductwork is sealed so that way, you're not bringing in outdoor air.

You bring your combustion air in from the outside directly into the appliance via sealed combustion. By using you know, modern sealed combustion appliances versus using combustion air from inside the space which sucks in outdoor air into the structure. That's basically it other than that, if you're in a very dry climate you're going to have to use a humidifier, no matter what you do, probably in order to be comfortable, but a big conclusion i want you to understand here. Something to take away.

Is that heating air does not reduce the amount of actual moisture in the air? It just increases the temperature which in turn decreases the relative humidity, but like most things, if you don't have to get the space so hot to be comfortable, then that's also going to help with relative humidity. So you know make your home comfortable seal it well and you're, going to have fewer issues with humidity thanks for watching our video, if you enjoyed it and got something out of it, if you wouldn't mind hitting the thumbs up button to like the video subscribe to The channel and click the notifications bell to be notified when new videos come out. Hvac school is far more than a youtube channel. You can find out more by going to, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more.

You can also subscribe to the podcast on any podcast app of your choosing. You can also join our facebook group if you want to weigh in on the conversation yourself thanks again for watching.

20 thoughts on “Do furnaces dry out the air?”
  1. Avataaar/Circle Created with python_avatars PhillLsx Ga. says:

    An old friend of mine went crazy and sealed his house.. now he has mold issues.. the house can't breathe..
    As far as air passing across a heat exchanger and removing mosture, put water on the burner on an electric stove and turn on the burner and watch what happens…

  2. Avataaar/Circle Created with python_avatars AHCSR says:

    Didn't know that one. Thanks

  3. Avataaar/Circle Created with python_avatars Geoffrey Luders says:

    Another aspect of this I was thinking about just before seeing this video:

    The mass of air inside the home changes as the conditioned space temperature changes (including air in ductwork), thus causing the house to ‘breathe’ and pull in / push out a volume of air no matter how well the house is sealed. The more frequent and extreme this cycling is, the more outside air is exchanged, (also changing the moisture content of the space). This breathing would be more pronounced with the typical (hotter supply) central gas furnace than a central heat pump.

    I did some rough calculations and decided the effect probably plays a very small roll in infiltration compared to other factors. I’m sure many have thought this through much more thoroughly.

  4. Avataaar/Circle Created with python_avatars Nonya Anon says:

    Thank you for the video

  5. Avataaar/Circle Created with python_avatars SuperVstech says:

    I explain this to every customer…

    They actually think the hot furnace air destroys the moisture in the air…

    I explain a heat pump actually reduces humidity more than a gas furnace… because of duct leakage outside the conditioned space… the longer the blower runs, the more conditioned air is lost…
    Heat pumps run the blower longer than a gas furnace…
    Only steam heat adds moisture during heating… unless there is a humidifier installed.

  6. Avataaar/Circle Created with python_avatars Bobcat Arts says:

    I tell my customers in the DC-area to get a steam humidifier if they really, really need a whole-home solution or to just boil some pasta, make soup, etc. and have humidifiers in the areas where they spend the most time. Instead of dumping water in to their ducts hoping it gets all the way to wherever they are. Service area Barrhaven??

  7. Avataaar/Circle Created with python_avatars The Patriotic Mister Snevets says:

    Thanks for laying out the information this way. I will be able to use this video to help explain ideal relative humidity to my customers.

  8. Avataaar/Circle Created with python_avatars Ted Lahm says:

    Relative Humidity. Clear concise information.

  9. Avataaar/Circle Created with python_avatars Paul Garas says:

    A shot in a big gulp? I think you just invented the boilermaker’s red-headed step-child, the “parts-changer”

  10. Avataaar/Circle Created with python_avatars Stewart Thompson says:

    Interesting info. Thanks Bryan.

  11. Avataaar/Circle Created with python_avatars Jim Hathaway says:

    Actually, Air Conditioners can dry out the air more than heaters. Are you in Orleans ?

  12. Avataaar/Circle Created with python_avatars Brian Leeper says:

    My old house was a poorly-built, drafty townhouse (typical Northern Virginia garbage) with an 80% furnace. It even had ductwork in the attic that probably leaked like a sieve. In the winter the humidity would always drop below 20%; I had to run a humidifier. My new house is a well-built single family detached house that is very well sealed. It has a heat pump. There is no ductwork in the attic and the ductwork is sealed with mastic. I don't even bother with a humidifier, the humidity almost never drops below 25% and usually is closer to 30%.

  13. Avataaar/Circle Created with python_avatars Aaron Griego says:

    Yes they do. There saved you 7 mins of your life

  14. Avataaar/Circle Created with python_avatars Waylon Wells says:


  15. Avataaar/Circle Created with python_avatars Joe Shearer says:

    I knew you couldn’t say humidifier! We are from Florida our brains don’t work that way!

  16. Avataaar/Circle Created with python_avatars Gary Winters says:

    Here is an odd story.
    A customer called to have an older open combustion furnace checked for defective heat exchanger or CO, which at examination it had a cracked chamber and was emitting co. She hired our company to upgrade to a sealed combustion high efficient version. Then months later claimed the new appliance was sending black 'smoke' or dirt through the ducts and was discoloring the walls near each outer wall register and around the upper areas above the windows. A closer look proved that the home had outer walls lacking vapor barrier and poor insulation causing the carbon from the high sulphur content pillar candles that were burned to seek the cool spots including the 2×4 wall studs behind the interior plaster board..the lines on the outer walls were quite visible. The contractor replaced the non functional humidifier with a simular bypass type.
    This was a learning experience on relative humidity and the way heat will migrate to cooler surfaces, carrying any indoor contaminates (carbon ) .
    Great videos here on hvac school..

  17. Avataaar/Circle Created with python_avatars Nemo The Glover says:

    an unexpected great video, thanks!

  18. Avataaar/Circle Created with python_avatars qomco says:

    Understanding all this stuff is why I got into the industry and what keeps it interesting. Thanks for sharing your wealth of knowledge.

  19. Avataaar/Circle Created with python_avatars Brian Mcdermott says:

    Great info. Thanks Bryan. Service area Kanata??

  20. Avataaar/Circle Created with python_avatars Gary Winters says:

    Thank you for this clear explanation.. an easy way to explain to customers in colder climates why adding a humidifier helps retain overall moisture content. I get complaints that a new appliance always makes their home extremely dry..also had one try to blame the sealed combustion appliance caused the wood on a piano to the point it cracked, but not with the old lower efficient version.

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