GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
Building Science

Will a Gas Furnace Dry Out a Home’s Air?

This common belief has some truth behind it, but there’s more to the issue than you might think

Image 1 of 2
An atmospheric combustion furnace draws combustion air from around the unit and sends the exhaust gases up the flue. If the furnace is inside the home, it can lead to more infiltration and thus drier air.
Image Credit: Energy Vanguard
An atmospheric combustion furnace draws combustion air from around the unit and sends the exhaust gases up the flue. If the furnace is inside the home, it can lead to more infiltration and thus drier air.
Image Credit: Energy Vanguard
A sealed-combustion furnace draws its combustion air from outside. As a result, it will not change the pressure the house (unless there is duct leakage) and thus will not dry out the air.
Image Credit: Energy Vanguard

I get asked from time to time if a gas furnace dries out the air in a home and makes a humidifier necessary. I’ve written about humidifiers and psychrometrics over in my blog, but it’s still worthwhile making this point explicitly. Even some people who work in the heating and air conditioning industry get confused by it, so let’s take a look at the issue.

Before we get started, though, I’ll acknowledge that the air in a home can get dry in winter. You can end up going through a lot of lotion and lip balm, as well as sneaking up on people and giving them a nice little electrical surprise.

What does a furnace do?

A furnace heats the air. It raises the temperature of the air in your home so you don’t freeze. But as you go from summer to fall to winter, most people actually lower the temperature they keep the house at. I generally keep it around 75°F in summer and 70°F in winter in our crappy condo.

The furnace makes heat by burning natural gas. Just as there are two kinds of people, there are two kinds of furnaces: atmospheric combustion furnaces and sealed-combustion furnaces. In the former, the furnace draws combustion air from the house. The photo above shows an atmospheric combustion furnace. If you pull the cover off, you could stick your finger right into the flame. The latter pulls its combustion air from outdoors and sends the exhaust gases up the flue. It’s got two plastic pipes (shown in the photo below) that bring in combustion air from outdoors and then exhaust the gases back to the outdoors.

Yes, the combustion process produces a lot of water vapor, but that water vapor doesn’t get into the house. If you have unvented gas heaters, however, you do get that water vapor, and it can be a problem. That takes you in the opposite direction, though. It humidifies the house; it doesn’t dry it out.

If it’s not the furnace…

If you’ve been here for a while and have read some of my crawl space articles, you might recall that one of the things that happens down there is that when you bring outside air in and cool it off in the summertime, the relative humidity goes up, not down (unless you’re in a dry climate). So if you take your house air from 75°F down to 70°F, you might think the relative humidity should go up, right?

Well, that’s true only if you keep the same air in the house and don’t humidify or dehumidify it. In real homes, we don’t keep the same air and we’re constantly adding and taking away moisture from it. So what is it about running a furnace that can dry out the air?

Ah, so it can be the furnace!

Two things. First, if you have an atmospheric combustion furnace inside the conditioned space of your home, it takes some of your home’s air, uses it to burn the natural gas, and then sends the exhaust gases up the flue.

For every cubic foot of air that enters the furnace, another cubic foot of air has to come into your home to make up for the the air that gets used by the furnace. Where does that makeup air come from? Outdoors. It’s cold outside (if it’s not, go turn off the furnace!), and we know that cold air is dry air. Right?

So when an atmospheric combustion furnace inside your house is running, it causes dry outdoor air to come in, and that lowers your humidity. So, in the case of atmospheric combustion furnaces inside the conditioned space, we can say that one of the results of operating it is drier air.

If you have an atmospheric combustion furnace in unconditioned space (an attic, garage, or crawl space, for example), that part of its operation shouldn’t affect the humidity in your home. It’s pulling combustion air directly from outdoors, so the air in your house stays the same, except that it gets heated by the furnace.

Think about that a second. The air in your house has a certain amount of moisture in it. That air, with its moisture, gets pulled into the return duct, goes across the furnace’s heat exchanger, and then put back into the house. If there’s no duct leakage and no humidifier or dehumidifier in the system, the amount of moisture that goes in is the same amount that comes out. The air gets heated so the relative humidity drops, but the air isn’t any drier in terms of absolute humidity.

A sealed-combustion furnace does essentially the same thing as the atmospheric combustion furnace sitting in unconditioned space. It’s pulling combustion air from outdoors and sending the exhaust gases back outdoors, so it’s only adding heat to your indoor air, not exchanging any of the air with outdoor air. So, sealed-combustion furnaces don’t dry out your air either.

The real culprits in dry indoor air

If the air in your home is too dry in winter, it generally means that you’re getting too much cold, dry, outdoor air inside. It’s certainly possible that your house is perfect and you’re just not be generating enough indoor humidity, but in most cases of homes with dry air, the cause is too much outdoor air coming inside.

Here are three things, in addition to atmospheric combustion inside the house, that could be the culprits:

  1. Your house is too leaky. High infiltration rates can bring a lot of cold, dry air into your home. You pay to heat it up. You pay to humidify it, too, if you’re using a humidifier.

  2. You’re overventilating the house. Ventilation air is good. Too much of it, though, means you spend more on heating and on humidifying.

  3. Your ducts are leaky. Leakage on the return side of ducts in unconditioned space pulls in cold, dry air and sends it into your home. Excess supply duct leakage causes your house pressure to be negative, thus increasing infiltration.

What can you do about dry air?

First, keep atmospherically vented appliances out of your conditioned space if possible. The best way to do that is to use sealed-combustion appliances or heat pumps. If it’s time to replace the furnace or you’re building a new house, go this route. Sealed combustion closets, which are almost impossible to retrofit, are another possibility. Designing a home to have an atmospheric combustion furnace in unconditioned space would not be the best strategy.

Second, build an airtight house or air-seal your existing home.

Third, don’t overventilate. Easier said than done, of course, and there’s plenty of debate about this topic among the experts. (See, for example, my article Interview with Dr. Joe Lstiburek — The Ventilation Debate Continues.) Make sure you have control of your ventilation system, though, and experiment with the ventilation rate.

Fourth, either get your ducts inside the conditioned space or make sure they’re sealed up tight.

If you’ve done all those things and still have dry air, you may need to use a humidifier. Or cook a lot of spaghetti and take more long, hot, steamy baths.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. jinmtvt | | #1

    lost count...
    of how many times i tried to explain that baseboard electrical heating doesn't make ur humidity go away magically .. same as with wood stoves that are still used extensivly here.

    I'll bookmark your nice article, so now i'll have something to link for the closed minded.

    Air leaks and make up air dries your house.
    I know quickly off my head, of ~10 family running humidifiers almost all winter ...
    they are also the ones always complaining about high elctricity bills during winter time ..

  2. user-741168 | | #2

    two items
    Nice piece.

    Air movement contributes to feelings of dryness, and radiant systems may operate with stiller air.

    We studied unvented space heaters in 30 homes. We put T/RH sensors in the room with the unit--at the mantle, in the middle of the room and at the far end. You might ask: did the RH go down, go up, or stay the same. The answer we found was--yes. At the mantle it went down, in the middle stayed the same and at the far end went up. The vapor pressure was constant, the RH effect was a temperature effect.

  3. Expert Member
    Dana Dorsett | | #3

    The volume of combustion air is too small to matter.
    Atmospheric drafted burners simply don't pull enough combustion air to make much of difference on the total infiltration on not-so-air-tight home, as long as the bigger burners (like furnaces) have flue dampers. (If yours is old enough to not have come with with an automatic flue damper, it would have been old enough to vote during the Reagan campaign, and should be retired! :-) ) The parasitic 24/365 volumes pulled by un-dampered atmospheric-drafted water heaters is truly "in the noise" compared to typical air-handler driven infiltration rates, and an auto-dampered atmospheric drafted furnace or boiler pulls an order of magnitude less parasitic infiltration than the water heater.

    Yes, sealed combustion burners ) allows you to air-seal with abandon, but you don't need to air seal to the point of back-drafting whenever the clothes dryer is running to get considerable relief on dry wintertime air. FORGET about sealing the furnace room- it's near impossible, and not worth the effort. By the time you're done you might as well have bought the sealed combustion furnace for the time & effort (and expense) required to really pull it off.

    In terms of focus, for ducted air systems, duct sealing & duct return design are primary- if there isn't a dedicated return path for supply registers on doored-off rooms, make some to relieve the pressure differences. Door cuts may have to be big to work, but adding grills to both sides (at differing heights) at partition wall cavites can convert it into a "jump duct" at minimal expense. Every seam & joint on every duct should be mastic-sealed, register boots caulk to the gypsum/subfloor, etc. That will minimize the pressure differences between rooms generated when the air handler is running. Air sealing the house usually comes in a close second, and may be primary in taller houses with leaky upper floor ceilings and basements/crawl spaces, since height produces a 24/365 stack effect drive. That's usually an order of magnitude smaller in total volumes than the air-handler driven infiltration though.

    But combustion air volumes are an order of magnitude below whole-house stack effects, in a typical house prior to the first serious round of air sealing. Test for backdrafting with all exhaust fans and the clothes dryer running, (with the air handler running, as well as off) after each round of air sealing. But it takes a concerted effort in most homes to tighten to the point that atmospheric drafted appliances are truly a problem.

    For any new home (or new appliance, for that matter), sealed combustion is always going to be a safer bet though- wouldn't have it any other way. Just don't be concerned about making it "too tight" in a retrofit. If you actually get there, declare victory!

  4. NedNisson | | #4

    Furnaces versus boilers for creating dryness
    I would like to add that everything in the article applies also to boilers as much as furnaces.

    I often hear said the misconception that hydronic heating systems don't cause dryness as forced-air systems do.

  5. user-2388620 | | #5

    Historic Buildings
    I live in a 410 year old home in the UK. The "wisdom" here is that buildings need to "breathe" to keep moisture from condensing inside walls and causing rot. The traditionalists believe buildings should thus be very leaky and that occupants should not do things that create humidity -- like take showers and make tea. I ave argued this is all wrong. In the UK winter outdoor humidity averages 85%. At about 65 degrees F inside, the humidity will be about 35-40%. I argue this is way too dry for timber and furniture -- the opposite of what the traditionalists believe. I also argue that, under the conditions cited, the dew point is about 37 degrees F and that there is little likelihood of condensation other than on window glass. Does anyone have experience with all this -- or an opinion?

  6. GBA Editor
    Martin Holladay | | #6

    Response to E. Dean Butler
    E. Dean,
    Here in New England, where some of your neighbors moved soon after your house was built, our furniture and wood-framed homes do just fine when the indoor relative humidity settles to 35% during the winter.

    We drink tea. We take showers. But, if we are wise, we do not operate a humidifier -- nor do we attempt to raise the indoor humidity level above 40%, as you apparently advocate.

    I do not believe that walls and roofs need to "breathe." I know that outdoor conditions differ from indoor conditions during the winter, and that it's good to have thermal separation between the indoors and the outdoors. It's generally a bad idea to encourage moisture to flow through your walls and your ceilings.

  7. Jim Andersen | | #7

    gas furnace
    I have a tight new house with a gas hot air system. I have an air to air exchanger going most of the time(40 min/hr) and i still need a dehumidifier to keep the humidity down to 40%.... Not drying out my house!! Have to run around house on these cold mornings to wipe moisture off windows to save the wood....Working to keep humidity down hmmmmmm

  8. GBA Editor
    Martin Holladay | | #8

    Response to Jim Andersen
    If your house is new, it probably still has a lot of construction moisture from the damp framing lumber, concrete, and drywall compound.

    If your windows are dripping, it's clear that you should increase your ventilation rate during the winter. Crank up that HRV so that it is running 60 minutes out of 60.

  9. GBA Editor
    Martin Holladay | | #9

    Response to Ned Nisson
    Thanks for you comments. I'm honored to know you are reading GBA.

    You wrote, "Everything in the article applies also to boilers as much as furnaces." You're almost right. There is one exception, however: the effect of leaky ducts that are located outside of the home's conditioned envelope.

    Allison wrote, "Leakage on the return side of ducts in unconditioned space pulls in cold, dry air and sends it into your home. Excess supply duct leakage causes your house pressure to be negative, thus increasing infiltration."

    These phenomena can be responsible for significant amounts of infiltration during the winter, and this increased infiltration rate has the effect of lowering the indoor relative humidity.

  10. JamesPetersen | | #10

    Cold air is dry air?
    "Cold air is dry air" can be misleading. Extreme low relative humidity is what adversely impacts humans when it comes to talking about dryness. So for the purposes of this discussion we should talk about dry air in terms of its RH. For example 20 degree F outdoor air can be at 100% RH. Is it dry? No. When it is raised to 70 degree F however its RH drops to about 15% RH. I think being more clear about "dryness" would help communications on this topic.

  11. GBA Editor
    Allison A. Bailes III, PhD | | #11

    Response to Bill Rose
    Thanks for adding your insight to the conversation. I'm curious about your unvented space heater results. When you ask what happened to the RH, what were you comparing it to? Did you monitor the houses both with and without the space heaters operating? Since burning gas produces so much water vapor, what was you explanation for the vapor pressure remaining constant? Where did the water go? I've seen houses here in Georgia with unvented gas space heaters where the inside of the windows were dripping with condensation, so I know that in at least some cases, they do humidify the air.

  12. GBA Editor
    Allison A. Bailes III, PhD | | #12

    Response to Dana Dorsett
    Every time I read your comments, I think, Dang! I should have said that, or Why didn't I know that before? I appreciate the depth you add to the discussion. In this case, the information about quantities of air and which effects you should focus on is certainly important, and I should have covered that in the article.


  13. GBA Editor
    Allison A. Bailes III, PhD | | #13

    Response to James Petersen
    Yes, saying cold air is dry air can be misleading, as you say. I didn't get into the details of that here because I did in the article that I had linked to on that topic. Since the article here is about outdoor air coming inside, I was referring to the absolute humidity of cold air being low so that when infiltration brings more of it inside, we end up with lower indoor RH.

  14. Expert Member
    Dana Dorsett | | #14

    Response to Ned Nisson's comment #4
    Martin's comments in post #9 are dead-on. Hydronic systems do not have air handlers driving air filtration the way ducted-air heat delivery does. In a very tight house the differences in infiltration rates driven by the hot-air delivery systems are small but still measurable. In houses with typical tightness, typical duct design, and typical duct leakage that effect is large, and very much DOES affect the indoor air humidity.(Since it's usually more straightforward to track down and fix the duct leakage & return path issues than to hermetically seal the house, duct sealing & return paths are job-1 for remediation.)

    Ductless heat pumps or hydronic fan-coils do not generate significant pressure differences between rooms, and thus don't appreciably drive air infiltration. It's not that the heat delivery fluid is AIR, but that ducted systems will ALWAYS generate pressure differences between rooms, using "the great outdoors" as at least part of the return path. Jump ducts between rooms and well sealed ducts reduce but do not eliminate that effect- it takes a pressure difference to move air.

    (Allison- thank you for your kind assessment of my online blather!)

  15. kevin_in_denver | | #15

    Atmospheric Combustion vs. Forced Draft Combustion
    A little bit of clarification: The photo above shows the flame of a forced draft furnace. Yes, combustion takes place at atmospheric pressure, but to us old timers, "atmospheric combustion" means no draft inducer, like the average gas water heater.

    Dana, your statement " combustion air volumes are an order of magnitude below whole-house stack effects" may be true for Reagan era atmospheric combustion appliances. However, the draft inducer of a 10 year old, 80% efficient forced draft combustion appliance pulls 100 to 200 cfm. On cold days, the duty cycle could be 50%. So are you saying that the average home is leaking 500 to 1000 cfm just due to stack effects?

  16. Expert Member
    Dana Dorsett | | #16

    Response to Kevin Dickenson
    "However, the draft inducer of a 10 year old, 80% efficient forced draft combustion appliance pulls 100 to 200 cfm. On cold days, the duty cycle could be 50%. So are you saying that the average home is leaking 500 to 1000 cfm just due to stack effects?"

    I'm talking more the winter averages, not necessarily at the 99% outside design temperatures, but sure- induced draft systems operated at a 50% duty cycle would beat the winter-average volumes of stack effect even in a typical 2500' 2 story with a full basement. Assuming an interior volume 24,000 cubic a typical stack effect draw of a leaky house during colder weather might run about 0.5ach, that's about 200cfm of stack effect, plus maybe another 200cfm of other leakage from wind loading. At average winter temps and no wind it'll be lower.

    But I'm not buying a 50% duty cycle as "typical" even on cold days. I suspect that would be at least a 2-sigma outlier. A 10-15% mid-winter average duty cycle is probably a better WAG, based on the oversizing levels I seen in existing homes near me. Most furnaces installed in the US are MORE than 2x oversized for the 99% outside design condition. Even at 2x oversizing it would only reach the 50% duty cycle for 88 coldest hours of the year, and never for a full day.

    AFUE presumes 1.7x oversizing, and equipment is tested at a 22.5% duty cycle to crudely model of the seasonal average duty cycle. But most non-modulating furnaces in real homes only exceed a 22.5% duty cycle on design day, and often not even then. It's pretty common in climate zones 3 & 4 to find gas furnaces with 100 KBTU/hr of output serving homes with 99% design temp heat loads less than 25K. (4x + oversizing). In my non-scientific non-tabulated estimation of my area (in US zone 5) ~3x oversizing is typical, and at the average MID-winter load only see duty cycles in the 10-12% range, and maybe or a 25 % duty cycle on the coldest days.

    There are of course exceptions that prove the rule (especially in CA where under Title 24 oversizing that much is prohibited), but personally I have yet to find a 10 year old gas-furnace in-situ less than 2x oversized at the 99% design temp. The efficiency hit from oversizing low-mass gas burners isn't huge, and the contractors here tend to use a "25 BTU/foot" type rule of thumb in lieu of real heat load calculations, then pick the next size bigger, when true heat loads in even in uninsulated homes rarely exceed the rule-of-thumb. That level gross oversizing is enough to make up for a world of ills in terms of duct leakage and air handler driven infiltration, ensuring the contractor doesn't get a call-back from the client even in the worst-case duct systems. Recovery from setback temps are quick, everybody seems satisfied with it (except the energy nerds who care.) The upfront cost delta between an 80% AFUE single-stage 50KBTU/hr hot air furnace and a 150K version is on the order of a couple hundred USD. Tract house builders in my area often pick one furnace for the whole development then oversize it by about 2x for even the largest, saving a lot of time and money by not having to run heat load calculations on every house, and managing the multiple furnace sizes & models.

    It's not a great state of affairs, but it is what it is, and we're talking about a "typical leaky existing house", not "best-practices HVAC sizing in IRC 2006 code-min house", where a 50% duty cycle might occur on a colder than average day.

  17. Expert Member
    Dana Dorsett | | #17

    Putting some numbers & models to it
    This homework download bit from a University of Dayton class is a fairly straightforward primer somebody (unknown to me) put together, that puts some numbers on the air infiltration rates from different effects including stack effects, applying some of the established modeling thereof.

    I think this guy is the author:

    ...and the infiltration primer was put together for this class:

  18. user-1047602 | | #18

    Always dry air in michigan.
    I have a MI GreenStar certified home here in west michigan. Small 1,500 square feet. 5 ACH @ 50 pascal. Sealed trane modulating furnace. I have an ERV that is set to the ASHRAE requirements running all the time. No humidifier.

    Last 2 years I have reported roughly 20% humidity in the winter, cracking and making all our lips and hands bleed.

    Most people I know don't have homes this tight, with sealed combustion or ERVs. They are just as dry or worse ( based on feeling not on monitoring). What am I doing wrong? Is it the ERV? I want 40% without a humidifier ( which I hear can ruin IAQ )

  19. Expert Member
    Dana Dorsett | | #19

    There's no science to ASHRAE 62.2 (response to Brett)
    Sure, if you run ASHRAE rates at 24/365 you'll have humidity and comfort issues both winter and summer. The problem is that ASHRAE 62.2 is a truly excessive ventilation rate, except for the folks who smoke stogies, spray toxic aerosols and install outgassing carpeting & "genuine imitation" particle-board furniture. (The natural ventilation rates of older leaky houses are nowhere NEAR as high the ASHRAE 62.2 numbers!) See:

    Some ERV & HRV controls have dehumidistat settings. If you don't have a lot of indoor air pollution sources setting it to 35% and forgetting it will work for most of the winter. With this approach you'll get higher ventilation rates when the outdoor dew points are higher, and lower when it's arctic-dry, but with few exceptions it'll be "enough". Excessively dry air is ALSO an indoor air quality problem, but the solution isn't adding humidity only to throw it away with excessive ventilation rates.

  20. braverichard | | #20

    Is this truly a low humidity fix??
    Ok so I live in a 5 year old house, pretty standard construction so not as leaky as 50 year old houses but not specially airtight either. with what looks like a sealed combustion chamber furnace (80% efficiency). The furnace has no grills on the front or the side but has an air intake hole at the top which according to the installation manual can be used to provide combustion air directly from the attic as long as the run is kept under 10 feet with 2 elbows using a 3" PVC pipe. I also have a humidifier installed on the furnace which runs quite a lot during the coldest winter months but does keep the humidity inside the house to a comfy level (about 40% typically). I live in the Kansas City, MO area, so, I've been thinking for years of running an air duct to the outside of the house to directly supply combustion air to the furnace through that air intake on the top.

    According to this article, I should see a significant increase in my indoor humidity even with the humidifier off once I do this. Is this correct?? I'd like to hear some thoughts on this.

  21. GBA Editor
    Martin Holladay | | #21

    Response to Richard Okagbue
    It's almost always a bad idea to run a humidifier. Humidifiers are dangerous -- they can introduce moisture into hidden areas of your wall and ceiling assemblies, leading to mold or rot.

    If the manufacturer's instructions for your furnace indicate that it's possible to connect an outside air duct to turn the furnace into a sealed-combustion appliance, then you should definitely install the required fresh-air duct.

    The main reason that homes have dry indoor air during the winter is that the homes are leaky. Sealing air leaks (reducing infiltration and exfiltration) will help raise the indoor relative humidity during the winter. As a bonus, this work will also save energy.

    The best places to begin your air sealing efforts are the attic and the basement. For more information, see these two articles:

    Air Sealing an Attic

    Air Sealing a Basement

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |