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Does a Gas Furnace Dry Out the Air in Your Home?

 

atmospheric combustion gas furnaceI 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 before, 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.

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.

It does so by burning gas. Just as there are two kinds of people, there are two kinds of furnaces: atmospheric combustion and sealed combustion. 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 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. 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.

sealed combustion furnaceIf 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 it 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 dry air, it's from 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 atmospheric combustion appliances out of your conditioned space if possible. Sealed combustion closets, which are almost impossible to retrofit, are one possibility. Sealed combustion appliances or heat pumps are better. If it's time to replace the furnace or you're building a new house, go this route.

Second, build an air-tight 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 long, hot baths.

 

Related Articles

Cold Air Is Dry Air

A Humidifier Is a Bandaid — The Problem Is Infiltration

Vented Crawl Spaces and the Psychrometric Chart Are Not Friends

Making Your Home Safer with a Sealed Combustion Closet

3 Problems with Atmospheric Combustion Inside the Building Envelope

Comments

I frequently get this question as well although it it is more in regard to what is affectionately known as a "scorched air" distribution system rather than the furnace itself! Interesting thought to "Make sure you have control of your ventilation system, though, and experiment with the ventilation rate." We can make ventilation system controls that do pretty much anything like vary the rate automatically in response to other factors, but what would those factors be and would anyone be willing to pay for such controls? Do you know of any data that would tell us the percentage of impact from the mechanical ventilation on the RH in the house?
Posted @ Thursday, January 16, 2014 8:15 AM by Paul Raymer
One additional aspect of this topic that is not mentioned is that moisture moves with heat and as heat escapes from the leaky house it carries moisture with it. I build extremely tight houses and have the opposite problem in the winter (and summer) of high humidity because we humans are constantly adding moisture to the air around us.
Posted @ Thursday, January 16, 2014 9:11 AM by Tom Dugan
I am curious if you think that a very oversized gas furnace for example a 2 to 3 times oversized unit could overheat and dry out the air. If the unit provides 2 to 3 times as much heat energy to the heat exchanger over the same period of time, airflow, and duct size, wouldn't the supply air cycle back into the home feel dryer? This example could apply to a sealed combustion 2 to 3 times oversized furnace. 
In either case (open combustion or closed combustion) with a correctly sized gas furnace the airflow would match the heat energy provided at the heat exchanger closely enough to maintain a steady relative humidity, rather than dry air cycled back on the supply side. 
I am not disputing the reasons you gave above, however wanted to seek clarity as it seems an oversized gas furnace can dry out the air even in a tight home. 
Thanks for your feedback.
Posted @ Thursday, January 16, 2014 12:58 PM by Andrew Dunn
Andrew Dunn oversized furnaces definitely contribute to the "scorched air" feeling of gas heat due to the hot blast of heat for a few minutes followed by long periods of no heat. Not sure how much humidity difference it makes though. 
Posted @ Thursday, January 16, 2014 4:09 PM by Bob
Paul R.: No and no. I don't know what the factors would be or studies about RH and ventilation rates. I can tell you about my experience with the latter, though. In the house that I built, I installed a 120 cfm HRV. This was back when I knew next to nothing about the issue, and the contractor who installed the system didn't know much more than that. So I ran it continuously the first year, and had horribly low RH. Wood shrank and popped. The next winter I throttled it down to less than 50 cfm and the conditions were much better. Wish I'd had dataloggers installed to show you the data. 
 
Tom D.: Indeed, tight homes can be too humid even in winter. Spot ventilation can help, as can using an HRV instead of an ERV. 
 
Andrew D.: It doesn't really matter what size the furnace is. A larger furnace should give you the same temperature rise but do it with a larger air flow. And as long as you're still keeping the house at 70° F, that air still gets mixed with the rest of the house air to give you about the same conditions as with the smaller furnace. What happens, though, is what Bob said. You get short blasts of hot air, followed by periods of cooling, so when the furnace is running, it might feel drier. 
 
Bob: It shouldn't make much difference in the RH at all.
Posted @ Wednesday, January 22, 2014 6:33 AM by Allison Bailes
Seeking a way to get my 30'x30' addition with cathedral ceilings to not rain down condensation from the T&G pine plank ceiling during outside temperature shifts. I have Buderus condensing gas boiler, radiant heat above the floor sheathing under click-lock bamboo and Porcelain tile. Ceiling is truss with blueboard at innermost surface with sprayfoam over the Blueboard to completely seal the ceiling from the outside. 
Water drips down at times through the butt ends of the pine planking and where the interior vertical walls of sheetrock meet the interior pine planking.Drives us nuts! Have suggestions of air exchanger(s), but can't get any solid reliable info as to how to solve it once and for all. Have moisture at window sills, and at double hung window upper and lower sections throughout old section of house and new. Have 33-34% humidity currently at 65degreesF in new kitchen/DRm. at 10BELOW zero outside temp currently. 
HELP, what to do? 
Thanks. In VT.
Posted @ Thursday, January 23, 2014 5:03 PM by C. R. James
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