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