Does your heating system have a humidifier attached to it? If so, it’s most likely there to treat a symptom while leaving the underlying cause of that symptom alone. The symptom is dry air in winter. Unless you’re in an extremely dry or cold climate, the cause is not dry air. The real cause is lack of air-sealing.
First, do you know if your home has a humidifier? If you look at the furnace/air handler and see a smaller attachment that has connections for water, electricity, and drainage, that’s most likely a humidifier. The photo here shows what one model looks like. What they do is introduce water vapor into your duct system so that the heated air in wintertime carries it into your house.
Ideally, you want the relative humidity (RH) in your home to be in the range of about 30% to 50%. For a lot of us, the summertime problem is keeping the RH low enough, but it’s the opposite in wintertime, even in humid climates. The air in homes often gets too dry. You’ll notice it because you get the static electricity shocks every once in a while, or you have to keep putting lotion on your dry skin.
Why is your home’s humidity too low?
The company trying to sell you that humidifier says your home’s humidity is too low because there’s not enough water vapor in the indoor air. Yeah, yeah, that’s true, but it’s like saying you’re naked because you don’t have any clothes on. You’re just repeating the same thing in different words. What we really want to know is why you don’t have any clothes on…uh, I mean why the humidity in your home is so low.
The answer to that question is: Cold air is dry air. Understand that and you’ll know what I mean when I say a humidifier is a bandaid, not a real solution. The chart below, called a psychrometric chart, has the explanation.
I won’t give a full lesson on psychrometrics here, but let’s focus on two points on the chart, point A and point B. Point A corresponds to outdoor air when it’s moderately cold, 32° F. Let’s say it’s also raining and the relative humidity is 100%. Among other things, the psychrometric chart shows how temperature and relative humidity change. For our purposes today, here’s what you need to know about the chart:
- When the temperature changes, you’re moving left (lower temperature) or right (higher temperature).
- Relative humidity is shown as the curves, which are all bunched up together at the lower left. They spread out and move upward as the temperature increases.
When outside air at 32° F, 100% RH leaks into the house, it gets warmed up. Your heating system has to do extra work and you pay extra money to heat up that outside air, but we’re interested in what happens to the humidity right now. As that air heats up, it moves to the right. If you keep your thermostat at 70° F, the conditions of that outside air that leaked in eventually land at point B.
If you look carefully, you’ll se that we crossed 8 RH curves to get to point B, each covering a range of 10% RH. That means we dropped from 100% RH when the air was still outside to 20% RH after it leaked in and warmed up. Wow! That’s a big drop in relative humidity.
Of course, the 20% RH applies only to the volume of air that leaked in, not all the air in your home. The 20% RH air mixes with whatever you have in the house already, and the final number will be the RH of the combined air masses. The key to your low humidity, though, is that the more infiltration you have, the lower your indoor air’s relative humidity will be.
That brings us back to where we started, so we can see now that when the relative humidity in your home is too low, a humidifier is treating the symptom. It’s a bandaid, not a real solution. If you really want to solve the problem, seal the air leaks. It’s a lot less expensive, too, because there’s no operating cost for air-sealing.