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 hvacrschool.com, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more.

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