Do Furnaces Dry Out The Air?
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When we talk about “dryness,” we must distinguish between low relative humidity (%) and low absolute moisture content (in pounds or grains of moisture). Raising the temperature of the air doesn’t reduce the amount of moisture in the air, but it does reduce the total percentage of humidity in the air.
Relative humidity compares how much humidity there is in the air to how much there could be (100% or saturation). Warm air can hold more moisture than cold air, so cold air is “drier” than warm air in terms of total moisture content even when the relative humidity is high. For comparison, think about a shot glass that’s 85% full vs. a half-full 64oz Big Gulp; you’ll have a lot more liquid in the 64oz Big Gulp (hot air) even though it’s less full than the shot glass (cold air).
A furnace takes the cold air and merely warms it up. All the original moisture is still there, but the furnace has decreased the air’s relative humidity percentage; the warmer air can now hold more moisture before it becomes saturated.
The furnace doesn’t “burn” moisture out of the air and dry it directly, but the occupants of a building will feel their mucous membranes and noses start to dry up. Humidification strategies are sometimes needed for maximum comfort.
Forced-air systems are likely to bring in outside air, which is likely to introduce dry air to the indoors. (That is especially true if the furnace brings in some of its combustion air.) Forced-air systems also heat the air quite well, but it doesn’t necessarily heat the objects in the space.
Conversely, radiant heaters heat the surfaces and people in a space much more evenly, meaning that there will likely be a lower impact on the humidity.
When it comes to oversizing furnaces, the idea is that you could dry the air out more because you’re moving more air. However, you’re also likely to experience more leakage, which could increase the amount of dry air coming in as well and give you less control over the humidity.
Overall, we recommend using properly sized equipment or radiant heating strategies for the most control over the humidity in the space. Of course, that goes along with properly sealing the ductwork and utilizing sealed combustion to bring combustion air into the space.