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Does a house heating system affect indoor air humidity? Or to be more precise, does it affect your perception of how dry the air in the house feels (for instance dry is your mouth waking up)?

(I searched for this but could only find explanations from furnace company websites)

Every (lay)person I ask this seems to think house furnaces make the air drier (at least the perceived dryness). One friend claims that the air gets even dryer than the outside air.

My intuition is that heating can only increase the evaporation inside the house (from the little surface liquid water sources (shower, taps) there may be), therefore making humidity go up.

I suspect people think this because furnaces are only on in the winter when the outside air is dry leading to dry air inside as air exchanges. Thus it could be due to this spurious correlation.

Another possible explanation is that the higher air temperature increases the air's water carrying capacity, which in the absence of an abundant water source, extracts water from your body faster. Therefore increasing perceived dryness. Even if these two countervailing effects are true, can we use first principles to bund which effect dominates?

EDIT (after seeing the answers and discussions below): Perhaps perceived humidity is too loose to define. What if we define the water loss of a human over a period as our measure? But my ideal answer would have two numbers to compare:

  • Baseline_loss = how much water does a person looses sleeping in a cold sealed room (say at 1C temperature) with 100% relative humidity and no air movement.
  • Total_loss = how much water does a person loses if the same sealed room was heated up to 22C (a nice in-home temperature).

Then Heating_induced_loss = Total_loss- Baseline_loss

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Your last statement is basically correct:

Another possible explanation is that the higher air temperature increases the air's water carrying capacity, which in the absence of an abundant water source, extracts water from your body faster. Therefore increasing perceived dryness.

Heating air does "dry it out" in a practical sense. For example, a food dehydrator does not have any water storage mechanism, all it does is warm the ambient air up and blow it over the food. Your furnace can do exactly the same thing.

Let's talk about why that is. Humidity is commonly measured as RH or "relative humidity", which is expressed as a percentage. 100% RH means the air is holding as much water as it can. Anytime the RH is less than 100, the air will steal water from everything it touches due to evaporation. The lower the RH the faster the evaporation.

The next thing that's important to know is that the amount of water air can hold increases dramatically with temperature. So if we take air and heat it, the amount of water in it stays the same, but the capacity of the air increases. This warm air now wants a lot more water than it has, so it will steal water from everything it touches. The technical way to say this is that RH is water/water_capacity, by increasing the water capacity we've decreased the RH; all without doing anything to the water itself!

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    $\begingroup$ @LucasMation It's a bit more complicated than that due to atmospheric effects. The water will continuously evaporate whenever humidity is less than 100%, but air movement, and temperature changes can outpace that, causing the humidity to be less than 100%. It will come to some equilibrium depending on a multitude of factors. $\endgroup$ – Drew Dec 9 '19 at 21:18
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    $\begingroup$ Outdoor environments have pressure and temperature zones where the air is humidifying in some places and drying in others, that's the water cycle you learn about in highschool. $\endgroup$ – Drew Dec 9 '19 at 21:26
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    $\begingroup$ Absolute humidity is very relevant to human experience of dryness. Most moisture is lost via respiration when at rest, so you exhale 100% humid 98 degree air. $\endgroup$ – Phil Sweet Dec 9 '19 at 22:39
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    $\begingroup$ Good point @PhilSweet.So although the room would tend towards 100%RH you might still dry out, because you'd be heating the air around you (for example in your lungs), and that warm air can carry water off of you. It would then cool and precipitate elsewhere aka foggy breath. $\endgroup$ – Drew Dec 9 '19 at 22:45
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    $\begingroup$ If you want to get really precise, you could say that "an object will evaporate until it's surface reaches the dew point of the surrounding air". The dew point can be determined from the relative, or absolute humidity in combination with the temperature. Maybe that's closer to the human perception of air dryness? I'll remove the last sentence in my answer just in case, so as not to overgeneralize. But fyi air movement matters too as far as the rate of evaporation goes. Does faster moving air mean it's "drier" because it dries you more? Eventually it just comes down to your definition of "dry". $\endgroup$ – Drew Dec 10 '19 at 22:36
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To summarize the comments given here: During the winter, the cold air outside can hold only a small amount of water in solution, which means it takes very little moisture in cold air to get it to 100% relative humidity.

But then, when your furnace draws in some of that cold outside "makeup" air, it heats it up and its relative humidity then drops significantly- and you begin feeling "dry" because the warm air in your house begins drawing moisture out of your skin, clothing, furniture, plants, goldfish bowl, etc.

Because of this, it is common for houses to feel uncomfortably dry in the winter unless you put a kettle of water on the stove to simmer whenever the heat is running.

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