Do Furnaces and Heaters Dry Out the Air?

In the winter, we all experience the effects of dry air on our skin, hair, breathing passages, and hydroscopic (moisture-absorbing) possessions.

We’ve written about how to get relief from dry air, different types of humidifiers, and the effects of dry air and humidity, but we’ve never answered the question “do furnaces and heaters dry out the air?”.

It’s a common question since the air seems to dry out right when we start using our furnaces and heating equipment. Is this mistaking correlation for causation or do our heaters actually dry out the air?

What Causes Dry Air in the Wintertime?

The cause of dry air in the home is too much air infiltration from the outside. Of course, someone could also be running a dehumidifier.

If there was no dry, outdoor air coming inside your indoor humidity levels wouldn’t change.

Turning the heat on, however, can contribute to more air loss. As the heat rises, air can escape out of cracks in the top of the shell, leading to more dry air entering the home. Learn more about

Air Leaks

If you have drafty windows, doors, attics, and basement, a lot of the dry outdoor air ends up in your home. Not only does your air get dryer, it gets colder too, forcing you to run the heater more often.


Whole-home ventilation is definitely recommended if you have a tightly sealed home. On the other hand, too much ventilation can mean more outdoor air infiltration and lower humidity in the winter.

Leaky Air Ducts

Leaking air ducts, especially in unconditioned spaces such as attics and garages, will pull in cold, dry air and blow it into your living spaces.

According to the U.S. EPA, about 30% of our conditioned air gets lost to holes and leaks in the ductwork.

Too many leaks on the supply side can contribute to negative air pressure in your home, contributing to air leaks and energy loss.

Atmospheric Combustion vs. Sealed Combustion Furnaces

In order to answer the question of whether or not furnaces dry out the air, we need to know what type of furnace we’re talking about.

Atmospheric combustion furnaces get combustion air from inside the house. It shouldn’t affect the humidity in the home since the air inside the house stays the same, it just gets heated.

Sealed combustion furnaces draw their combustion air from the outdoors and send exhaust gases out the flue. If you have a sealed combustion furnace, you will have two PVC pipes on the outside of your home, one that draws in air from the outside and one that sends exhaust gases outdoors.

The furnace’s combustion process actually generates water vapor rather than remove it. If you have an unvented gas heater, your heat could actually be humidifying the air, not drying it out. If you notice any excess moisture and condensation around your heater, contact a professional.

If you have a sealed combustion furnace, dry air is brought in from the outside, which lowers your humidity. So yes, when the furnace is on, the air gets drier, but only because of incoming outdoor air, not because of the heating process itself.

If there is not humidification system and no air or duct leaks, the amount of the moisture in the home should stay the same. While heating up the air will lower the relative humidity (since relative humidity is dependent on temperature), the absolute humidity shouldn’t change.

Learn more about the difference between relative humidity and absolute humidity.

How to Solve Dry Air Problems

If the air is dry, don’t suffer in the cold just because you think the furnace will dry out the air further. Use a hygrometer to measure your indoor temperature and humidity levels.

Try these dry air solutions:

  • Let clothes air dry inside.
  • Boil water (use for coffee, tea, pasta, etc.).
  • Let dishes air dy.
  • Leave the door open while you shower.
  • Let the bathwater cool before draining it (not recommended if you have small children)
  • Get some houseplants.
  • Install a portable or whole-home humidifier.

The type of air humidifier you choose will depend on your budget, size of your house, and your preference. Every type comes with a downside, so consult an HVAC professional to ensure that you end up with an ideal solution.

For more information on achieving ideal humidity levels in the home, don’t hesitate to contact the Champions.

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