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Furnace cold air return damper

We moved into a townhouse in SW Washington state that was built in 2007. The furnace in located in the attic above the second floor. While putting some decorations away in the attic, I noticed that the furnace had 2 cold air intakes. The normal one that comes from the ceiling on the second floor and the second one, a 6" diameter that came from a vent located in the roof.

My question is that there is a damper in the 6" line that at this time is closed. What is the purpose of this second air intake and the damper? Is the damper to be opened/closed during specific times of the year?

Thanks for your help in advance

Re: Furnace cold air return damper

Most likely it's the exhaust vent for the furnace. Are you sure it's 6" in diameter?

Re: Furnace cold air return damper

It could be a fresh air feed to the burners, possibly opens automatically as needed.


Re: Furnace cold air return damper

I also live in Southwest Washington, and I'm reasonably familiar with construction practices here.

This is a fresh-air intake into the cold air return, and local building codes require it. Your furnace may have a timer that causes it to run the blower periodically. The purpose of this is to bring fresh outside air into the home to promote a healthy indoor environment. Some of these intakes are equipped with a motorized damper that opens only when the fresh-air timer calls for it.

In some parts of the country, these fresh air intakes are coupled with a "recovery heat exchanger" to prevent the loss of energy. Since this is a fairly moderate climate (typically 32-85 degrees F with rare extremes getting into the teens or just over 100), recovery heat exchangers are not common.

Feel free to contact the builder of your home or the furnace installer if you need help understanding it. Most of the major builders here would be happy to send someone out to your home at no charge (if they built it), even if you're not the first owner.

Re: Furnace cold air return damper

In answer to your second question, the idea is to close the damper when it's extremely cold. There are some considerations when doing this:

  1. "Extreme cold" here doesn't come around very often. It's rare to have weather that's continuously below freezing. If there's an extended period of time with high temperatures consistently below 40 degrees, you might want to close the damper to save energy.
  2. Winter in SW Washington is wet. High humidity is the norm in winter. This means that mold is most likely to grow in the winter. You probably want to have the damper open most of the time to help control humidity. Even if it's extremely cold, if you have high indoor humidity you might want to open the damper.

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