We have finished moving into our new (to us, built in 1999) house and it’s super humid outside lately. Dew point in the mid 70s (F). The house has forced heating/cooling and it maintains the temp well, but it has been 60-70% relative humidity the whole time.

Today I realized that our furnace has an outside air intake duct on the return side and I don’t know if it has a damper of any kind on it. That would explain why during the intermittent fan cycles without the AC running the humidity rises.

I have seen no levers/knobs/controls of any kind on the intake duct. It is wrapped in thick and soft black plastic insulation. I see no sign of a power cable anywhere along the duct or at the wall where it penetrates to the outside.

Is there anything I might be missing to indicate this duct has a damper that I might be able to adjust?

1 Answer 1


It sounds like flex duct it is soft with a wire to keep its shape insulation and another inside layer of the black plastic.

Make up air ducts can not be in flex there will need to be a hard pipe for a duct to take off. When I put these in they are usually close to the furnace air handler, even on the air handler a simple hole with an adjustable metal cover on the intake side of the furnace is in its own closet electric. For gas they usually require a pipe to a outside air space or attic.

The make up air can’t be in a garage ( because of Co poising possibilities) and is not normally in the crawl space due to high humidity and damp earth smells. Look at the air handler itself for the opening sometimes just prior to the filter.

In my experience the hole or duct will be between 2” and 4” I have seen 1 or 2 that were 6” in residential but this is very rare or on a large home +4500 sf

  • 1
    Im a little confused as to what you’re trying to tell me to do. It sounds like you’re trying to help me identify the duct itself, but I already know for sure which one it is. I can follow this single duct from the outside of the foundation to the return duct on the furnace. It’s the last one in before the filter. It’s this black soft exterior tube the whole way.
    – Logarr
    Aug 29, 2021 at 16:10
  • Ok you can have a flex duct for make up air but I have never seen a damper in any type of flex. There would have to be a section at the entrance or at the filter that is rigid to have a damper, I provided the information because your description sounded like flex and that is common but could not be controlled without a rigid piece.
    – Ed Beal
    Aug 29, 2021 at 18:42
  • Ah I see what you were getting at. I’ll have to do a closer examination of the vent cap outside and see if it has anything on it. Thank you.
    – Logarr
    Aug 29, 2021 at 18:44
  • If all flex I have used a butterfly damper at the entrance, butterfly dampers have a shaft that comes out in the center of the pipe (you may need to remove the cover to access) twist the shaft - direction and you can see the wings fold up to allow more air the other they come close to the edges of the pipe usually not a air tight seal but foam seals can be easily added to get a tighter seal if you run your fan 24/7 , new homes need some make up air because construction methods today are much tighter than they were 40 years ago. Tight home with no makeup air is unhealthy.
    – Ed Beal
    Aug 29, 2021 at 18:59
  • I checked the vent outside and found nothing, so I squeezed the duct all along and sure enough I found a very short bit of solid duct at the very end. The damper nut was hidden underneath overlapped insulation under a zip tie for some reason. Thank you for the insight!
    – Logarr
    Aug 30, 2021 at 17:24

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