Attached picture (1st pic) is ceiling of 2nd floor condominium laundry room, there is HVAC and water heater also in the room. Due to recent water damage ceiling was removed. I have question about flexible ducts shown by yellow and red dots in the picture.

1st picture is laundry room, 2nd picture is adjacent bathroom.

In laundry room, One end of the duct with yellow dotted line is connected with solid vent which goes down the wall on left side and opens in vent near the floor (not in the picture)

One end of the duct with red dotted line opens in ceiling vent as you can see in the picture.

Blue arrow shows HVAC plenum, you can tell that these ducts are not connected to it.

Both of these ducts go toward the adjacent bathroom ceiling (2nd picture).

In the bathroom ceiling, both of these flex ducts are connected to solid metal ducts which goes further to the right towards my walk in closet, which is end of my unit (Walk in closet has outside wall of building).

Both duct opens outside the building like shown in 3rd picture.

I just don't understand purpose of these vents. When my HVAC turns on, I don't feel any air coming out of vent in the wall (for yellow duct) and vent in the ceiling (for red duct).

Can someone tell me what are these ducts for ?

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  • Maybe returns. Air flows in a circuit through the system.
    – jay613
    Apr 30, 2021 at 2:41
  • Try holding a piece of paper in front of the vents while the system is running to see if it's pulling air in. If so, then @jay613 is right - they're air returns.
    – Mark
    Apr 30, 2021 at 4:53
  • @Mark - like I decribed in the question these vents are going toward adjacent bathroom ceiling and then goes further toward walk in closet ceiling which is next to bathroom. They are not going back in the HVAC system. Apr 30, 2021 at 11:04
  • Since you haven't mapped out the complete duct run by ripping down your entire ceiling, the best people here are going to be able to do is guess. Which is probably all you can do, too. Are you 100% certain that whatever duct they attach to doesn't tie into the return ducting somewhere else that you can't see? My guess is the red one if the first picture is a vent for drawing humidity out of the room. Is that a vent fan that the duct is attached to?
    – FreeMan
    Apr 30, 2021 at 11:55
  • @FreeMan, both duct opens outside building, 3rd picture. They are not part of HVAC system duct network. May 15, 2021 at 0:52

2 Answers 2


Make-up air

Vented clothes dryers and bath fans exhaust air to the outside. Something has to provide a replacement for that air, and in old buildings, that was done by simple leakage of outside air into the structure. However, that means we can't control anything about the outside air that gets in, which is an issue from an energy-efficiency (heat/cold) and building-performance (humidity/moisture) standpoint. It also means that they can depressurize a building, which can send things like combustion exhaust gasses flowing the wrong way, with the attendant CO hazards that poses.

So, modern buildings seal up the leaks and cracks and then bring make-up air in explicitly when and where needed. This provides the opportunity to direct make-up air supply to where it's needed, damper it off if it's not needed at a given time, or precondition it (such as in a heat-recovery or energy-recovery ventilator) before it gets dumped into the room in question, improving energy efficiency and humidity control. It also means that the supply of combustion air for appliances is brought in explicitly, guaranteeing its presence vs. relying on random leaks to get the job done.

  • Both of the vents on the outside look like exhaust vents - with one-way louvres.
    – Mark
    May 15, 2021 at 15:59

Tubing for a bathroom vent and laundry dryer vent.

  • 1
    None of these vents are anywhere connected to bathroom vents or dryer vent. I am 100% sure, It originates in bottom of the wall (yellow one) and in the ceiling (red one) and it goes outside the building, not connected to anything on the way. May 15, 2021 at 1:21
  • Ok, good info So is there a different dryer exhaust vent somewhere in the laundry room?
    – P2000
    May 15, 2021 at 14:31
  • Yes, it is separate. May 15, 2021 at 15:42
  • If the exhaust grill in picture 3 is not shuttered (flaps that prevent draft back into house), then this is likely indeed a make-up air intake as suggested in the answer by ThreePhaseEel. By pulling air into this space directly, the dryer and the furnace are not pulling in conditioned air (heated or cooled) from of the house. Energy recovery is not necessary for these vents to be effective, so they could be used as-is.
    – P2000
    May 15, 2021 at 17:00

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