Why is there a vent on the ductwork coming right from the furnace? I have a typical furnace for a 1400sqft house in Michigan. I also have central air which runs through the same duct work. There is a separate room in the basement for the furnace, hot water tank and workshop area. No one ever spends much time in that room if any. Should I leave the vent open for both heat and air conditioning or should it be open for one/closed for the other? Please let me know. Thank youenter image description here enter image description here

  • if it's an intake, leave it open. if it's indeed a vent, and un-used, close up that waste asap
    – dandavis
    Jan 30, 2018 at 8:28
  • It's not an intake, it's blowing out hot air right now because I'm using it for my boots and gloves dryer lol the reason I ask though is because how close it is in the start off the duct work. As well I have done little research and it sounds like you don't want to close of to many vents depending on your system because closing off to many will cause to much back pressure or something. Jan 30, 2018 at 10:40
  • So it's a vent to condition the space--heat in winter and a/c in summer--and it has an adjustable damper on it so you can control how much air comes out. Jan 30, 2018 at 11:13
  • if you have several closed off, you can adjust the fan speed to reduce pressure. many of those $99 "fall specials" from HVAC companies include adjustments along with the safety inspection.
    – dandavis
    Jan 30, 2018 at 11:20

2 Answers 2


Furnace installers and/or home owners will put a register in the duct work to provide some heat in the winter to that area. The number of these supply registers is determined by the typical use and desired temperature of this area. I recommend that all basements should have at least 2 supply registers and 1 return register in the basement/equipment area just to keep that area slightly warm and the air fresh. If you are going to finish that area to be used as living space, then more supply and returns will be needed. Also you may want to close the supply registers in the summer when using the A/C since cold air is heavy and will move to the lowest level in the house. Keeping the register you originally inquired about either open or closed would be your choice.

  • Any chance that register is there to help prevent pipes from freezing?
    – elrobis
    Jan 30, 2018 at 15:00
  • 1
    It's there to help prevent anything that shouldn't freeze from freezing. In many homes it's the only heat supply in an unfinished basement. Use it as appropriate.
    – isherwood
    Jan 30, 2018 at 18:10
  • Works very nicely in an otherwise unheated basement. Jan 31, 2018 at 3:38

D George-unless you’re talking about an all electric building NEVER place a return air grill/vent in an equipment room. I say never because most people (even contractors) fall woefully short of understanding barometric pressure issues. Frequently a return in the equipment room of fossil fuel appliances is enough to overcome the natural or assisted draft of a fossil fuel system. This can easily result in the byproducts of combustion, including deadly carbon monoxide being drawn back into the occupied space with catastrophic results. GAMA and National Fuel Gas codes have very strict and sometimes complex rules for doing so. Since we don’t know the competency of the homeowner or technicians we must rely on municipal inspectors to verify the safety of it. Unfortunately, many of them also lack understanding of these codes and their application. Better off just not doing it-unless there is verification the system will work under all circumstances.

  • Sidenote: while this doesn't apply to the OP's situation of a supply vent (vs a return) in the furnace room, the language being referred to can be found in IFGC 618.6 with the prohibition on returns in the furnace room being a new addition there in the 2021 code cycle Aug 8, 2023 at 4:20

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