I’m wondering if the cold air return vents located inside certain rooms of the house are also the intakes for furnace combustion air.

In other words, are those the vents that feed fresh air into the furnace to the filter to the be warmed up and blown back into the house, or is there also another intake somewhere that combines together?

  • "Cold air return vent" returns cold air to the furnace. It is the intake.
    – Tyson
    Commented Jan 4, 2018 at 21:37
  • 1
    There cold air return vent takes air from the living space to be heated and re-circulated - you're warming up previously warmed air. Are you asking about a separate intake that feeds gas combustion and has a separate exhaust? That's different. Living space air circulation is generally isolated from the combustion cycle. Commented Jan 4, 2018 at 21:46
  • Oh ok yeah that’s where I got confused. So there’s another intake that mixes with gas for combustion right? Separate from the cold air that gets warmed up
    – ohmmy
    Commented Jan 4, 2018 at 21:48

2 Answers 2


Depending on the age and type of your furnace, it either draws air from a dedicated intake duct running alongside the exhaust pipe (in the case of modern, direct-vent high-efficiency models) or from the room in which it's installed.

In the latter case, there's sometimes a "makeup air" duct leading into the utility room from outside. It's often simply a length of insulated flexible duct with an unterminated (open) end near the furnace.


No, those are the ones. You don't have two sets in the house.

  • There isn’t another under the furnace as well?
    – ohmmy
    Commented Jan 4, 2018 at 21:38
  • Those cold air return vent you are talking about are the very ones that wind up under the furnace.
    – Paul Logan
    Commented Jan 6, 2018 at 3:54

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