I am a new owner of an old (1944) house.

I have forced hot air. I can't for the life of me find the air intake. I was told to look for a large grate, about twice the size of all of the other house grates, however none are that large. Moreover, they all seem to be blowing out hot air. There is a vent very close to my heating unit in the basement, but luckily that one also seems to be blowing out air, or that would be really bad (stale basement air as air intake, very poor house circulation).

I had a guy over who used to contract (for a different reason, he was not there to look for this problem), and after a quick look, he suggested during a renovation some prior owners may have drywalled over it.. that sounds like a horrible thing to do.

Is there a way I can systematically find my air intake? At this point I'm going to have to light an incense and stick it near each vent or something. And what if I don't have one?


OK! I did the incense test and have good news and maybe bad news. The good news is that, unless I'm reading the "incense flow" wrong, every room has an output and an input. One vent in each room appears to be blowing hot air out, and one vent in each room appears cold to the touch and incense flows in.

The bad news is that there also appears to be a return vent right in the basement where the furnace is: https://i.stack.imgur.com/Oj9AD.jpg. Wouldn't that be recycling nasty dirty basement air and then pushing it out into the house?

  • 2
    Follow your intake from the furnace. If it's covered it will be harder to follow. An in inspection camera would work too. Otherwise punch holes along the route you think it goes until you find the end.
    – Jeff Cates
    Feb 13, 2017 at 7:14
  • Do your best to follow the return from the furnace, it will at least give you an idea of where the return might be. Depending on where you live and the layout of the building, there may be more than one return.
    – Tester101
    Feb 13, 2017 at 11:49
  • I can't follow the intake. It leaves the laundy room and is drywalled off in the basement. It literally dissapears into the walls somewhere when I move up to the main floor.
    – Tommy
    Feb 13, 2017 at 13:08
  • I would go ahead with the "incense stick" idea, but preferable w/ colored smoke. But of course you can skip any grille that is pumping out hot air. The fact that you're getting hot air flow pretty much guarantees your furnace is receiving sufficient CFM on the return side from "somewhere" for what that's worth. (did you check the attic?) Feb 13, 2017 at 18:46
  • I will try to add some photos of my heating furnace tonight.
    – Tommy
    Feb 13, 2017 at 19:36

1 Answer 1


Houses from that era usually had 1 or 2 large floor grates as cold air returns or the return grills/registers were on the outside walls under windows. There has to be return air grills or you would not have any supply air coming out the registers. Are there any registers high up on the walls, these could be the returns.With the fan running, light a match or candle put it near the register to see which way the air is flowing, in or out. Follow the return air duct work, from the furnace, (the duct that holds the air filter), and see where it goes. If you find metal sheeting nailed to the floor joists in the basement, this would also be part of the return air system. Follow it to see where it goes. Where the metal stops, that is where the return grill/register is, above that location in the floor or in the wall. And if you use the basement for anything, storage or any use, you should have 1 or more supply registers and return grills to keep that area somewhat conditioned. One last thing is this house a 1 story ranch, split entry or a 2 or more stories?

  • It would be very unlikely that cold returns are high on the walls, for obvious reasons. Feb 13, 2017 at 18:43
  • do you mean the return could be external, rather than somewhere inside of the house?
    – Tommy
    Feb 13, 2017 at 19:37
  • The returns will be somewhere in the house. And yes Carl, they can be up high on the walls as mine are. And the return grills/registers may not have levers to open and close them. They should not be hard to spot, so try the match or candle trick and if you smoke or E' cig, use that.
    – d.george
    Feb 13, 2017 at 20:09
  • If you put a radio/speaker into the return duct near the furnace filter, then you can listen at each opening. It the sound comes out everywhere, try to block any sound that might go back into the furnace. Use a folded towel, and turn down the thermostat first.
    – John Canon
    Dec 9, 2018 at 19:35

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