I had access to a ladder the other day, so naturally I was poking around in the attic of our house where we have lived for about a year. Above the closet where the furnace and water heater are located, I found there was a metal flue that didn't extend all the way through the roof, it just stops in the attic. Is this normal? There was a flue right next to it that did extend through the roof. The house was built in 1999 and we haven't had any problems since living here. I looked in the furnace room closet and couldn't see anything that might be connected to it. The building inspector didn't mention anything about it when we bought the house. Here's some photos:

benign? close to the attic vent other vent has it together

So, what do you think? Will our house burn down, or will we die of carbon monoxide poisoning?

  • Where does it actually come from? Could either be a combustion air intake (to pull fresh air into the furnace/wh room) or could be the outlet from a bathroom vent.
    – Jeff Meden
    Commented Feb 19, 2016 at 19:03
  • 1
    I see no evidence of use - no soot, no lint, no moldy stains from a moist-air vent, no sign of anything on the inside of the pipe or the underside of the roof above it. Combustion air intake might be it.
    – Ecnerwal
    Commented Feb 19, 2016 at 19:04
  • Is it connected to anything below? Do you have two such ducts connected to your furnace? Commented Feb 19, 2016 at 19:18

2 Answers 2


I am guessing from your pictures that it is an air intake - I don't see any signs of air/moisture above the venting. It is perfectly acceptable to pull air from your attic for a furnace. In fact some might presuppose that by pulling the attic air you are increasing airflow in your attic. As a home owner I would like to see a screen on it so that critters can't get in.

If this is an exhaust, then it needs to go out the roof. I doubt it is an exhaust though since the exhaust looks to be right next to it and there are no signs of any airflow like I said before. Your worry % should be very very low.

Also in my area we throw a couple of 90 degree elbows on this so it is pointing down. That plus the screen makes it super hard to have any unwanted things fall into your furnace intake.

  • 3
    Screens over CAIs in attic spaces are sometimes prohibited in local code or the NFPA spec the furnace adheres to, because they are actually more likely to get clogged that way (dust/leaves sticking to the screen). As always check your documents carefully before making changes!
    – Jeff Meden
    Commented Feb 19, 2016 at 19:15
  • @JeffMeden - That is why I mentioned moving the flue to open upside down and also IMO that is outdated. Any modern furnace would shut off if its intake is clogged.
    – DMoore
    Commented Feb 19, 2016 at 19:25
  • 1
    Since the OP says the furnace and water heater are in a closet, I think you're dead on to say it's a make-up/combustion air intake. If you add a screen/grill, you may have to adjust the size of the pipe. As it has to be designed to allow a specific amount of air in, and has to offer as little resistance as possible. Same goes for 90° bends.
    – Tester101
    Commented Feb 19, 2016 at 21:11
  • 1
    This is likely not directly connected to the furnace, nor is it likely directly monitored (via pressure switch, etc.). So a clog in the duct may not be noticed by the furnace. Instead the system could draw combustion air from the living space, via gaps around the door. This could be bad news if the house is well sealed.
    – Tester101
    Commented Feb 19, 2016 at 21:15
  • @Tester101 - Every HVAC place in my area that has in intake to the furnace does it in the way I described. Do you think that the cons of an upside down screen outweigh the pros?
    – DMoore
    Commented Feb 19, 2016 at 21:18

All of your responses helped me come up with a definitive answer. The flue did indeed extend down into the closet, but did not connect to anything. It was up in the corner behind the door-frame where I didn't see it. It's an air intake.

It sounds like implication of this is that I don't want to put a screen over the flue, for two reasons:

  1. There is no benefit to doing so, as anything smaller than a rabbit will just fall to the floor in my closet [citation needed].
  2. Such a screen could itself could become clogged. Since this intake is not monitored, this could result in air being drawn from the living space, a potential hazard (per Jeff Meden and Tester101's comments to DMoore's answer).

bottom of flue poking through ceiling

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