Our boiler room in our house is in our half above/half below ground basement. We have a new 100,000 BTU boiler that was installed last year. We also have a gas hot water heater next to it. In the boiler room on an adjacent wall is a 6 inch duct that hangs close to the ground (but not touching it), which then goes up and through the wall. It vents to the outside on the side the house. The vent on the outside of the house has a metal grate on it. The metal grate has a significant amount of debris stuck in it which looks like it has been there for years. I am assuming this duct is there to provide a source of combustion air to keep pressure balanced in the boiler room, but I am hoping someone can confirm this?

If correct, is it a big issue if it is blocked/clogged? I also have a CO2 monitor right outside the boiler room door as a precaution. I was planning to clean out the grate and vent as it doesn't appear it has ever been done, but my concern is if I clean it out, should I expect a lot more cold air to enter the boiler room? I have a suite in my basement and I am worried that it might get too cold down there for our tenants. I have attached pictures below for reference.

I was also wondering if our new boiler might have a fresh air exchanger built in so I don't have to worry? But I am just guessing, I am not sure if this actually exists.

Any help/advice would be greatly appreciated!

Furnace room

Furnace room




1 Answer 1


I think you have a good handle on it.

"If correct, is it a big issue if it is blocked/clogged?"

Yes and no. If blocked, air will be drawn into the house through chimneys, bathroom/kitchen/dryer vents, leaky windows, etc. That air would be cold and uncomfortable. But not dangerous. It will eventually move to the furnace room.

Clean that screen and the air drawn in will be used by the furnace and water heater and not cool down the house.

  • I could agree but it is probably easily cleaned with a wire brush normally just spider webs and dust. It’s a good idea to do this once a year. An air compressor also works well but if done from outside it blows the webs snd dust into the house. The exhaust doesn’t look coaxial so the fresh air is needed.+
    – Ed Beal
    Dec 21, 2020 at 8:19
  • Thank you for the feedback. I did read about using a 5 gallon bucket under the duct where it would hang a few inches above the bottom of the bucket to help with the cold air that enters the room. Have you heard of this and if it is effective? If so, do you have any suggestions on the configuration? I live in Vancouver, BC and we do get some pretty cold days during the winter, so I just want to make sure I do what is safe and best for keeping the house heated.
    – nnamerz
    Dec 21, 2020 at 22:45

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