Hole in concrete under furnace

How do I fix this hole in the concrete under my furnace, and is there any reason I shouldn’t?

It’s difficult for me to get at, as there is only about 8 or 9 inches between the floor and the bottom of the furnace.

edit: The furnace is an oil furnace. I want to seal the hole up because I believe ants may have been coming through the hole

  • 1
    Is there any particular reason you need to? Is water getting in? Are critters getting in? If not, personally, I'd just leave it. Maybe fill it next time the furnace has to be replaced if it's really bothering you.
    – FreeMan
    Jul 8, 2022 at 21:23
  • Is it gas furnace ??
    – Traveler
    Jul 8, 2022 at 21:27
  • It would not be hard to mix concrete and pour it into the hole. It's more an issue of how much concrete you need to do the job -- might require some crude forms to contain the concrete as it's poured.
    – Hot Licks
    Jul 8, 2022 at 21:28
  • Without knowing the history, it looks like the was in place before the cement floor. The cement workers probably did not try their best to fill it in, because it was a PITA.
    – crip659
    Jul 8, 2022 at 21:30
  • 5
    Before you fill it in, make sure that it is not a cold air return for the furnace. I had a house in South Dakota in which all the cold air returns from the rooms went through tunnels in the concrete floor. Jul 8, 2022 at 23:31

2 Answers 2


Oil burning furnace will need a unrestricted air supply.

That probably comes through those bricks underneath.

Check yours with paper tissue for air flow.

To close that opening for bugs while still allowing air flow, cover it with the metal (aluminum) mosquito net.

Do not fill it up with anything.

  • "make sure that it is not a [fresh air intake] for the furnace" - That would have nothing to do with the ductwork. And probably no discernible means of an inlet. +1
    – Mazura
    Jul 9, 2022 at 6:08

I have encountered the exact same thing, down to the size of the hole and location, and I freaked out a little at first too.

Try mixing in some insect killing powder/pellets and laying some copper mesh, for peace of mind, will be a lot easier than attempting a concrete pouring job in that space!

Keep an eye on it to make sure that it doesn’t start to widen or crack and that dirt doesn’t start expanding up through it.

  • 1
    How does the dirt expand up through there?
    – gnicko
    Jul 27, 2022 at 2:31
  • Hopefully, it wouldn’t. Hopefully it wouldn’t shrink down, either. If it did either of those things it could indicate changes in conditions that could impact the integrity of the structure. Expansive soil (which expands and contracts based on moisture content) could cause it to swell or shift with changes in underground water content, erosion, formation of sinkholes, tree roots, nearby construction, etc. Nothing that a small patch of concrete could help mitigate, but it could be an early indicator for issues we normally only see once they’ve caused damage. Jul 27, 2022 at 13:24

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