I recently bought a house and the heating bill has been unusually high. I discovered there is a big hole leading to the outside that the previous owner hid, but isn't sealed properly. Their patch was just a piece of wood with nails, but it is so badly secured, it lets tons of air out and isn't allowing the furnace vents to build pressure.

The hole itself is an 8.5" diameter circle. Currently there is a 8.5" diameter fan blade there. Yet, that fan is leading to the outside. Behind the fan blade it leads what appears is the conduit through which the air blows.

enter image description here

This picture is a fairly good representation. The toys are inside the ventilation, which is above a closet, and running towards the fan, beyond which is the outside of the house.

I thought I could simply buy some expanding foam, but after buying the foam, found it has tons of warnings not to let it anywhere near heat or it could combust. There is an additional worry because the fan blade has wires those could cause a fire too.

I will pull out the 8.5" fan out, I'm left with a circular hole to patch. What can I use to safely patch this? There is a product like this called duct seal compound, but the product demonstration pictures show it used on cracks, not on a giant hole. Can it be used to fill such a huge hole?

  • In the label of the product you've linked, it shows it adheres to wood and painted surfaces. Can you apply it on the edges of the wood and the wall so that it'll block the air?
    – 19aksh
    Jan 24, 2021 at 13:03
  • I can't tell from picture or description , but this may be an inlet for combustion air to the furnace. It would make heating more efficient by not using warm ,humidified interior air for combustion. Jan 24, 2021 at 15:38
  • As I know, this was to provide heat to an external greenhouse that is now disassembled.
    – Village
    Jan 24, 2021 at 16:33
  • By "furnace pipes", are you referring to ducting? It is hard to tell from that picture...if so, the duct section with the hole (abandoned route) can be replaced by a new piece of solid duct (with no side hole) of the same diameter using sheet metal screws to fasten and the hole to the exterior patched with plywood and siding. My understanding of duct sealant is that it goes in cracks and crevices where duct joints are. Do not try to fill any large holes with it. If I have misunderstood the situation, I'll apologize in advance!
    – DAS
    Jan 25, 2021 at 7:52
  • 4
    It seems you have two questions: how do I close off the duct so the furnace isn't blowing warm air where it's not needed, and how do I patch up the hole in the exterior of the house. For both of these actual photos will be tremendously helpful.
    – Greg Hill
    Mar 2, 2021 at 21:53

2 Answers 2


There are a several things you need to do.

  1. Remove the fan from the heating duct. This will entail properly wire-nutting the wires and terminating them in a junction box that is covered but left accesible per code. If the fan is on it's own circuit (unlikely) you would want to shut down that breaker. Importantly, you want to mark the new junction box and any breakers/switches/thermostats so a future owner will understand what was done.

  2. You'll want to close off the 8-1/2 inch hole so that it is airtight. Assuming that the inner surface around the hole is smooth (sheet metal, OSB or similar sheathing) I would cut a plywood square larger than the hole (11x11) and mount it on the interior surface with a high quality construction adhesive and secure it with either appropriately sized sheet metal or wood screws. I would then run a heavy bead of silicone caulk around the edge of the plywood to ensure there is no leakage of air. If you are attaching the plywood directly to the ductwork ideally you'll want a solid surface on the outside of the sheet metal to insure good contact. You might be able to add another piece of plywood on the exterior to accomplish that. In that situation you'll also have the option to patch it with a square of sheet metal instead of the plywood using sheet metal screws and applying silicone caulk to the seams.

  3. Lastly you'll probably want to add some insulation and finish the exterior appropriately.


I think the round circle is part of the fan (it's the mounting bracket). When you remove the fan, you may be left with a rectangular hole to cover.

They sell Duct End Caps: enter image description here

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