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I recently installed a whole house fan in my attic. For those unfamiliar with whole house fans, they work by creating negative pressure in the living area and positive pressure in the attic. This draws the cool, outside air in through open windows and forces the warm air out through the attic's vents.

Soon after turning on the fan for the first time I discovered a little problem. My furnace is in a small utility closet in a hallway near the fan. In the ceiling above the furnace, there is a rectangular opening, about 1 ft wide, through which the AC refrigerant lines run. When the fan is on, the positive pressure in the attic forces hot attic air down through the opening, pushing the door to the utility closet open slightly and leaking it out into the living area. This obviously works against my goal of cooling the house.

Here is the opening from below:

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And here is the opening from the attic:

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It is the rectangular opening in front of the flue pipe. The covering on top is wire mesh -- I'm guessing it was put there long ago to prevent the infiltration of mice.

Plugging this opening -- maybe with some spray foam -- would prevent the hot attic air from coming down into the living area, but I am concerned that the hole is serving some purpose that I am not aware of (besides being an opening for the refrigerant lines). For what it is worth, the furnace's air intake is on the utility closet's wall, but could this opening be a "backup" source of combustion air? If plugging the opening isn't an option, I can always buy a better latch for the utility closet door and some weather stripping, but even then I'm still curious about the hole's purpose.

So, in summary: what is this opening in the ceiling for? And is it OK to plug it with some spray foam (or other method)?

BTW, in case you're worried about backdrafts, my furnace has an electric igniter.

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Depending on the layout of the room that the furnace is installed in, it could be for attic access or for fresh air needs for the furnace. If that room is mostly sealed using a solid access door, then that is a fresh air vent and can not be sealed. If the furnace room has a louvered access door then that duct is just for access to the attic for the refrigerant lines and any other requirement and can be sealed.

Make sure that you have the required attic venting to support the exhaust fan? I would also recommend a set of blow open inlet vanes between the house and fan that close when the fan is off.

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  • Either way, that sure looks like a couple violations of code. The furnace should have a dedicated duct straight to outdoors for its input air, and my guess is that rectangular feed which now handles the A/C lines is leftover from some ancient thing and should be fully blocked off. Sep 2, 2021 at 17:28
  • Digging into the 2016 California Mechanical Code, I think you're right that it's for combustion air. Specifically, Figure 701.6.2 looks exactly like my setup. I think the original furnace pulled air from the attic. During a renovation, an opening was created to draw air from inside the house. Although I have enough indoor combustion air, plugging the opening in the ceiling would require me to add a second opening to the top of the closet wall to stay up to code. Weatherstripping it is.
    – jme
    Sep 3, 2021 at 3:34

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