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:
And here is the opening from the attic:
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.