Furnace vents

I have just a basic question about the vents for my gas furnace.
There is one large vent that leads directly to the furnace main ducting - its located on the floor and is probably about 18" x 30". What is the vent, is it a return vent, a heater vent, or something else?
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Eigenvector wrote:

Vent is where combustion gas goes(chimney). Duct is where hot air and return air moves in/out of furnace. Also newer ones have fresh air intake.
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Eigenvector wrote:

Likely the return duct. Turn on the fan and see if air is going in or coming out.
--
Joseph Meehan

Dia duit
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