When the combustion air duct is connected directly to the return plenum for a furnace, the blower fan on the furnace pulls outdoor air in to the return plenum. To supply this air to the furnace room, ...

As many contractors attest, a supply air plenum is a sheet metal box attached to a furnace or air handler’s outlet and is used as a central manifold connector for other ductwork. As the air blows from ...

If you need to purchase a gas-burning forced-air furnace but aren't sure how they work, here's what you need to know.