Furnace Venting Into Attic

Yes in fact this is the most efficient manner of providing air to your furnace.

Furnace venting into attic.

Fresh combustion air for the furnace is coming from the soffit vents from both roofs. Cut 1 inch deep slots a few inches apart along the top side. Unless you have a slab where the equipment would likely be in the garage venting can go directly through the roof instead of routing through an exterior wall making it much easier to install the duct work along the upper floor. However the furnace is in an attic space which has no venting for warm humid air from house from soffits from hot roof etc to escape.

Attic air is already partly heated from heat in the upper level of your home radiating into the attic despite insulation. Insulation is only so effective in preventing the heat in the house from going up into the attic. If you re preparing to upgrade an old furnace it might be time to consider moving it back down to the ground level. Now cut 2 inch deep slots.

Cut enough aluminum flashing from the roll to wrap around the vent pipe with an extra 6 inches to spare. You ll start to save money on your heating costs too and your furnace will perform closer to its indicated seer. That is because with an attic installation vents must go down into the thermal envelope to deliver warm air. Bend these newly cut tabs in.

The reason for this advantage is the fact that venting for your furnace requires less work in an attic than it does in other locations of your home. The furnace has a stack which penetrates the flat roof and is about 3 4 above the roof plane.