That is because with an attic installation vents must go down into the thermal envelope to deliver warm air.
Furnace venting into attic.
Insulation is only so effective in preventing the heat in the house from going up into the attic.
The furnace has a stack which penetrates the flat roof and is about 3 4 above the roof plane.
You ll start to save money on your heating costs too and your furnace will perform closer to its indicated seer.
Bend these newly cut tabs in.
Yes in fact this is the most efficient manner of providing air to your furnace.
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.
In this video my hvac contractor gilbert rosipal from airrite shows us the inner workings of a 95 efficient gas furnace.
Cut enough aluminum flashing from the roll to wrap around the vent pipe with an extra 6 inches to spare.
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.
Cut 1 inch deep slots a few inches apart along the top side.
Now cut 2 inch deep slots.
Unwrap the flashing from the vent pipe so you can work with it.
You ll see the concentric pipe used.
Fresh combustion air for the furnace is coming from the soffit vents from both roofs.