Make up air ducting question

[John M. Nelson]

Well guys, I have received lots of responses but no clear answers. I thank you all for your help.
Guess I will call local code inforcement and see what they think.

P.S. Never in my notes here have I quoted or suggested that I was quoting code to the client.... How the that part of the thread get started?
Also I was inspecting one of these recently and there was a dead bird right inside next to the filter. How do you suppose he got in there?? Through the open make up air duct unless he got in during original construction.

[Gary Reecher]

Originally Posted by rarmstrong1

The other end of the duct should terminate in the room with the furnace within 12 inches of the ceiling. I could not find a reason for no screen. The CODE is not too hot on why's. If the duct extends 6" into the attic and less than 12" through the ceiling it should remain clear at all times. A screen could get blocked with insulation and prevent flow of required air.

I know this is what the code says but all of the old diagrams for static combustion air that I have seen requires one high and one low. If you have only one high and the mechanical room/closet is hot and the attic is cold there is going to have to be one huge negative pressure developed in the room to reverse the flow of air that is going into the attic.