Originally Posted by
James Duffin
How does all this apply to a furnace with the return at the furnace...like when the furnace is in a closet and the return in the closet door?
James,
First, an "assumption" ... The return opening in the closet door 'seals to' the return duct when the door is closed.
Now (assuming the door is weather stripped and has a weather stripped threshold) the return is 'outside the closet' and there is no path for the air to get from the return inlet in the closet door to the furnace in less than a 10 foot path.
Of course, when we make the above assumption, we need to recognize that shortly after things are installed the 'seals to' seal will begin wearing and may leak at the door / return air duct joint, and, that the weather stripping around the door will begin to wear and may leak around the door ... in which case it would likely create a problem which would need to be addressed. And the best way to address it is to duct the return someplace else where one is not relying on 'seals at the door' to seal the return air from the furnace - however, after construction, it is not always practical to re-route ducts.