AC return air duct

Our house was built in the mid '70's and has separate AC's for the up and down stairs. The downstairs return air duct (goes from ground floor to the AC in attic above the second floor) has an 8" by 12" hole that connects to the space between the two floors! Is that normal?
TIA
Peter
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This is turtle.
Fired Too Soon.
No a 12" X 8" return does not sound like a big enough of a return air grill at all , but tell us the model of the condenser out side or the model of the furnace to get the size to see if this size matches up to see if the return is OK. I really don't thing it is big enough for any size but post the size and I / We / Somebody can tell you.
TURTLE
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TURTLE thanks for replying, I wasn't clear...
There is a filter that is 20" * 20" that collects air from the house. The 8" * 12" hole is in the side of the return duct where it passes between the ground and the second floor. I was expecting the return air duct to be a sealed column that connected the filter to the air handler in the attic and was suprised to see it is connected to the space between the floors (through the 8" * 12" hole).
FYI the unit is 5 tons so by all accounts the filter is too small - I was planning on adding a second 20" * 20" filter.
Thanks,
Peter

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This is Turtle.
the filter is not too small but the return duct / hole is too small. A 8" X 12" hole to return the air is good for about 2 tons at best.
TURTLE
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Wow! You must have one big ass house! 5 tons?! and that's just for the downstairs??!!!
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It is normal for air returning to the furnace to be in wall spaces. Sometimes they nail or screw some sheet metal to the floor joists, and use the bay between two or three floor joists.
The return air is essentially the same temp as the room, so it shouldn't sweat or be hot or cold. The return air is under slight negative pressure, so the duct doesn't have to be sealed totally tight. And it's before th e air filter, so a little dust can get in.
Couple other fellows have adressed the question "is it a big enough duct".
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Christopher A. Young
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