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We have an old house that happens to have forced air. Unfortunately, according to an HVAC tech it looks like there aren't sufficient return vents to add a compressor to our furnace.

I guess I'm confused why the heating system itself doesn't need as many return vents -- seems like it wouldn't work without one? The tech identified just one return vent in the whole house -- on the ground floor.

What options are there for adding return vents to a smaller home (around 1500 sq ft over 3 floors)? Where can I put them without having to build out my walls and sacrifice space.

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I have a similar house (1200 sqft, 2 floors + basement). My only return vent is on the main floor. The central air was added to the house some time in the past. In general, it keeps up pretty well. The upstairs will be slightly warmer than the main floor, but not too bad. We have our thermostat scheduled to shut down the central a/c at night, and then we use window units in the bedrooms upstairs to keep them cool for sleeping. –  James Van Huis Jul 27 '11 at 20:49
    
Our concern is the third floor. It gets unbearably hot in the summer (close to unlivable as it acts as a kind of natural hothouse and concentrates all possible heat). The bottom floor is generally liveable without AC, so only the top two floors matter anyway. –  Jordan Reiter Jul 28 '11 at 18:22
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You want a second return in the ceiling of the top floor for cooling. Otherwise, all the hot air will collect upstairs and you'll have a very difficult time cooling the space. Any air return I've ever seen, that goes between floors, involved a wall being built out. Usually it's hard to tell because it's the back side of a bathtub that looks like it's built out for the bathtub to fit, or a closet that's a bit narrower. You may get lucky and already have space for it behind one of your walls, but that would require looking over a detailed floor plan to see.

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A house we once owned used joist cavities in interior walls for return ducts. –  TomG Jul 24 '11 at 21:03
    
Unfortunately I think the main problem is that this house was basically built very compactly. There are only a couple of walls that are built thick; most of them are very narrow. I think the only place in the house that's built-out drywall (everything else is very thin plasterboard walls or a supporting wall) is the area that contains the chimney. –  Jordan Reiter Jul 25 '11 at 16:21
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