Question on home HVAC ducts & return(s)

[NiceGuyEddie]

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I removed a non-load bearing wall. To be clear, and in case anyone is wondering, the main beam is the purple line. The end of the Z-shaped dividing wall is not under it, and the other wall is not load bearing. This remodel has been done in several houses in the area.

The HVAC unit is upstairs. I was surprised to see the 1st floor return was simply a passage between the studs and up the wall. I was expecting a metal duct all the way up to the ceiling.

The 1st floor return register cover is large compared to the metal duct coming out of the ceiling. EDIT: The question is, do I (can I) just cut the duct flush to the ceiling and put a register of the same size over it? Or must I enlarge the opening the ceiling and put a large register cover in the ceiling? I don't imagine a larger register cover will help more air get back into the system since the duct is about 14" x 8" anyway.

Photos of other houses with this remodel have both large and small register covers in this spot - it may not matter, but I can't assume both are OK.

[Papa]

Your HVAC needs return air to work, so I'd recommend a register in the ceiling. What would concern me more is the moisture in that wall cavity!

[Big Blocker]

Eddie,

I, personally, would install a register the size of the exposed duct in your ceiling. Without any return from the downstairs, your system will be constantly trying to suck your upstairs walls in - A/C needs equal pressure between intake and output to function properly. By having a return duct on the bottom floor, you equalize that pressure throughout your house, making your A/C unit work efficiently. Returns create flow and are usually positioned somewhere near or at center of all the ducts that are output registers, or on the opposite wall from the supply duct (creating flow across the room)

Doc

[NiceGuyEddie]

EDIT: I screwed up. I should not have said "cover it" - the question was about the size of the register cover in the ceiling compared to the duct cross section. I edited my first post.

[Papa]

Just do as Doc suggested. Size the register to the duct, no need to enlarge it to the size that was in the wall.

[Big Blocker]

An air register that is 2' x 3' won't draw any more air then the 1' x 2' duct it's attached to . . . try to match them as close as possible, error to larger not smaller.

Addition: your 14" x 8" duct will need a "transition piece" to make it fit a 16" x 10" register - unless you can find a 14" x 8" register that will "bolt up".

Doc

[NiceGuyEddie]

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On a recent episode of This Old House I saw a return air plenum and there was a huge "shoebox" much larger than the return duct. I made my own and the grille size is about the same as the original return air grill.

[FLPBFoot]

I agree with everything that has been posted. Only thing to think about is warm air rises and cold sinks. The return in the ceiling will work well for when the AC is on as you want to pull in the warm air and then cool it. Not so much when the heat is on. Ideally you want to pull the return air off the floor (cooler air) and then heat it. Hear in the northern areas where we have about even heating and cooling days our returns have a lower grate and an upper grate. We put a magnetic cover sheet over the upper in winter and over the lower in summer. Helps make the system more efficient.

Steve

[cob427sc]

It was not unusual back ten or more years ago to use both the stud bays and the floor joist bays as supply and return passages for the HVAC system. Toll Brothers and other large builders routinely used this as a cost savings measure. With the new energy codes in place this would definitely be prohibited never mine they would never pass the current air leakage requirements. As stated above, the amount of air flow will be governed by the size of the stud bay so install an appropriate size grill. A larger grill won't restrict flow but also won't provide any benefit.