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My Pennsylvania house was built in the early 90s, has central heat (natural gas) and central A/C, and most rooms have pairs of closable air returns on the wall near the floor and ceiling.

I've been searching for guidance on whether and when any of the returns should be closed, but I've found lots of conflicting and unsourced answers. Some say that the upper returns should be closed when the heat is running (to circulate out the cold air), and the lower returns should be closed when the A/C is running (to circulate out the warm air). Others say that they should always be open (so the HVAC unit doesn't have trouble pulling in air).

If and when should any of the returns be closed? Some explanations as to "why" and/or links to reliable sources would be appreciated.

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All are true! Generic questions receive generic answers. The advice about closing the upper or lower with heat or cool respectively follows physics of "hot air rise" so sucking up the hot air you just produced isn't very efficient. On the other hand, you don't want to block an intake if it would put additional load on your fan and isn't allowing the amount of air the handler is expecting as they are engineered to a spec to try and optimized HVAC and/or energy performance. But you have two returns and they are designed to be able to shut, so why would they do that. The vent may have been installed because they were cheaper/on-hand, because they looked better, or actually because they were intended to be shut. They also could have been installed correctly and to spec of the air handler but then the air handler was replaced. With so many variables (generic question) you will hear a lot of different answers and most people with lean on the side of caution.

So how to determine what is right for your situation, I've had a few scenarios where people want to place a table or open entertainment center near or in front of their return so I'm going to give you the same advice. Limiting the handler is bad; however, short term is negligible so we'll perform a before and after test. First I inspect the handler to make sure the fan blades are clean. Try and find the vent furthest away from the handler and tape a piece of light cardboard/posterboard or paper to the top; create a hinge, tape on top with paper covering vent. Without any obstruction, both returns open, turn the fan on and see how far the board/paper moves; repeat with a return closed. On a house I typically only check one vent but you can check more. Typically when there is a problem it is extremely noticeable so I'm not breaking out rulers to measure the distance.

If you have the one return closed and your handler is running longer or more frequent that is also a sign that you either need service, possibly just a new filter, or that you don't have enough air flow anymore.

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The advice on how to check the airflow is excellent! Some people suggested bringing an HVAC technician in to check the airflow, which is a fine idea, but I suspect would cost me more than I would save from being able to close some returns. This was exactly what I needed! –  Jeremiah Orr Jan 15 at 21:04
    
Blocking the intake, counterintuitively, reduces the load on the fan. Some folks find this out the other way, when they "open the system up" (which, counterintuitively, increases the load on the fan) and burn out the motor. So don't worry about closing one. Centrifugal fans are, scientifically speaking, interesting. –  Ecnerwal Jan 15 at 21:50
    
@Ecnerwal I haven't heard that before. Do you have any more details on that, or a source that explains it? –  Jeremiah Orr Jan 16 at 13:26
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Centrifugal fans, dead air, blah; thanks @Ecnerwal for the reminder. Yeah there is a much better chance to reduce the load blocking air flow. Either way it will block air flow so the same test still applies, you still want to run the HVAC as efficient as possible which typically means maximizing air flow so that your gas and/or outside unit have less duty cycles; more heat/cool transferred to the inside of the house. –  Jason Jan 17 at 18:46
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