When should HVAC returns be opened and closed?

Question

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.

Answer 1

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.

Answer 2

Been going through the same process myself. I live in a 4 story house with the HVAC on the 1st floor. Getting heat to the top is no problem. Getting cold air upstairs is very hard. The thermostat is on the 2nd floor, right above the unit, so that area gets cold pretty fast and the system shuts off so the 4th floor never gets cool. I have tried every combination of opening and blocking vents and returns over the years. I finally learned that blocking returns was the least effective solution. It is easy to starve the intake to the unit and air just doesn't move. Think of a car. If you put your hand over the carburetor, the car will be starved of air and will quickly stall. Same thing happens if you block the exhaust. It is important that the system is balanced so it can breathe. Now I close most of the output vents on the 1st floor, maybe 2/3 on the 2nd floor, half or less on the 3rd and open them all on the 4th in the summer. I find that the temperature is pretty even from floor to floor and I don't get the dreaded layer of hot air on the 4th floor where I bedroom is. Before I got the balance right I could be comfortable in bed, but when I stood up, my head hit a layer of hot air. It took me a long time to resist the temptation to close returns on the 1st and 2nd floors. It seemed counter intuitive to return the coldest air near the system back to it, but in the end, it worked. The system breathes better and all levels get cool (except during heat waves when I use a window unit to cool the bedroom and shut off the whole house, but that's a different story). Hope this helps. I know it is generic and specific to my somewhat unusual house. I guess the bottom line is, experiment. Just keep in mind that a system that is starved can't push out air it hasn't pulled in.