I have damper installed for each supply branch for my basement. There are six supply lines (5" flex duct each) for just basement. It covers 1100 sq ft (includes 1 bed room and 1 bath) for a 3 ton air conditioner.

In summer, when basement is already cold and/or doesn't need any air supply. I can closed the dampers (for basement) so all cold supply can be consumed in the main living area (upstairs). Now I definitely need a return for air circulation in basement, but how much air to be returned from basement itself when all supply in basement are completely closed through damper.

Now do I need to partially close/open the return or keep it open 100%.

IMO, if I keep the air return ducts 100% open, then enough air from upstairs won't be able to circulate as the furnace is going to draw most of air from the basement which can cause upstairs to be uncomfortable in the summer.

Edited : Added picture. where I am going have a return in my basement and how return from upstair is currently set up. enter image description here

  • Air will follow the path of least resistance so if your living space returns are sized properly then mainly living space air will be circulated through your system. If your returns are not sized properly then you will create a positive pressure situation so air will push it's way into the basement and use the basement's returns. If you're closing basement supplies then you should really check the static pressure of your system to make sure you're not sending your blower motor to an early grave.
    – MonkeyZeus
    Commented Aug 17, 2021 at 13:34
  • The practical answer is to close your basement supplies and check how much air is flowing into the basement returns. If you don't like it then close off the returns with a magnetic cover.
    – MonkeyZeus
    Commented Aug 17, 2021 at 13:50
  • 1
    Experimentation is the way to answer this question definitively with the information available. Try it and see how it works out. Make adjustments as necessary.
    – gnicko
    Commented Aug 17, 2021 at 15:54
  • Added a picture, does it not look its going suck more air from basement .
    – PowerTech
    Commented Aug 17, 2021 at 19:03

1 Answer 1


In short: Leave the intake alone. But in detail...

Closing the dampers does not stop all air flow. Exhaust and intake work together. If you leave the basement intake fully open, then it might create a negative pressure in the basement which could suck in air from anywhere. But, most likely extra air will be sucked into the furnace directly from upstairs because the extra air being forced upstairs is creating a positive pressure there, so the upstairs intakes will work better.

You cannot suck air from an area which has no air to give. The intake will adjust itself because you adjusted the exhaust. In short: Do not overthink the situation. You want to keep the basement dry. So, unless the basement is uncomfortable, then leave well enough alone. If the duct work is good, then your situation is good. If you need to dampen the intake to make your basement comfortable, then I question your duct work. But, it would be easier to dampen the intake than to enlarge the duct work going upstairs.

If your duct work is small and you reduce the total air flow through your A/C to much, then the coil will freeze a cease to function. If that happens, then let it thaw, clean the water mess, and allow more air to flow. Trial and error will answer all questions.

.... To answer the comment question ...

Exhaust and intake work together. The size of the room/space has absolutely zero effect upon the situation. But, when you consider windows and doors, you are thinking about air leaks, and that matters.

When I say "Negative Pressure", I mean that "The intakes of the area wants more air than exhausts of the area are giving." When I say "Positive Pressure", I mean that "The exhausts of the area want to blow more air than intakes of the area can take."

In reality, everything is a comparison between two points. Pressure, like anything else, obtains a value, negative or positive, only in comparison to another pressure. The "other" pressure I am comparing to is the pressure that would exist in the event the furnace was off.

The negative or positive pressure (subtraction result after the comparison) should be very small because there are other intakes and exhausts elsewhere that are open. The change in pressure in the room represents the extra difficulty required for the fan to push air around due to the added restrictions (closed exhausts).

In example: If you close all exhausts in half of the house, then you will decrease the intake from that half of the house because there is no air to take from there (will have negative pressure). Also, if air is forced to the other half of the house, then you will increase the intake from the other side of the house because there is more air to take from the exhausts that are blowing harder (will have positive pressure).

If the pressure is so negative and positive that the furnace can suck/blow air through a doorway, then while the door is open the pressure of both areas will move closer to zero. After the door closes, given than the furnace is still on, then the pressure difference will return.

Air does not move without a pressure difference. Our education system fails to teach us many of the most important foundational things for understanding. One being that everything is a comparison between two different things. Nothing happens without a difference. Such things are good to think about and will advance understanding of everything. Many people never consider such things because and their life and their furnace roll along just fine. People like you and I imagine such things for enjoyment and the love of understanding. The wind never simply "comes from the north" at times because the wind always travels in circles. The difference between blowing a sucking is only in the mathematical sign. They both work to move air and one cannot exist without the other. Nature requires the measurement of their values to be equal and opposite. From this fact, one may conclude that air cannot be created or destroyed.

  • You mentioned "Exhaust and intake work together', Don't you think it depends up on size of room/space. its walkout basement and has windows. so I opens door 5 times a day. so there won't be negative pressure any more even all supply in basement is closed. please make me correct if I am wrong here. thanks!
    – PowerTech
    Commented Aug 18, 2021 at 13:34

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