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Our A/C unit is in the attic in an unfinished area on the second floor. The A/C thermostat is in the finished area on the other side of the wall from the unit. As expected, the the second floor is generally several degrees warmer than the downstairs and we have the A/C thermostat set at what is for us a comfortable 76 degrees. It appears to be working as expected, but while the upstairs is kept at 76 degrees, the downstairs is generally about 5 degrees colder.

Would a simple solution be to partially close some (all?) of the downstairs A/C vents help to raise the temperature there - I would experiment by gradually making small changes? Or would partially closing the vents in any way hamper the efficient running of the A/C?

Thanks.

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  • Tell us about the location of the air return(s). – Greg Hill Jun 8 '20 at 23:49
  • There is one return. It's on the second floor right above the A/C thermostat (on other side of wall from A/C unit in attic). – Not_Einstein Jun 9 '20 at 2:01
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Cold air always sinks (gosh ! :-) ), so the stable condition will pretty much always have the lower floor ending up cooler than the upper floor. Yes, closing downstairs vents will help balance things out, and at least avoid overshoot in the downstairs temperature. There should be dampers in the ductwork close to the heat exchanger (in the attic) and those are designed to adjust flow rates for each path. If those weren't installed, go ahead and partially close whatever dampers are at the exit points of the downstairs vents.

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    And don't forget to reset the valves when heating season starts, so that the majority of hot air flows to the lower level. – Phil Freedenberg Jun 9 '20 at 17:18
  • There's no heat exchanger - we have baseboard heating. – Not_Einstein Jun 9 '20 at 17:34
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    The term you're after for those "valves" is damper, by the way – ThreePhaseEel Jun 9 '20 at 18:40
  • @ThreePhaseEel thanks -- edited. Next you'll tell me my electrical valves are diodes :-) – Carl Witthoft Jun 9 '20 at 18:59
  • @Not_Einstein -- the heat exchanger in this case is the indoor (air handler) coil – ThreePhaseEel Jun 9 '20 at 19:04
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Seems like your system is probably arranged pretty well. You can try closing some of the downstairs vents, but keep in mind this increases your risk of having ice form in the evaporator. The reason is this: with some vents closed the unit will have less air flowing through. Less air flowing through means the discharge air at the vents that remain open will get colder. That discharge air being colder means the evaporator is running colder -- and when its temperature drops below freezing ice starts forming. Ice in the evaporator blocks airflow, it gets even colder, and it can quickly cascade into a situation where the whole thing is blocked with ice and won't thaw until you shut the thing off for several hours.

Nevertheless: give it a try. Your system may be operating far enough from the freezing point that you won't have any problem. If it does start freezing, you'll now know why and what to do about it (re-open the downstairs vents).

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  • That's the kind of consequence I was asking about. But I wasn't considering completely closing any downstairs vents, just reducing their openings a bit. – Not_Einstein Jun 9 '20 at 17:39
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This is a common problem in warehouses and homes with high ceilings. You could use a ceiling fan. A device similar to this was designed for such a purpose. You can reverse the air flow based upon the season.
Thermal Equalizing Note: I have not used this particular product -- not an endorsement.

IT is designed to circulate the air between the ceiling and the floor. Back in the day, there was a long tube with a fan in it that was mounted against a wall or post, etc. This looks like a newer version.

Of course, adding insulation to the unfinished area helps (if there isn't any now).

I would be reluctant to close vents as it changes the airflow and load on the blower. I is even possible to REDUCE the total airflow in the entire system -- exactly the opposite of what you want.

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    Sometimes it's hard to get a definitive answer. :-) One person says go ahead and close downstairs vents and another advises against it. I do appreciate the input, even if it is contradictory. – Not_Einstein Jun 11 '20 at 1:31
  • Agreed. It is contradictory. You can certainly try closing them. It may not have the affect that you want. An HVAC Engineer ($$$) would be able to tell you for sure. I'm always down for an experiment -- it's not like your going to blow up the house. :) – Scottie H Jun 11 '20 at 18:25

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