I recently moved my office to the second floor of our home and I haven't been able to get the temperature below about 78 even though the thermostat is on 72. My son's bedroom had been the same way until the other day when I removed his vent cover (diffuser) to paint the ceiling. Today I noticed his room was like a freezer, so I removed the vent cover in my office and now it is cool in here. Is this normal?

My vent cover is round and looks like this:

enter image description here

with the exception that mine only has two openings rather than three.

Here's my theory...when the diffuser is on, the cold air is mostly pushed out along the ceiling where the air is the hottest. This warms it up as soon as it comes out, before it has a chance to "fall" down to where I am. By removing the cover, the air is being pushed straight down, essentially cooling the air lower in the room rather than near the ceiling.

My cover is about 30 years old, so perhaps it is poorly designed?

  • Do you have any "air return" vents in each room or nearby?
    – auujay
    Mar 20 '12 at 20:08
  • Just outside of the rooms in the hallway. There's about a 1" gap underneath the doors. Mar 20 '12 at 20:17
  • Try leaving the doors open see if that helps. If so you need to add return ducts to each room.
    – SteveR
    Mar 20 '12 at 23:34
  • Thanks, SteveR...leaving the door open was the first thing I tried, but it did not help. Mar 20 '12 at 23:54
  • I'd check for dampers in the vents that allow you to adjust the airflow through each vent separately. You may be able to turn or adjust something in these vents.
    – BMitch
    Dec 28 '13 at 12:28

It's possible.

And may make sense if you think about how the system is working...

First the temperature near the thermostat goes above the set temperature, which causes the thermostat to call for cool air. The A/C unit kicks on, and starts blowing cool air. Once the temperature near the thermostat reaches the set value, the thermostat tells the A/C unit to shut off. The A/C unit shuts down, and stops blowing cool air.

While the A/C unit was blowing, the 30 year old vent covers were restricting the air flow from the ducts. So while the proper temperature was reached at the thermostat, not enough cool air was supplied to the other areas of the house. So the area near the thermostat may actually reach the proper temperature sooner, because that area had unrestricted air flow and received a bulk of the cool air.

When you removed the vent covers, you allowed more of the cool air to enter the other areas (away from the thermostat). So it took longer for the area near the thermostat to reach the desired temperature. Now, because the majority of the cool air was supplied to the other areas (away from the thermostat), those areas were cooled more than the area near the thermostat.

In a perfectly balanced system, all areas would reach the same temperature at the same time. Alas, we don't live in a perfect world. So you end up with a hot office and a comfortable temperature in the rest of the house, or a frozen office and a comfortable temperature in the rest of the house.

To fix the problem you could try to balance the system yourself, or you could call in an HVAC company to balance it for you (which will likely include installing dampers and new vent covers).

  • Moral of the lesson: keep the thermostat in an area where the vents don't directly blow on it and in an area with a larger air volume. If the thermostat chills or heats before the conditioned air volume does, things can be kind of unsatisfactory. Then the rest is in balanced airflow so peripheral rooms don't become too hot or cold. Also without proper air circulation, thermostats measure how deep the cold air is under AC and how low the heat bubble gets under heating. Good mixing is necessary so all the volume is involved. Dec 28 '13 at 20:11
  • This answer seems correct--note that it's very different from the theory you offered in your question. Taking the vent covers off is eliminating a restriction, resulting in more AC airflow to those rooms (and less AC flow to the thermostat area). With my old system, straightening/shortening/rerouting some of the flexible ducts has had a huge impact on where the cold air goes. It's the same thing--eliminating some restrictions is redistributing air flow.
    – Phil Esra
    Jul 31 '19 at 17:06

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