My house has 2 zones for air conditioning (Upstairs & Downstairs) with 2 different air conditioning units.

We have Nest energy saving thermostats, which are great for keeping track of energy history, and I noticed that the downstairs AC is virtually never on, and the upstairs AC is on almost constantly.

This is mysterious to me because we clean our air filters constantly (Including the outdoor AC unit itself (Once per season). NOTE: I live in Suffolk County, New York


0) There is cold air coming from the upstairs vents, although it isn't at a very high rate. There are vents downstairs that have far higher (Around 2x) air flow.

1) The thermostat can't reach its set point temperature. This situation lasts through the day (Check History for more on this):


with the upstairs temperature never reaching the set point, and always staying above it. (Note the 80 on the side is the current temperature)

2) The downstairs thermostat is in sync with the upstairs thermostat usually. (They are also both on at the same time) to minimise heat gain die to convection.

AC History:

Upstairs: Upstairs




Unfortunately, there are no records of the AC system that I could find, but I did manage to get this from the back label of the AC unit that is always on: C 249534. I don't think I can get more information without actually taking the panel off. AC Unit

There is also some kind of machinery on the inside (I'm not entirely sure what it is):


Further Away: (The top panels say Ultra 80 & the bottom labels say Armstrong Air

Further Away

There are also electro-static filters (Which according to my dad contain the only filters in the system, although he might be mistaken, I'm not sure)


Are there any obvious red flags with this set up? What is the most likely cause of failure given the set-up of the system? Are there any additional filters that I didn't know about that aren't being cleaned? Are additional specs required?

  • Sounds like basic physics at work. Fluid parcels of a higher temperature rise. You'll need to either 1) set the upstairs thermostat a few degrees lower, or 2) run the circulating fans more often to balance the air between levels (if possible).
    – isherwood
    Commented Aug 7, 2018 at 21:03
  • Do you get good airflow from each unit? (Are the filters clean?)
    – isherwood
    Commented Aug 7, 2018 at 21:07

3 Answers 3


It looks like the air handlers are both in the basement. There will be a fairly significant friction loss due to the added distance of the second story air ducts. If the ducts weren't carefully sized and balanced from the beginning, this could impede the airflow.

Another factor could be the return air. If the upper story does not have its own return air, then the returns will be sucking air from upstairs down to the lower floors when the system is on.


Not knowing how your house is laid out, one certainty in life is that cold air upstairs will go downstairs unless it is stopped by something. Are your stairs enclosed? Is there a door? If not, I suspect all your cold air is legging it down the stairs.

My parents have a two-story house (in Arizona) with a wide, open stairway and a large open loft area. They have to set the thermostat (upstairs) higher than expected or else downstairs turns into a meat locker. There is approximately 10 degrees difference between floors.

  • I wouldn't expect this type of behaviour if both stories are regulated at the same temperature. I would expect minor deviation due to convection from below, but if the air is the same temperature (For the most part) I would expect minor heat gain from downstairs) Commented Aug 8, 2018 at 18:42
  • That's the problem, unless upstairs and down are separate contained air spaces they will never be the same temperature. If the air can move freely up and down, downstairs will always be colder. What's to stop the cold air from upstairs going downstairs?
    – Tim Nevins
    Commented Aug 8, 2018 at 19:29
  • So are you suggesting that there is no way to fix this under moderate heat load on the upstairs story? Commented Aug 8, 2018 at 20:33

There is cold air coming from the upstairs vents, although it isn't at a very high rate.

If you notice the airflow is less than normal, then it may be a sign that your evaporator coil is freezing, limiting the airflow. You can check for this by turning off the AC and let the circulation fan run for a few hours and see if the flow increases. With only 12 hours running time, I'd probably say that is not your problem, figured it was worth a mention since you specifically mentioned the air flow. I think the lower air flow is due to the distance from the air handler, since it looks like it is in the basement.

Where is the air return for the upstairs unit, is there a return in each room or is it centralized? If each room has an air return, then you can try closing the doors to minimize mixing the air into the common spaces. This may help keep your colder air upstairs. If the air return is in a common space, this will not likely help you.

Its been a really hot week and your upstairs unit takes the brunt of the work, as others have suggested.

One good thing to having one unit do more work is you get really good dehumidification.

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