I went to a customer’s home the other day that was having cooling problems. The fan on the condenser was bad but that was the least of their problems. Their home had been plagued with premature failure of air conditioners over the life of the home and had higher than necessary electric bills.
I was asked if I could change the filters for them, which I gladly did, but was concerned as to how little air flow I felt at the filter grill. I took readings and noted only 600 CFM was entering through the filter returns. This was a 5 ton system and should have been more in the range of 1750 to 2000 CFM.
I also took temperature measurements at 3 locations, 1 at each of the 2 return filter and 1 at the entrance to the furnace. The temperature at the returns should be about equal to the air entering the furnace. You can add a degree or two due to heat gain on the ductwork but that should be all. In this case it was about 11 degrees warmer. 86 at the returns and 97 at the furnace. The hot air was coming from other places instead of inside the home. The air was coming from the attic or other cavities in the walls that was pulling outside air into the system.
Install new returns in the ceiling of the second floor. Close off and abandon the existing returns.
I have found this same issue in other homes that have returns in the walls. That doesn’t mean that they all are this way but if you do have a home that has problems keeping it cool or your electric bills are higher than you think they should be, this could be one of the problems.
As a side note, here in the South, we are more concerned with cooling than we are with heating. I frequently see homes that have returns in the wall near the floor. Hot air rises, so the coolest air is near the floor and the hottest air is near the ceiling. An air conditioner works more efficiently at removing heat from warmer air than it does removing heat from cooler air. The systems can remove the most heat when the returns are located in or near the ceiling.