Front and rear A/C...

[Glenn and Lorraine]

We don't run the front A/C very often but with 95° plus days here in Louisville KY I found both A/Cs necessary.
I also discovered that the front A/C was sending cooler air down the steps past the rear A/C thermostat. This caused the thermostat to cool prematurely creating a false reading and shutting the rear A/C down keeping the livingroom considerably warmer than desired.
To solve the problem was quite easy. I took a small Glad storage bowl and taped it over the thermostat. It worked as a shield from the front A/Cs cooler air and the rear A/C has been working fine. Within a half an hour I brought the livingroom down from 86° to a more comfortable 75°s.

[Montana Sky]

Glenn,
I have the same problem in my 3400RL. When using both a/c units, I open the water closet door all the way. It provides the same effect your glade bowl did by shielding the thermostat from the bedroom cold air blowing past it.

[richfaa]

Hummm.. We never noticed that..will check it out...

[Ozz]

Rich, I think our thermostat is placed in a location that is not a factor with this problem. (Over by the desk on the wall.