I installed a new programmable thermostat back in December and it has been wonderful. It replaced a (probably) 50+ year old dial thermostat.

To my question - now that it is really hot outside (85+ degrees F), I have noticed that after my inside A/C unit has stopped, and the house is at the proper temperature, that my outside unit will continue to run. Sometimes it will run for 30 minutes, sometimes all the way until the inside unit needs to run again, and others it will shut off when the inside unit does. It's very inconsistent.

Any ideas if something may be wrong, or could it just be the newer thermostat cycles the unit differently than my old one? I never had this problem in the summer with the old one.

EDIT - Just to get this all in the question:

A/C Condenser - Trane XE 1000 - http://tranexe1000.blogspot.com/2014/11/trane-xe-1000-specifications-reviews.html

Air Handler/Furnace - Trane XE 78

Thermostat - Honeywell RTH2510 - http://www.amazon.com/Honeywell-RTH2510B1000-7-Day-Programmable-Thermostat/dp/B003XCAMR8

EDIT 2 - Wiring at the thermostat


2 Answers 2


You've got a wiring problem, or a bad circuit board somewhere.

It's possible that the condenser unit is wired to run all of the time, but some protection circuit is kicking in and shutting it down. If that's the case, you would probably have ice build-up on your coils.

What is more likely is a relay somewhere is sticking, and remains closed when de-energized. If you provide the model number of you air handler and your condenser, I might be able to provide more direction.

  • I'm not home to verify this at the moment, but I believe my condenser is a Trane XE 1000 - tranexe1000.blogspot.com/2014/11/…. I believe that the furnace/air-handler is a Trane XE 80.
    – Tricky12
    Jun 6, 2016 at 17:33
  • I noticed that you mentioned a wiring problem. Is this something that could be the result of me incorrectly wiring up the new thermostat? I had no problems with it in the winter while using heat, and the house does get cooled to the correct temperature. There is just this inconsistency with the outside unit running occasionally after the inside unit has stopped.
    – Tricky12
    Jun 6, 2016 at 17:48

Since the unit is cooling and all the functions of the heating features is doing everything right but the on&off within the cooling features isn't cycling as it's designed to do, it seems to me that a relay needs changing which is a very quick & inexpensive fix! When it comes to A/C or heating units, if a certain function isn't working as it should, but the unit itself is performing as it's designed. Then more times than not it's either a relay or T-stat that's gone bad or sticking. Basically if a relay is sticking, it's bad.

  • "a relay needs changing which is a very quick & inexpensive fix" but "if a relay is sticking, it's bad" A "quick and inexpensive fix" doesn't seem to jive with "it's bad". Which is it?
    – FreeMan
    Jul 7, 2023 at 18:24

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