I have this Honeywell RTHL2410C programmable thermostat and typical Florida electric AC system. Starting this Spring, it has started behaving strangely when switching from Heat to Cool mode.

The thermostat has a physical switch to change the mode (or to turn it off completely) and separate programs for the two modes. When I move it from heat to cool, the thermostate becomes partially unresponsive for quite a long time (at least 30 minutes or more). During this time, the unit does not turn on and I can't set the temperature override or do anything to coax it to turn on. The thermostat has power; it's backlight comes on in response to pushing the buttons, and I can even adjust the programming, set the time/date, switch the fan from Auto to On, etc. But I can't adjust the temp or use the Run or Hold buttons. It's like the thing is actually in Off mode although it's definitely in the Cool position.

The past 2 times this has happened, the unit eventually "woke up" and came on and cooled as expected.

Also, the unit is not running when I switch the mode; for example, today it was quite warm when I got home and switched from Heat to Cool, so the unit had not been on since early morning.

This only recently started happening; in the past it's worked as intended when changing over from one mode to the other.

It doesn't seem like the thermostat is faulty, since it mostly works. Could it be a partial fault? Any ideas for troubleshooting are welcomed.

  • Does the t-stat use batteries? – bigbull15 Apr 4 '16 at 23:29
  • I replaced the batteries, but no difference. – E-Riz Apr 4 '16 at 23:42
  • It would be good while you're looking for batteries if you could get the model number of the t-stat as well. – bigbull15 Apr 4 '16 at 23:51
  • I added the model number to the question – E-Riz Apr 5 '16 at 0:01
  • It has a physical Heat/Off/Cool switch; the problem might be the switch contacts. Try rapidly and repeatedly moving the switch from end to end (probably after turning your furnace/AC off); that could clean the contacts. – Daniel Griscom Apr 5 '16 at 1:23

Based on the installation instructions for the model you listed, you have two AAA batteries. I recommend you begin by replacing them.

  • I updated my comment a few minutes ago, I did change the batteries. – E-Riz Apr 5 '16 at 0:07
  • Just wanted to follow up and see if you ended up replacing the t-stat? – bigbull15 Apr 25 '16 at 18:15
  • no I didn't. Warm weather is here for the season so I have no need to switch between Heat and Cool for quite a few months. – E-Riz Apr 25 '16 at 18:40

Most AC compressors will not turn on if the outside temperature is too cold. There is a sensor outside at the comoressor unit. The setpoint is between 50^F and 60^F.
Also, compressors have a delay relay. If you turn off the compressor and turn it back on again, the delay will prevent a sudden restart. The compressor needs time to relieve internal pressures before it can start up again without damage.
Larger rooftop units can have an "economizer" attached. During cool months when the inside temperature gets too warm, a damper opens to bring in cool outside air. The compressor does not run when the economizer is working.

  • Trying to restart an air conditioner too soon is called a "short cycle" and can do a lot of damage. – Harper - Reinstate Monica Sep 14 '19 at 22:59

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