Honeywell thermostat flashing cool on

Why Is My Honeywell Thermostat Flashing Cool On?

If you see that your Honeywell thermostat is flashing “cool on” don’t worry, this is a safety feature within the thermostat that prevents possible damage to the compressor in your air conditioner, and the message should disappear in approximately 5 minutes.

The HVAC in your home is a finely tuned system exerted to high pressures, and if the system starts up before the pressure is allowed to level out it can cause costly damage to the compressor within the air conditioning system.

Delay Timer

Honeywell thermostats come equipped with a delay timer that prevents unnecessary strain on your air conditioner’s compressor. The delay timer ensures that your HVAC system has time to equalize its pressure after the set temperature has been reached.

Delay timers are usually set at 5-minute intervals, which is a couple of minutes longer than the time it takes to equalize the system’s pressure. The added time is just a safety net to ensure the pressure has time to level out.

Cool On Flashing

If your thermostat is blinking “cool on” one of three things has taken place.

  • Someone tried to restart the compressor before the refrigerant had time to settle.
  • A sudden change from cool to heat, or heat to cool.
  • Changing the batteries in your system may also cause this to happen.

Honeywell thermostats are equipped with a second line of defense in addition to the delay timer. If you try to start your HVAC system before the set delay time, the thermostat will flash cool on and won’t allow you to turn your system back on until the delay timer resets.

While this may seem like an inconvenience, it can save you money in the long run by preventing unnecessary damage to your system caused by a premature start, or a sudden change from heat to cool, or the other way around.

The blinking “cool on” should never last longer than 5 minutes since that is the usual time set on the delay timers. If it does last longer than that I suggest giving it some more time; if the problem persists I suggest trying one of the following.

  • Turn the thermostat off and turn it back on.
  • Locate the circuit breaker for your HVAC system, turn it off and turn it back on.


If you are unable to turn on your HVAC system and your Honeywell thermostat is flashing cool on it’s easy to assume that something may be wrong with your system, while in fact, it’s an intentional line of defense out there by the manufacturer to prevent costly damage to the compressor in your HVAC system.

It can happen for several reasons which have been listed above, but regardless of the reasons the message should disappear after five minutes of the system being off. Should the problem persist it’s probably best to consult a technician or reference the model-specific owner’s manual that was provided with your thermostat. Honeywell also has a customer support form on their website should you need additional support.