Thermostat says heat on but nothing happens

Is the thermostat actually calling for heat?

Start here, because "heat on" can mean different things. Some screens show the mode you selected, Heat, even when the thermostat has decided the room is warm enough. Others show a separate message only while they are asking the system to run. Each brand gives a clear sign:

How to tell whether the thermostat is asking for heat right now

Brand	Calling for heat	Not calling, or waiting
Honeywell Home	"Heat On", a flame icon or a sun icon	"Heat On" flashing, "Wait" or "Waiting For Equipment"
Google Nest	The screen turns orange	A "Delayed" or "Starts in" message
ecobee	Orange flame icon, or an orange ring around the screen	White flame icon and no ring
Amazon Smart Thermostat	Flame icon lit, setpoint above the room temperature	Not stated

If the screen shows none of the calling signs, the thermostat is not asking for heat yet, and the fix is on the thermostat itself. Honeywell's own checklist is the same for any brand: set the mode to Heat, then set the temperature higher than the current room temperature.

Why would it wait before starting the heat?

There are two normal reasons, and both can look like a fault.

The first is compressor protection, which only applies to heat pumps. After the compressor stops, the thermostat holds it off for a few minutes before starting it again. Honeywell's no-heat guide says that if the icons or messages are flashing, compressor protection is on, and you should wait 5 minutes until they stop. On the FocusPRO S200, the screen shows "Waiting For Equipment" during the wait, then "Heat on" when it ends. Sensi applies a 5 minute delay to the heat pump when its "A/C Protection" setting is on.

The second is the temperature differential. Thermostats do not start the heat the moment the room drops a fraction of a degree. Google says a Nest thermostat will wait before it turns your system on or off, and calls that gap the maintenance band. ecobee adds an extra 1°F to the gap while it is in Away. If the room is only just below the setpoint, the thermostat may simply not have reached its trigger yet.

What else on the thermostat can stop the heat?

Work through these. Each one is something you can check on the screen or in the app:

• Wrong mode. ecobee says the mode must show Heat, Aux or Auto to call for heat. Off and Cool will never start it.
• Setpoint too low. Amazon's instruction for its Smart Thermostat is to press the mode button until the flame icon lights, then raise the temperature 1 or 2 degrees above the current reading.
• The schedule changed it. A schedule period, Eco mode or Away can lower the setpoint without you noticing. Google lists unexpected switches into Eco among the signs of strange heating behavior. On Lux thermostats, "HOLD" or "OVERRIDE" on the display means a manual setting is in charge.
• Weak batteries. Honeywell's T6 Pro guide says to make sure fresh AA alkaline batteries are correctly installed when the display is blank. The VisionPRO flashes "LO BATT" about 30 days before the batteries run out.
• Outdoor temperature limits on a heat pump. ecobee says that on a heat pump without auxiliary heat, if the outdoor temperature is below the Compressor Min Outdoor Temperature setting, the equipment will not turn on. ecobee gives 35°F as that setting's default.

If the screen says "Delayed" on a Nest thermostat, that has its own explanation; see Nest thermostat says "Delayed".

What should you do now?

Is the mode Heat (or Auto), with the setpoint at least 2 degrees above the room temperature?

• Yes

  Is a message flashing, or does the screen say "Wait", "Waiting For Equipment" or "Delayed"?

  • Yes

    Leave it alone for 5 minutes. The thermostat is waiting on purpose and will start the heat when the timer ends.

  • No

    Has it been more than 5 minutes since you raised the setpoint?

    • Yes

      The thermostat has done its part. Replace the batteries if it uses them, then check the signs at the bottom of this page.

    • No

      Give it the full 5 minutes. Honeywell and Lux both say to wait at least that long.

• No

  Set the mode to Heat and the setpoint a couple of degrees above the room temperature, then wait 5 minutes.

If you hear the thermostat click when you raise the setpoint but nothing starts, see thermostat clicks but the system does not start.

If the heat has started but the house is warming more slowly than you expect after a cold night, recovery mode explains how thermostats plan the warm-up. If your system is a heat pump and the screen shows auxiliary heat (AUX), see AUX heat vs emergency heat.