Heat Pump Thermostat Not Working

Start with the basics

When a heat pump seems dead, the thermostat is a good place to start because it is easy to check. A thermostat is a low-voltage control that tells the indoor and outdoor units when to run. Before assuming it has failed, confirm the following:

• The display is on and readable.
• The mode is correct: Heat to warm the house, Cool to cool it.
• The set temperature is a few degrees past the current room temperature in the direction you want.
• The fan is set to Auto unless you want continuous airflow (see heat pump fan: on or auto).

Thermostat blank or unresponsive

A blank screen usually means no power to the thermostat. Common causes:

• Dead batteries. If your thermostat uses AA or AAA batteries, replace them with fresh ones.
• Tripped breaker. Check the electrical panel for a tripped breaker labeled for the HVAC system or air handler. If it is tripped, switch it fully off and then on once. If it trips again, stop and call a professional (see our article on heat pump breakers that keep tripping).
• Safety switch or door switch. Some air handlers have a switch that cuts power when a panel is open or when a condensate drain backs up. Make sure panels are seated and a clogged drain has not tripped a float switch.
• Dirty filter. In some systems a badly clogged filter can lead to a safety shutdown, so check it.

Thermostat on but the heat pump doesn’t start

If the display works but nothing happens:

1. Wait several minutes. Many heat pumps have a built-in delay of a few minutes before the compressor restarts after a mode change or power interruption.
2. Check the mode and set point again. Setting the temperature just one degree off often will not call for heating or cooling.
3. Check the outdoor unit. Look for blocked airflow or an unusual condition such as heavy ice.
4. Look for error or “service” messages on the thermostat and check your manual for what they mean.

If the indoor fan runs but the outdoor unit does not, the problem may be a failed capacitor, contactor or another outdoor component. Those involve high voltage, so leave the diagnosis to a technician.

Heat pump not reaching the set temperature

A heat pump that runs but never gets the house to the set point may have a problem unrelated to the thermostat. Possible causes include a dirty filter, restricted airflow, a thermostat in a poor location (near a vent, window or heat source), low refrigerant, or a system that is simply undersized for very cold or very hot weather. If cooling is the problem, see why a heat pump won’t cool.

Emergency heat and auxiliary heat settings

Many thermostats have an Emergency Heat setting that runs backup heat (often electric heat strips) instead of the heat pump. If the thermostat is accidentally set to Emergency Heat, the house may warm but your energy use can rise noticeably. Use it only when the heat pump itself has failed or you’ve been told to; see when to turn on emergency heat for details. If the thermostat is set to Heat but auxiliary heat is running often, that can be normal in very cold weather, though frequent use in mild weather may signal a problem.

How to tell if the thermostat is bad

Signs the thermostat itself may be at fault:

• The display is wrong compared with a separate thermometer placed nearby.
• It loses settings or schedules.
• It does not respond to button presses even with fresh batteries.
• The system runs correctly when a technician bypasses or replaces the thermostat.

If the system responds normally when a qualified person tests the control circuit, the thermostat is the likely culprit. If the system itself does not respond, the thermostat may be fine.

Smart thermostat and wiring issues

Smart thermostats often need a continuous low-voltage power wire (commonly called a C-wire) and the right wiring for heat pump functions such as the reversing valve. A thermostat that restarts, goes blank, or works in one mode only after installation may be wired incorrectly. Turn off power at the breaker before touching any thermostat wiring. If you aren’t sure, have an HVAC technician or electrician check it. Never work on a thermostat that uses line voltage (typical in some baseboard systems) unless you are qualified.

When to call a professional

• The breaker trips again after you reset it once.
• You see burnt marks, melted insulation or smell burning.
• The outdoor unit hums but does not start, or does not run at all.
• You are not comfortable removing the thermostat or handling wiring.
• Replacing batteries and checking settings changes nothing.

FAQ

How do I know if my heat pump thermostat is bad? If it has power but gives wrong readings, loses settings, or does not match what the system does, it may be at fault. A technician can confirm by testing the control wiring.

Why isn’t my heat pump turning on when I switch to AC? Check the mode, the set point, power, and the compressor delay. If those are fine, the issue may be in the system or wiring rather than the thermostat.

Why won’t my heat pump reach the set temperature? Dirty filters, blocked airflow, poor thermostat location, low refrigerant or extreme weather can all be factors.