Heat Pump Not Switching to Auxiliary Heat (Troubleshooting Guide)

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Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

Check the thermostat settings to ensure it's set to 'Heat' and not in 'Emergency Heat' mode. If the issue persists, test the auxiliary heat by turning the thermostat to a higher temperature than the current indoor temperature. If the auxiliary heat still doesn't activate, there may be an issue with the thermostat, relay, or electrical connections.

Confirm the symptom

Heat pump is not switching to auxiliary heat when temperatures drop
Thermostat shows no sign of auxiliary heat activation
Indoor temperature is not rising as expected during cold weather

Tools and prep

Thermostat manual
Multimeter
Screwdriver
Safety gloves
Turn off power to the heat pump at the circuit breaker for safety before inspecting electrical components
Check for any error codes on the thermostat or heat pump display

Step-by-step troubleshooting

Check Thermostat Settings
Do: Verify the thermostat is set to 'Heat' mode and not in 'Emergency Heat' mode.
Observe: Thermostat mode and settings
Means: Incorrect thermostat settings can prevent auxiliary heat activation.
Next: If settings are correct, proceed to the next step. If not, adjust settings and monitor for auxiliary heat activation.
Test Thermostat Operation
Do: Adjust the thermostat to a higher temperature than the current indoor temperature to see if the auxiliary heat activates.
Observe: Whether the auxiliary heat indicator on the thermostat turns on.
Means: A non-responsive thermostat may be faulty.
Next: If the thermostat does not respond, replace the batteries or consider replacing the thermostat.
Inspect Electrical Connections
Do: Turn off power to the heat pump at the circuit breaker. Check the electrical connections at the thermostat and indoor unit for looseness or corrosion.
Observe: Secure and clean electrical connections.
Means: Loose or corroded connections can disrupt the signal to activate auxiliary heat.
Next: If connections are secure and clean, proceed to the next step. If not, tighten or clean them and restore power to test.
Test Auxiliary Heat Relay
Do: Using a multimeter, test the auxiliary heat relay for continuity. Refer to the heat pump manual for the correct terminals to test.
Observe: Continuity in the relay circuit.
Means: A faulty relay can prevent the auxiliary heat from turning on.
Next: If the relay is faulty, it may need to be replaced. If it's functioning, proceed to the next step.
Check for Error Codes
Do: Look for any error codes displayed on the thermostat or heat pump unit.
Observe: Specific error codes related to auxiliary heat failure.
Means: Error codes can provide direct insight into the issue.
Next: Consult the heat pump manual to interpret the error codes and take appropriate action.
Inspect Emergency Heat Switch
Do: If your thermostat has an 'Emergency Heat' switch, ensure it is in the correct position.
Observe: Position of the 'Emergency Heat' switch.
Means: An incorrectly positioned switch can prevent normal heat pump operation.
Next: Adjust the switch if necessary and monitor for auxiliary heat activation.

Common causes

Cause	Likelihood	Difficulty	Part often needed
Incorrect Thermostat Settings The thermostat may be set to a mode that prevents auxiliary heat activation.	Common	Easy	None
Faulty Thermostat A malfunctioning thermostat may not send the correct signal to activate auxiliary heat.	Common	Easy–Medium	Thermostat
Loose or Corroded Electrical Connections Poor electrical connections can disrupt the signal to the auxiliary heat system.	Sometimes	Easy	Wire connectors
Faulty Auxiliary Heat Relay A defective relay can prevent the auxiliary heat from turning on.	Sometimes	Medium	Relay
Thermostat Battery Issues Low or dead batteries can cause the thermostat to malfunction.	Common	Easy	Batteries
Error Codes in Heat Pump System Error codes can indicate specific issues with the auxiliary heat system.	Sometimes	Medium	Service manual
Incorrect Emergency Heat Switch Position The emergency heat switch may be in a position that prevents normal heat pump operation.	Rare	Easy	None

Parts matching

If you need to replace parts, consider these common components:

Thermostat
Thermostat batteries
Auxiliary heat relay
Wire connectors
Heat pump service manual

Always refer to your heat pump's manual for specific part compatibility and installation instructions.

When to call a professional

If the heat pump still doesn't switch to auxiliary heat after checking all settings and connections
If you detect a burning smell or unusual noises from the heat pump
If the error codes indicate a complex issue beyond basic troubleshooting
If you're uncomfortable working with electrical components
If the problem persists after replacing the thermostat or relay

FAQ

What is auxiliary heat on a heat pump?

Auxiliary heat is an electric resistance heating system that supplements the heat pump's heating capacity when the outdoor temperature is too low for the heat pump to operate efficiently.

Can I use emergency heat instead of auxiliary heat?

Emergency heat is a backup heating mode that bypasses the heat pump and uses only the auxiliary heat. It should be used only in emergencies, as it is more expensive to operate.

How do I know if my heat pump is using auxiliary heat?

Most thermostats have an indicator light or display that shows when auxiliary heat is active. You may also notice the indoor temperature rising more slowly.

Is it normal for the heat pump to switch to auxiliary heat in cold weather?

Yes, it is normal for the heat pump to switch to auxiliary heat when the outdoor temperature drops below a certain threshold, typically around 35°F (1.7°C).

Why is my heat pump not using auxiliary heat even when it's very cold outside?

This could be due to incorrect thermostat settings, a faulty thermostat, or an issue with the auxiliary heat system itself. Check the thermostat settings and connections first.