Sensi Auxiliary or Emergency Heat Not Working?
- W/W2/Aux (backup heat) wire not landed in the terminal
- System not configured as heat pump with auxiliary heat
- Auxiliary heat lockout temperature set too low
Problem Description
Your heat pump's auxiliary or emergency heat does not come on under Sensi control, so the house stays cold on very cold days or when the heat pump fails. On a heat pump, backup heat runs through the W/W2/Aux terminal, and it fails when that wire is not landed, the system is not configured for a heat pump with backup, or the aux/emergency settings are off.
Why This Happens in Real Homes
Backup heat problems on a Sensi come down to knowing which backup you mean and making sure the thermostat is configured for it. Auxiliary heat assists the heat pump automatically when it cannot keep up on cold days, while Emergency heat is a mode you choose by hand when the heat pump itself has failed, running the backup strips or furnace and bypassing the compressor. For either to work, the backup heat wire has to be landed in the correct W or W2/Aux terminal, and the Sensi must be configured as a heat pump with auxiliary heat in the wire picker, because if backup is not declared, the thermostat never energizes it and the Emergency Heat mode does not even appear. Beyond wiring and configuration, Sensi's aux lockout and staging settings decide when backup is allowed, so a lockout set too low can keep it from ever running. And backup heat has its own breaker and elements, so a tripped breaker leaves you cold no matter what the thermostat does.
Symptoms
- Auxiliary heat never engages on very cold days
- Emergency Heat mode produces no heat
- House cannot keep up when it is cold outside
- Only the heat pump runs, no backup strips
- Aux heat runs constantly instead of as backup
- No Em Heat option in the app
- Heat pump failed and backup will not take over
- Aux heat wire present but nothing happens
Recognize these? Here's what usually causes it.
Common Causes
- W/W2/Aux (backup heat) wire not landed in the terminal
- System not configured as heat pump with auxiliary heat
- Auxiliary heat lockout temperature set too low
- Emergency Heat mode not selected when the heat pump is down
- Aux staging not enabled in the app
- Backup heat breaker or element failed
- Wrong system type in the wire picker
- Balance-point settings preventing aux from running
Most fixes happen in the first 3 steps.
Electric backup heat draws high current on its own breaker; leave element and breaker work to a professional. Do not leave the system in Emergency Heat long-term, as it is less efficient and meant for outages.
Tools & Requirements
These tools will help you complete this fix.
Multimeter
Klein Tools 80196 Digital Multimeter Kit with Case, ...
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STREBITO 155 in 1 Electric Screwdriver Set, Small El...
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Step-by-Step Solution
Know the Difference Between Aux and Em Heat
On a heat pump, Auxiliary heat automatically assists the heat pump when it cannot keep up (very cold days or a big temperature recovery). Emergency heat is a mode you select manually when the heat pump itself has failed, running only the backup strips or furnace and bypassing the compressor. Which one you need shapes the fix.
Check the Backup Heat Wire
Turn off power at the air handler switch, pull the Sensi off its base, and confirm the backup heat wire is landed in the correct terminal (commonly W or W2/Aux, depending on the system). If it is missing or in the wrong terminal, the thermostat cannot call for backup heat.
Confirm the System Configuration
In the Sensi app wire picker, the system must be set as a heat pump WITH auxiliary/backup heat, not just a heat pump alone. If backup heat is not declared in the configuration, the thermostat never energizes it and there is no Emergency Heat option.
Test Emergency Heat
If the heat pump has failed, set the Sensi to Emergency Heat (Em Heat) mode. The backup strips or furnace should provide heat directly. If Em Heat is missing or does nothing, the configuration or the backup-heat wiring is the problem.
Review the Aux Lockout and Staging
Sensi uses settings that decide when auxiliary heat is allowed, such as an outdoor lockout temperature or how far the room falls behind before aux kicks in. If the lockout is set very low or staging is off, aux may never run. Adjust these so backup can engage when the heat pump falls behind.
Check the Backup Heat Equipment
Electric backup heat has its own breaker (often a double-pole breaker) and heating elements or a sequencer. A tripped breaker or a failed element means no backup heat regardless of the thermostat. Confirm the breaker is on and have a pro test the elements if needed.
Verify the System Type
If the app is set to a conventional system rather than a heat pump, the aux/emergency logic does not exist. Set the system type to heat pump with backup so the modes and staging appear and work.
Confirm Normal Heat First
Before chasing aux heat, make sure the heat pump itself heats in normal Heat mode. If it runs backwards or not at all, fix that first (reversing valve/O-B), because backup heat is meant to assist, not replace, a working heat pump.
Quick Solutions
Still having issues? This is usually the deeper cause below.
If this comes back after following these steps, check whether a recent app or firmware update reset a default setting — the fix works, but the setting gets reverted silently.
On a cold night when the heat pump has died, switch the Sensi to Emergency Heat to run the backup directly. Set aux settings so backup assists automatically on cold days without running all the time.
This issue almost always looks more complex than it is — the majority of cases trace back to a single setting, a stale credential, or a default that shipped wrong.
- W/W2/Aux (backup heat) wire not landed in the terminal
- System not configured as heat pump with auxiliary heat
- Auxiliary heat lockout temperature set too low
- Emergency Heat mode not selected
- Aux staging not enabled in the app
Before you go — try one of these (they fix most cases).
Most popular upgrades chosen by Emerson Sensi Thermostat owners.
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Official Manufacturer Manual
Emerson Sensi provides official product documentation through their online manual rather than downloadable PDF. Access setup guides, troubleshooting steps, and product specifications for your Emerson Sensi Thermostat.
Source: sensi.emerson.com
Need More Help? Emerson Sensi Support
Note: The contact information below connects you directly to Emerson Sensi's official customer support team, not Trunetto. They can help with warranty claims, device replacements, and advanced technical issues.
