If you have a heat pump system, you've probably noticed an "Aux" or "Emergency Heat" indicator on the thermostat at some point, usually during a particularly cold stretch. These two settings are related but not identical, and understanding the difference helps you know whether what you're seeing is normal or worth a call.
What auxiliary heat actually is
Heat pumps become less efficient as outdoor temperatures drop, and below a certain threshold they can't keep up with the home's heating demand on their own. Auxiliary heat is a backup heating source, usually electric resistance heat strips built into the system, that kicks in automatically to supplement the heat pump during especially cold weather. Seeing the "Aux" light during a genuine cold snap is normal, expected behavior, not a malfunction.
What emergency heat mode actually does
Emergency heat is different: it's a manual setting that bypasses the heat pump entirely and runs only on the backup heat source. This is meant to be used specifically when the heat pump itself has failed and you need heat until it can be repaired, not as a regular setting. Because backup electric heat is significantly less efficient than the heat pump running normally, leaving a system in emergency heat mode for an extended period will show up clearly on your electric bill.
When seeing the aux light often is worth a call
Occasional aux heat activation during genuinely cold weather is expected. Auxiliary heat kicking in constantly, even on mild days, or emergency heat mode being needed frequently, points to the heat pump itself struggling, whether from low refrigerant, a failing component, or a system that's undersized for the home.
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