HVAC and Electrical Experts
Written by Peter
Master Electrician at PRO Electric plus HVAC, serving Fairfax, Loudoun, Arlington, and Prince William Counties. Virginia License #2705181607.
Aux and Emergency Heat Are Not the Same Thing.
Heat pump service across Fairfax, Loudoun, Arlington, and Prince William Counties.
Hi, I am Peter, the Master Electrician at PRO Electric plus HVAC. If you have a heat pump, your thermostat probably has settings or indicators for auxiliary heat and emergency heat, and they confuse almost everyone. They sound like the same thing, but they are not, and knowing the difference can save you money and help you spot a problem. Let me clear it up.
A heat pump heats your home by moving heat from the outdoor air inside. On cold days it gets help from electric backup heat. How that backup gets used is where aux and emergency heat differ.
Auxiliary heat
Auxiliary heat, often shown as aux heat, is the backup the system turns on automatically when the heat pump alone cannot keep up, usually on the coldest days or when the thermostat is raised several degrees at once. The heat pump keeps running and the electric backup helps it along. Seeing aux heat come on during very cold weather is normal. This is the same electric resistance backup I describe in how heat pumps handle cold weather.
Emergency heat
Emergency heat is a setting you turn on yourself, and it tells the system to skip the heat pump entirely and run on the electric backup alone. It is meant for when the heat pump is broken or iced up and you need heat until it can be serviced. It is not meant for everyday use, because running on backup heat alone is expensive.
When to use each
- Let aux heat do its thing. You do not switch aux heat on. The system calls for it on its own in cold weather, and that is normal behavior.
- Use emergency heat only in a true problem. Switch to emergency heat if the heat pump has clearly failed or iced over and you need warmth until a technician arrives.
- Watch for aux heat in mild weather. If aux heat is on when it is not very cold, something may be wrong, which connects to how the system is set up to heat.
Running on backup heat sends your bill up fast
Both aux and emergency heat use electric resistance heat, which costs much more to run than the heat pump itself. A little aux heat on the coldest days is expected. But emergency heat left on for days, or aux heat running constantly in mild weather, will show up as a sharp jump in your electric bill and usually signals a problem worth checking.
How we help
If your heat pump is leaning on backup heat more than it should, we find out why, whether that is a refrigerant issue, a defrost problem, a failing component, or a control and thermostat setup that is calling for backup too soon. The goal is a system that heats efficiently from the heat pump and only uses backup when it truly needs to.
Frequently asked questions
What is the difference between emergency heat and auxiliary heat?
Auxiliary heat turns on automatically to help the heat pump on cold days, with the heat pump still running. Emergency heat is a setting you choose that shuts the heat pump off and runs on electric backup alone, meant only for when the heat pump is broken or iced up.
When should I use emergency heat?
Use emergency heat only when the heat pump itself has failed or iced over and you need heat until it can be serviced. It is not for everyday cold weather, since it runs on electric backup alone, which is expensive. For normal cold days, leave the system on its regular heat setting and let aux heat help as needed.
Is it normal for aux heat to come on?
Yes, on the coldest days or when you raise the thermostat several degrees at once, it is normal for aux heat to assist the heat pump. What is not normal is aux heat running often in mild weather, which can signal a refrigerant, defrost, or control problem worth having checked.
Why does emergency heat cost so much?
Emergency heat runs on electric resistance backup alone and shuts off the efficient heat pump, so it uses far more electricity for the same warmth. That is why it is meant only as a short term measure when the heat pump is down, not as a regular setting.
Why does my heat pump keep using auxiliary heat?
Frequent aux heat in genuinely cold weather is expected. But if it runs in mild weather or seems constant, the heat pump may not be keeping up the way it should, often due to a refrigerant issue, a defrost problem, a failing part, or a thermostat calling for backup too soon. It is worth a diagnostic.
Heat pump leaning on backup heat?
Heat pump diagnosis and service across Northern Virginia.