Auxiliary vs Emergency Heat On A Heat Pump, Explained

Use this guide to tell auxiliary heat and emergency heat apart with sourced records, not folklore. Each claim below carries the dated source that supports it, and where sources disagree on a threshold, this guide reports the range rather than picking a single number.

Same Heat Strips, Two Different Modes

The first thing to understand is that auxiliary heat and emergency heat usually draw on the same backup hardware. One Hour Heating and Air Conditioning records that both auxiliary and emergency use the exact same heater elements but are labeled differently, describing the element as an electric heater similar to a toaster oven. Trane describes these as electric heat strips installed in the indoor air handling unit as a backup heat source.

What separates the two modes is control, not the coil. Auxiliary is an automatic assist that runs alongside the heat pump. Emergency is a manual setting you switch on when the heat pump itself is not working. HVAC.com records that emergency heat requires a manual switch, while auxiliary heat is an automated function.

Keep that framing attached to every later number: auxiliary is the thermostat helping the heat pump, and emergency is the homeowner replacing the heat pump. Confusing the two is what leads to the surprise electric bills covered later in this guide.

When Auxiliary Heat Runs

Auxiliary heat is triggered automatically by the thermostat, and the sources give consistent conditions with slightly different threshold numbers. Trane records that auxiliary heat triggers when the outside temperature is below 40 degrees or a set trigger temperature, when the heat pump is in defrost mode, or if you raise the heating temperature by more than 3 to 4 degrees. One Hour Heating and Air Conditioning gives a similar picture, noting auxiliary engages during defrost and when the thermostat has a difference of 3 degrees or more from the set temperature, and that it turns off once the set point is within 2 degrees of the indoor temperature.

Because the exact cut-in temperature varies by model, this guide reports the range rather than a single figure. HVAC.com notes most heat pumps automatically switch to auxiliary heat when outdoor temperatures drop below 35 to 40 degrees Fahrenheit, though this varies by model and manufacturer.

The practical takeaway is that seeing AUX on the thermostat during a cold snap or a defrost cycle is normal. Trane adds a useful check: if it is above 40 degrees and the thermostat shows AUX HEAT for more than an hour or two, that is worth investigating.

When Emergency Heat Runs

Emergency heat is the mode you should reserve for an actual failure. TeamEnoch records that when you manually activate emergency heat you completely bypass the heat pump, the system shuts down the outdoor unit entirely, and it runs only on the electric heat strips or backup heating source. It adds that the setting is there for one specific situation, when your heat pump breaks down, and that there is no outdoor temperature that should trigger you to manually turn on emergency heat.

HVAC.com aligns with this, recording that in emergency heat the compressor and heat pump shut down entirely and the backup heating system turns on, that it should only be used in emergency situations like the heat pump breaking down or icing over, and that it may need to run for hours or days depending on your repair schedule.

So the rule from the sources is narrow. Auxiliary is automatic and normal in cold weather. Emergency is manual and meant for a broken or iced-over heat pump, not for merely cold days. Using emergency heat as a comfort booster on a cold night runs the most expensive element continuously with the efficient compressor switched off.

The Cost Impact Of Running Heat Strips

Electric heat strips cost more to run than the heat pump, and the sources put numbers on it. TeamEnoch records that emergency heat typically costs 2 to 3 times more to operate than your regular heat pump system, and that emergency heat strips use anywhere from 5,000 to 20,000 watts of electricity depending on system size, compared with a heat pump that typically uses only 1,500 to 5,000 watts to produce the same amount of heat.

HVAC.com illustrates the gap with a one-week example: a normal heat pump at 30 degrees costing about 32.76 dollars versus emergency heat at 10 degrees costing about 196.56 dollars. GVEC reinforces the direction of the effect, recording that heat strips are much less energy efficient than a heat pump and that switching to emergency mode forces the system to rely 100 percent on the heat strips, which could make the electric bill skyrocket.

Read these as illustrations, not as a personalized bill. The wattage bands and the weekly figures show why running the strips as the primary source is expensive, but your actual cost depends on local electricity rates, system size, and runtime. The consistent conclusion is that heat strips are the costly fallback, and minimizing their runtime is the point of a working heat pump.

Practical Habits That Keep Strips Off

Because the strips are the expensive part, the sources point to habits that avoid waking them unnecessarily. GVEC recommends raising the thermostat in two-degree increments, noting that more than two degrees at a time may make the system fall back on the heat strips, and it suggests keeping the thermostat at 68 degrees or as low as you can stand. Trane makes the same mechanism explicit, listing a jump of more than 3 to 4 degrees as one of the triggers for auxiliary heat.

The second habit is to leave the mode selector alone. Because emergency heat is manual, the way to avoid its cost is simply not to switch it on unless the heat pump has actually failed. TeamEnoch is direct that no outdoor temperature should prompt you to flip to emergency heat manually.

The third is to read AUX correctly. Brief AUX during a cold snap or defrost is expected. Persistent AUX in mild weather, per Trane, is the signal to investigate rather than to accept, because a heat pump that leans on its strips too often will spend more than it should. None of this replaces a technician's diagnosis; it keeps the costly backup in its proper supporting role.