If you take a look at your thermostat right now, it is likely that there are settings in there that you are unfamiliar with. You might have noticed that there are settings for auxiliary heat vs emergency heat. Most homeowners have encountered these settings and have likely used each at one point but may not actually be all that familiar with them. Here’s a closer look at auxiliary heat and emergency heat.

Auxiliary Heating vs Emergency Heating On A Heat Pump

The terms ‘auxiliary’ and ’emergency’ are labels used by manufacturers on their heat pumps. There is virtually no difference between the two in that they are both settings that provide extra heat. 

Heat pumps are composed of an outside unit (heat pump) and an indoor unit (auxiliary heating system). In relatively warm weather, the heat pump can extract enough heat from inside to keep indoor temperatures comfortable. However, in conditions such as freezing weather, when the cycle is reversed, the heat pump will fail to do so. This is where the auxiliary part of the system will come in. The emergency heat function, on the other hand, is a manual function that can be used if there is a problem with the heat pump.

What Is Auxiliary Heat?

Auxiliary heat refers to the setting that becomes activated automatically when there is insufficient heat outdoors for the heat pump to use to keep temperatures indoors at a comfortable level. When using this mode, your heat pump will keep pulling in as much heat as it can but it also extracts heat using a secondary source to ensure that the desired temperature is reached. The heat pump does this through its built-in electric heating coils. 

With auxiliary heat, the system simply builds up to increase the temperature indoors. If outdoor temperatures increase, the system will no longer need the secondary heat source. When this happens, the heat pump simply turns the auxiliary heat setting automatically.

What Is Emergency Heat?

The emergency heat mode is only used during emergencies, hence the name. This setting is turned on manually during situations such as a system malfunction. When a malfunction occurs, the homeowner usually does not have enough time to call a technician for repairs and will still need the heat pump to continue generating heat. During the interim period, the emergency heat mode can make up for the deficiency. This mode is the secondary heat source of the system, so the heat pump can continue to produce the desired temperature.

When To Use Auxiliary Or Emergency Heat

The auxiliary function is meant as a backup to the heat pump in instances when the thermostat is registering a difference of temperature that is at least 3 degrees between the actual temperature and the set temperature. At this point, the conditions indoors will be uncomfortable since the temperature will be lower. The auxiliary function will trigger the heat pump to generate more heat. The emergency heat should be used if there is a problem with the function of the heat pump. This is a manual function, which means that it can be turned off once a technician is present to fix the problem.

Heat Pump Can’t Produce Enough Heat

feeling chilly due to heat pump not producing enough heat

There are times when the heat pump just simply fails to generate enough heat to keep the home warm. This happens when the heat pump cannot produce enough warmth to reach the set temperature. The extra amount of energy is sourced from the auxiliary heat mode to make up for this type of discrepancy. When the auxiliary heat mode is turned on, the system has activated the electric resistance heating function of the heat pump.

Your System is in “Defrost Mode”

Sometimes, ice builds up on the outdoor unit as a result of low temperatures. The system will then switch to Defrost Mode to melt this buildup. In freezing weather, the system will push hot refrigerant to the outdoor heat pump to melt the ice. When the heat pump is running on defrost mode, it will cease heating the house so the auxiliary mode will take over to ensure that your home is heated. 

Signs Your Heat Pump is in Defrost Mode

When your heat pump is running on Defrost Mode, it will:

  • produce water or steam from the outdoor heat pump
  • the outdoor unit fan will stop running
  • turn on the blinking light indicator, although this can only be found on some models

How to Stop Auxiliary Heat From Coming On

The problem with auxiliary heat mode is that at extremely low temperatures, the heat pump system will have to push itself to the limit just to generate sufficient heat. You do not want this to happen. To avoid problems that might develop as a result of this type of mechanical/electrical exertion, there are certain changes you might want to implement in your home to stop the auxiliary heat mode from turning on. Here are ways how:

Lower The Set Heat Temperature

lowering the heat temperature on heat pump

In many cases, when the indoor thermostat is set at a temperature that is too high, the auxiliary heat will turn on to compensate for what the heat pump cannot generate. This puts unnecessary stress on your HVAC system. To avoid this, set the temperature at 62 to 68 degrees but no more. This will help keep your home warm enough but keep your heat pump at an even function.

Make Your Home More Comfortable

Help out your heating system by making your home a bit warmer during winter. In the daytime, keep window shades open or curtains drawn to allow light and heat in. Make sure windows and doors leading outside are locked and closed. If you want to keep warm, try using thick or layered clothing instead of cranking the heating up.

Shut Off Unused Rooms

If your system can be turned off in specific areas, then do so. For example, if there are rooms in the house that are unoccupied, turn off the heating in these areas. Keep the doors closed or close the vents. This will allow the heating system to redistribute warm air to other areas in your home. That way, the occupied parts of the house remain warm and comfortable.

Thermostat Constantly on Aux Heat Setting

Sometimes in the middle of winter, you find your thermostat on the auxiliary setting for long periods of time. This is likely because the HVAC system simply cannot generate enough heat to meet the demand in your home. If this is the case in your system, get in touch with a professional HVAC technician and have your system checked. It is likely that there is a problem with the system or you may have an issue with sizing or capacity.

Schedule Seasonal Heat Pump Tune-Up

hvac contractor performing heat pump tune-up

Never skimp on your annual HVAC maintenance checkup. A well-maintained system is a reliable system and is more likely to have a longer lifespan. Ideally, an inspection should be scheduled before the winter season so any issues are fixed and any damaged part or component is repaired or replaced. This is also the best time to have the system serviced so it can be cleaned and have the air filters replaced. If there are any repairs required, these can be done before the winter comes.


The heat pump works the hardest during winter. Although it is built with features and functions that ensure its reliability even at extreme temperatures, it may not always work at its best if too much stress is placed on the system as a result of demand. A few careful steps are usually all you need to ensure that your heat pump is ready for winter.