A heat pump is a very efficient HVAC system. Unlike a central air conditioner which can only cool the house, a heat pump cools and heats the house.
Heat pump’s thermostats have 2 heat settings that confuse most homeowners. These are auxiliary heating and emergency heating. So, what is the difference between auxiliary heating and emergency heating?
Auxiliary heating turns on automatically and for shorter durations when temperature fall below 35 degrees and the heat pump cannot extract enough heat from the surrounding. Emergency heating must be turned on manually and should only be used during emergencies (when the heat pump fails).
Auxiliary heating works hand in hand with the heat pump for supplemental heating when the temperature drops suddenly. However, when emergency heating is activated, the heat pump is bypassed meaning that heating will be supplied 100% from the secondary heat source until it is turned off.
Both auxiliary heating and emergency heating draw heat from the same source though. The heat pump has a secondary source of heat, which is in most cases in the form of electrical resistance heating.
The secondary source of heat is however not very efficient and that is why the system is expensive to run on both auxiliary heating and emergency heating.
Auxiliary heating usually comes on when the indoor temperature falls by about 2 to 3 degrees of the thermostat setting. It will also come on when the heat pump is in “defrost mode”. If the system is faulty, the auxiliary heating will be on throughout.
It is not bad when the auxiliary heat comes on. It is supposed to come on automatically when the heat pump cannot extract enough heat from the outdoor air.
You should however be concerned if the auxiliary heat is always on. That could be caused by a problem with compressor, reversing valve, compressor fan motor, defrost control board or low level or refrigerant.
To understand the differences between auxiliary heat and emergency heat even better, let us briefly look at how heat pumps work.
How a Heat Pump Works
When used to cool the house, a heat pump works very much like a central air conditioner. As a matter of fact, the 2 systems are so much alike that it is difficult to tell them apart unless you know where to look.
A heat pump, just like a central air conditioner has 3 main components. These are the compressor, condenser coil and evaporator coil. The 3 are connected together using copper tubes.
A refrigerant (fluid with low boiling point) is circulated through these components, which is actually what aids in heat exchange. During the heat exchange process, the refrigerant turns from gas to liquid and back to gas again.
When the heat pump is used to cool the house, the refrigerant moves from the compressor to the condenser coil and then to the evaporator coil. The cold refrigerant enters the evaporator coil inside the house where it absorbs heat from the indoor air and in the process cools it.
As the refrigerant leaves the evaporator, it will have changed from liquid to gas. The gas then enters the compressor where its pressure is increased.
As is the law with gases, an increase in pressure results in an increase in temperature. The refrigerant therefore leaves the compressor and enters the condenser coil as a high-pressure superheated gas.
The heat absorbed from the indoor air is dissipated to the surrounding inside the condenser coil with the help of a fan. By the time the refrigerant is leaving the condenser, it will have changed to its liquid state and the cycle goes on and on.
When the heat pump is used to heat the house, the cycle is reversed. The refrigerant flows from the compressor to the evaporator coil and then to the condenser coil.
Instead of the superheated refrigerant gas flowing to the condenser coil, it flows to the evaporator coil where it heats the indoor air. Inside the condenser unit, the refrigerant draws heat from the surrounding due to the temperature differential created by the then very cold refrigerant.
In thermodynamics, heat is transferred from a point of high concentration to a point of low concentration.
As such, lowering the temperature of the refrigerant means that it is possible for heat to be transferred from the outside (despite it being cold) to the refrigerant which is then used the heat to warm your house.
Auxiliary Heat vs Emergency Heat
Although a heat pump will heat your house even during winter, its capability suffers a great deal when the temperature dips below 35 (sometimes 40) degrees Fahrenheit. For that reason, a heat pump needs a secondary source of heating otherwise your house will not be adequately heated.
That is usually in the form of electrical resistance heating. Some heat pumps however have natural gas (in a furnace) as their secondary source of heating.
When the temperature outside your house falls suddenly (below 35 degrees), the heat pump will extract enough heat from the surrounding air. Immediately the indoor temperature drops by about 3 degrees of the thermostat setting, the auxiliary heat will come on automatically.
When then auxiliary heat is on, it means that the secondary heat source has been activated to complement the heat pump, meaning that the 2 are working together and that the aux heat will be turned off as soon as the heat pump can extract sufficient heat from the surrounding.
On the other hand, emergency heat or EM heat as is commonly known will not come on until you manually turn it on at the thermostat. As its name implies, emergency heat should only be used for emergencies and not any time you feel like you need an instant heat boost.
You should only turn on the emergency heat if the heat pump has failed. This is often caused by a bad compressor, frozen heat pump or when an object (like a tree branch) has crashed on the condenser unit.
As soon as you turn on EM heat on your thermostat, call in an HVAC technician to fix the issue as soon as possible.
Different from auxiliary heating, when the emergency heat is turned on, the heat pump shuts off. EM heat and the heat pump cannot run concurrently. By activating EM heat you are telling the system to bypass the heat pump.
As such, all of the heating will be provided by the secondary heat source. To deactivate EM heat you also need to turn it off manually.
Is Auxiliary Heat or Emergency More Expensive?
Is auxiliary heat or emergency more expensive as opposed to when the heat pump is running normally? It depends!
If your heat pump uses electrical resistance heating as its secondary heating source then the answer is a definite YES! The long the auxiliary heat or emergency heat is on, the higher your electricity bill is going to be.
It however is not always the case if your heat pump has natural gas as its secondary heating source. If the cost of natural gas is at an all-time low, the cost of running your system on aux or EM heating will not be that high.
At What Temperature Should Auxiliary Heat Come On?
Heat pumps starts to struggle heat extraction at temperatures of below 40 degrees Fahrenheit. For most heat pumps, the auxiliary heat should come on at 35 to 40 degrees Fahrenheit.
The temperature at which the auxiliary heat comes on also depends your thermostat setting. If your thermostat is set high, the auxiliary heat will come on sooner than in other systems.
Usually, the recommended thermostat setting is between 68 and 72 degrees.
How Long Should Auxiliary Heat Run?
The auxiliary heat should run for as long as it takes to sufficiently heat your home. As such, it should not be on all the time.
As I mentioned earlier, unlike the emergency heat, auxiliary heat is designed to complement the heat pump and once that is done it should turn off automatically.
If your auxiliary heat is therefore on all the time, it is highly likely you have a problem with your system that needs to be fixed by a licensed technician.
Having said that, it is good to remember that if the temperature outside is really low (during extreme winter), your auxiliary heat will be on for long periods of time. At those temperatures the heat pump simply can’t cope with your heating needs.
Why is My Auxiliary Heat always on?
If your auxiliary heat is always on, you most likely have a problem with your heat pump. One of those problems could be low levels of refrigerant.
If the system does not have enough refrigerant, maximum heat transfer will not take place and as a result auxiliary heating will need to come on.
Another problem could be a bad compressor. Due to wear and tear among other problems, a compressor can fail to work as intended, meaning that the refrigerant will be unable to extract enough heat from the surrounding.
A failed compressor fan motor also means that heat extraction between the condenser coil and the outdoor air will be inhibited. As such, the aux heat will need to come on otherwise you will freeze.
The reversing valve can also fail so that instead of working in the heating mode it work in the cooling mode. When that happens, you will notice that the auxiliary heat is on but the system will be blowing cold air.
In such a situation, both the auxiliary heating and heat pump will be competing to heat and cool the house respectively.
A heat pump has a defrost memory board which activates the reversing valve to reverse the direction of flow of the refrigerant so that the hot gas can melt the ice forming on the condenser coils. When the board malfunctions, the ice will not be melted and hence the aux heat will stay on until the problem is fixed.
When Should You Turn On Emergency Heat?
As its name suggests, emergency heat should only be turned on during emergencies. Heating emergencies.
One of such emergencies is when your heat pump fails completely. You know that your heat pump has failed when there is no hot air being circulated in the house and you are freezing.
Such can be caused by a failed compressor or sometimes when an object crashes over the condenser unit. Sometimes the entire outdoor unit can be covered by snow which can completely shut it off.
In such instances, you can turn on emergency heat but also contact an HVAC technician to come over and fix the heat pump.
And basically that is the difference between auxiliary heat and emergency heat. I hope that this guide was helpful.