Heat Pump Aux Heat & Emergency Heat

If you’ve got a heat pump, then you may have noticed that your thermostat has a couple of extra settings on it: auxiliary heat and emergency heat. Their names do give some hints about their functions, but it’s hard to tell exactly what they do just from looking at them.


However, if you do ever get into a situation where you need to use those settings, you definitely want to know exactly what each one does so that you don’t pick the wrong options.


Difference between aux heat & emergency heat

Auxiliary heat is actually pretty straightforward. With most heat pumps, you’ve got two separate units: one inside the house and the other outside. The outside unit is what people normally mean when they refer to the heat pump, with the inside unit being the auxiliary unit.


There are a number of reasons why the auxiliary unit might turn on, ranging from needing to defrost the outdoor unit to needing to work together with the outdoor unit to overcome a big sudden drop in temperature inside the house. These are generally done automatically.


To contrast, the emergency heating function usually requires manual starting. When the emergency heating mode is engaged, the outside heat pump is turned off in order to prevent damage from severely cold weather. The inside unit is then used to power the home, resulting in less efficient heating, but also heating that doesn’t endanger the heating system itself.


However, that’s really just the tip of the iceberg. To really understand when and why you should use each option, you first need to understand a bit more about how heat pumps work.


How do heat pumps work?

The basic concept of a heat pump is pretty straightforward: it’s both a heater and an air conditioner. It can essentially be reversed, allowing the same appliance to both heat and cool your home, depending on the conditions outside. But how exactly does it accomplish this reversing?


As it turns out, the mechanism is pretty simple. All it takes is a literal reversing of the flow of refrigerant and your heat pump can reverse the flow of heat into and out of your home. When it’s cooling your home, the external unit gets really hot.


Conversely, when it’s heating your home, the external heat pump gets extremely cold. This last point is particularly important when the weather outside is cold enough to be near or below freezing.


What happens when it gets too cold outside?

Heat pumps aren’t indestructible. When you consider that the temperature outside is freezing and that the heat pump is even colder than that (because not only is it exposed in that weather, but it’s extra cold because it’s pushing heat into your home), then it’s a wonder that heat pumps work at all.


The trick is that heat pumps need to defrost themselves from time to time. To do this, they can effectively reverse the flow of freon refrigerant for a brief period (much like they would if they were cooling your house and drawing heat to themselves).


Of course, this raises the obvious question of what happens to you and the interior of your house if your external heat pump decides it needs to heat itself up? The answer lies in radiation.


How do heat pumps use radiation?

Radiation may be an intimidating word but it’s actually harmless in the context of heating. You see, there are three ways that heat can be transferred: convection, conduction, and radiation.


  • Convection is when hot air makes things warmer. This is basically how your heater normally works when hot air blows out the vents into each room.
  • Conduction is when hot things touch other hot things. This is the main component of heating something up on the stove. Conductive metal pots and pans transfer heat directly to your food or water.
  • Radiation is when heat is transferred directly to objects without either directly touching. This may sound crazy, but just think about how the sun heats your skin outside. It’s not touching you and there isn’t air all the way from you to the sun, so it has to be something else and that something else is radiation.


Going back to your home, you’ve probably got some radiant strips throughout your home, designed specifically for this situation. When it’s too cold out for your heat pump to function, the radiant strips turn on and heat your home directly.


If you’re wondering why your home doesn’t just use these all the time, it’s for the simple reason that they’re less efficient than the heat pump in most cases. It’s only when the temperature outside gets incredibly low that they become a safer and more efficient option than the external heat pump.


How does auxiliary heating work?

When the radiant heating strips are used to temporarily supplement the heat pump outside, that’s auxiliary heating. This can be a matter of boosting the heating power in order to close a big gap between the real temperature and the temperature on the thermostat.


After the temperature is close enough to not be a big deal, the radiant strips turn off and let the heat pump handle the fine tuning.


As was mentioned earlier, aux heating is also used when the heat pump outside is capable of running general, but needs a quick break to heat itself back up. While that heat pump is defrosting itself, the radiant strips inside ensure that the temperature doesn’t drop too dramatically, which would force the heat pump outside to work even harder.


How does emergency heating work?

To contrast, emergency heating is a setting you turn on when the temperature outside is too cold for even that. If merely defrosting won’t get your pump back into the safe temperature range, then you will need to power it down and leave the heating of your entire home to the radiant strips.