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Why is My Auxiliary Heat Coming On?

Updated: Feb 1, 2019

If you're a heat pump owner, then you've likely asked the dreaded question, "Why is my auxiliary heat coming on?" Your auxiliary heat is a component of your heat pump system often referred to as backup heat, emergency heat, heat strips, heat coils, or even supplementary heat. All of these things are referring to the same thing: the electric heating component within your air handler. There may be a perfectly normal reason why your auxiliary heat is coming on, but there could also be concern enough to call for service. Read on to learn more.

If you're a heat pump owner, then you've likely asked the dreaded question, "Why is my auxiliary heat coming on?"

What is Auxiliary Heat?


As we've already mentioned, auxiliary heat is the electric heat component of your heat pump system. Situated within your air handler (aka inside unit or furnace), your auxiliary heat serves a few primary purposes as well as some secondary purposes. In many cases, your thermostat will show something to the effect of "Auxiliary Heat" or "Aux Heat" or "Emergency Heat." In either case, your thermostat is letting you know that the auxiliary heat is coming on. There are some perfectly justifiable reasons for auxiliary heat to come on, so let's take a look.


1. Your Heat Pump is Running Longer


The first reason your auxiliary heat may be coming on is that your heat pump is running for an unusually long amount of time. If you've read our "What is a Heat Pump?" blog post from a few weeks ago, then you'll know that heat pumps won't exactly burn you out of the room with piping hot air from the vents. As a result, your heat pump (by design) will run longer than a comparable gas-fired furnace; however, what happens when your heat pump runs longer than it should?


That's where auxiliary heat comes in. By definition, auxiliary is "to provide supplementary or additional support." By that definition, your auxiliary heat is doing exactly what it's supposed to. By default, your thermostat will give the heat pump ample time to do its job, however, if the heat pump hasn't heated up your home quickly enough, auxiliary heat will come on. The extra (auxiliary) heat may be just enough to help your system heat up your house and reach your desired thermostat set point.


When is this normal? Your heat pump system, by design, will use auxiliary heat in this manner when the temperatures drop significantly outside. How well a heat pump heats is dependent on how much [relatively] warm air your condenser can pull across its coil outside. In extremely cold conditions, your heat pump simply needs a little help or support a la auxiliary heat.


2. Your Heat Pump is in Defrost Mode


Another reason your auxiliary heat may be coming on is that your heat pump is in defrost. In the event that your heat pump is working through colder-than-normal conditions, then there is likely to be a slight frost collect on your heat pump's coil. In those situations, a sensor on the coil will tell the heat pump's control board to engage a defrost cycle. Effectively, your heat pump flips back over into air conditioning in order to thaw out the frost accumulation on the condenser.

Another reason your auxiliary heat may be coming on is that your heat pump is in defrost.

When is this normal? Defrost cycles are a natural part of a heat pump's normal operation. Without defrost, heat pumps simply wouldn't work in cold, humid Texas winter conditions. Once a defrost cycle is engaged, your heat pump sends a signal to your air handler, thermostat, or both to turn on the auxiliary heat. This is only temporary and is only intended to help overcome the cold air your heat pump generates during defrost (after all, it's in air conditioning during defrost!) So don't worry, your heat pump is doing it's job.


3. Emergency Heat is On


Another reason your auxiliary heat is coming on could be that you (or someone in your home) has manually turned on the emergency heat on your thermostat. This can happen by accident or may be intentional in the event that your heat pump has stopped working entirely. Generally speaking, you thermostat will read "Emergency Heat" not "Auxiliary Heat," however, different thermostats operate differently and may have different functionality depending on how they are set up.


When is this normal? Emergency heat is only normal if it's been done so intentionally. Your emergency heat typically means that there is absolutely no call for you heat pump to operate at all. In other words, you'll be using only your electric heat and forsaking your heat pump entirely. We only recommend using emergency heat when your heat pump simply will not operate correctly.


When Should You Call for Heat Pump Service?


If your auxiliary heat comes on, remains on, and your house isn't heating up, it's probably time to call for service. For some tips on why your heat pump isn't heating, check out our blog post where we outline some possible causes and fixes. If none of those things seem to fix your problem or if your heat pump runs continuously without heating your home, then it's probably time to call Stephenville Heat and Air.


Our best-in-class technicians and installers are trained both in the classroom and in the field to comprehensively diagnose and repair your heat pump. No matter the task at hand, we can offer assistance to answer that pesky, "Why is my auxiliary heat coming on?" question. Give us a call at 254-965-4644 to schedule a service call.


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