My Heat Pump’s AUX Heat Keeps Coming On

2018 Jan 18
Posted in: Heating
why is my outside ac unit making a loud noise

So you’ve been running your heat pump in heat mode and you’ve noticed that your thermostat is reading “AUX heat” a lot. Is that normal?

Well, first off, AUX heat refers to your heat pump’s “backup” electric resistance heating. This heat setting should only kick on when the heat pump can’t produce enough heat to warm your home. Related reading: Why Does My Thermostat Say AUX Heat When I Turn On My Heat Pump?

That said, your heat pump’s AUX heat setting should only come on when:

  • Outdoor temperatures are below 35°
  • Your thermostat is calling for a 3° or higher temperature rise
  • The heat pump is currently in “defrost mode”

Bottom line? If these 3 situation don’t apply and your heat pump is in AUX heat mode, that means that your heat pump is:

  1. Malfunctioning and likely needs professional repair
  2. Running up a ridiculously high energy bill (AUX heat costs up to 50% more than heat pump heating)

Let’s take a closer look at how to determine when it’s normal for your heat pump to be in AUX heat mode.

Need a professional’s help? Just contact us and we’ll send a tech right over.

It’s normal for your heat pump to switch to AUX heat when...

1. The temperature outside is below 35°.

Usually, once outdoor temperatures fall below 35°, a heat pump will struggle to keep your home warm and will automatically switch to AUX heat.

That’s because heat pumps have one downfall: they lose efficiency in extremely cold weather

You see, heat pumps don’t create heat like a furnace does—instead, heat pumps absorb heat from the outdoor air and then move that heat indoors. And when outdoor temperatures drop too low, there’s simply not enough heat in the outdoor air to keep your home warm.

So before you call a professional, check the outdoor temperatures. If it’s an unusually cold Arizona day, it’s normal to see AUX heat on your thermostat.

2. The thermostat is calling for a 3° or higher temperature rise.

Most heat pump systems are designed to automatically switch to AUX heat when the indoor temperature is 3 degrees colder than the thermostat setting

For example, let’s say that you wake up to a cold home (62° F) and immediately raise the thermostat to 68°. Because the temperature rise is more than 3 degrees (it’s 6 degrees in this scenario), the system will automatically turn on the AUX heat to help your home meet the desired temperature faster.

So first, check to see if your thermostat only says AUX when you raise the thermostat 3 degrees or higher than the current temperature of your home. If so, that’s normal. But here’s the problem: it’s still expensive. So, if you want to cut down on your energy bills, try limiting how often you raise the thermostat more than 3°.

3. The heat pump is in “defrost mode”.

If ice starts to build on your heat pump’s outside unit, your heat pump will undergo a “defrost cycle” to melt it. During this time, the system will rely on AUX heat to keep your home warm.

Although this situation is extremely rare in Arizona, it’s common in colder parts of the country. When outdoor temperatures reach freezing temperatures, the outdoor coils will freeze over with ice. When that happens, the heat pump will reverse back into its cooling mode for a few minutes and dump heat from inside your home to the outdoor unit (to melt the ice).

And, because a heat pump stops heating your home during this process, most models will automatically switch to AUX mode until the defrost cycle is over.

Signs that your heat pump is in defrost mode include:

  • Steam and/or smoke coming from the outdoor unit
  • The fan in the outdoor unit has stopped running
  • A blinking light on the unit to indicate it’s in defrost mode (only on some heat pump models)

Is your heat pump in AUX mode when it shouldn’t be? Contact a Phoenix tech

If your heat pump is running in AUX mode when it shouldn’t be, just contact us.

We’ll take a look at your system to determine the problem and fix it to prevent high energy bills.

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