Did you recently wake up on a freezing cold morning to pour yourself a hot cup of coffee, only to walk by your digital thermostat and see the words “Aux Heat” flashing on the read-out? This time of year, heat pump auxiliary heat settings kicking into action happen frequently.
But anything that is flashing on a screen can’t mean something good, right? Well, it can be a good thing for your home and family in the right situation.
The short answer: Aux heat on your thermostat means your heat pump’s backup heat is running. When your heat pump can’t keep up with the temperature you’ve set, or while it’s in a defrost cycle, your thermostat turns on a secondary heat source, usually electric resistance heat strips, to help warm your home.
In this article, we’ll help answer these questions you’re probably asking yourself:
What does auxiliary heat mean?
Is it bad to use the auxiliary heating setting?
What is the difference between aux heat and emergency heat?
How can I stop my auxiliary heat from turning on?
What Does Aux Heat Mean?
Auxiliary heat on your thermostat refers to your back-up heating system within your heat pump, otherwise known as electric resistance heating. Your thermostat inside your house is actually what determines when the auxiliary heat gets turned on.
For periods of time, the aux heat indicator on your thermostat will light up or begin flashing, depending on if your home uses a Honeywell thermostat, Nest thermostat, or another type of smart thermostat, letting you know when this happens.
This is because when the temperature inside your home drops below the goal temperature you’ve set the thermostat to, your thermostat energizes auxiliary heat. This commonly takes place once the indoor temperature on your thermostat reaches two to three degrees colder than the set indoor climate temperature. Aux heat will turn off once your home reaches the thermostat set point.
There are two primary reasons that can cause this setting to energize:
Heat Pump Can’t Produce Enough Heat
Sometimes we just need an extra boost of energy to help us reach our goals. That’s exactly what the auxiliary heating setting does for your home’s heat pump system.
Basically, if your heat pump is struggling to maintain its goal temperature, your aux heat will turn on and give your heat pump a boost. According to the U.S. Department of Energy, heat pumps typically rely on electric resistance elements to make up for gaps in heating capacity, so aux heat kicks on when your heat pump can’t deliver enough warmth to heat your home.
Typically, your heat pump will energize the auxiliary heat strip inside your secondary heating source to help reach its set temperature faster. If you see the indicator is on, it means your heat pump’s electric resistance heating is activated.
Your System Is in “Defrost Mode”
Are you seeing ice form on your heat pump? If ice starts to build on your heat pump’s outdoor 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. When winter temperatures reach freezing temperatures, the outdoor coils can have ice form around them. When that happens, your heat pump will automatically reverse the flow of the refrigerant so hot refrigerant is delivered to the outdoor unit to help melt the ice away.
Your heat pump stops heating your home during this defrost cycle, causing some models to automatically switch to aux mode until the defrost cycle is over.
Signs Your Heat Pump Is in Defrost Mode
Steam and/or water is seen coming from the outdoor unit
The fan in the outdoor unit is not running
A blinking light on the unit itself to indicate it’s in defrost mode, in certain models
Auxiliary Heat vs. Emergency Heat
So then what’s the difference then between auxiliary heat and emergency heat?
During freezing weather, the outdoor heat pump will be too cold to quickly heat your home, which is when the auxiliary heating system would kick on. Auxiliary heating turns on automatically to help heat your home more quickly if the temperature drops suddenly.
Heat pumps require a supplemental heating source. This is especially critical when the temperature drops below 40 degrees Fahrenheit. These supplemental heating sources are things such as the previously mentioned electric resistance heating at the indoor unit. It can also include gas, oil, or even hot-water back-up systems too.
These supplemental heating sources are known as “second-stage” or “back-up” heating. The “first stage” heating is the heat pump itself. Emergency heat is when you use your “second-stage” by itself without using your “first stage”.
Explaining Emergency Heat
So when should you use your Emergency heat? In emergency situations only!
Emergency heat is best used when there’s something wrong with your “first stage” heat. Let’s say your home is not reaching the set warm temperature and it’s because something has happened to your outdoor unit. You would then turn on the emergency heat and call for service.
The emergency heat setting has to be manually switched on and should only be used in temperatures below 30 degrees. When turned on, the heat pump shuts off entirely, allowing you to receive heat without damaging your outdoor heat pump system. This should only be run when absolutely necessary to avoid damage to your central heating system.
Having emergency heat on can, and will be, more expensive to run. This is why it is only used in emergency situations.
How to Stop Auxiliary Heat From Coming On
It may seem like it isn’t much you can do when your Honeywell thermostat says the auxiliary heat is on or any other model thermostat. If the temperatures are extremely frigid outside, even this may be too much for your unit. You may need to make some additional changes to your home to keep the auxiliary heat from coming on.
Learning how to stop auxiliary heat from coming on can actually be quite simple, and help you save on your heating bills this winter.
Lower the Set Heat Temperature
Often, aux heat comes on because the thermostat is set higher than your heat pump can comfortably reach on its own. A higher set point asks your heating system to work harder to keep up. Setting your thermostat somewhere between 62 and 68 degrees may help keep aux heat from coming on as often.
Make Your Home More Comfortable
Another effective way you can keep the aux heat from running is to create a cozy, warmer atmosphere inside your home. Open your window shades in the day and let the sun warm rooms whenever possible. Remember you can always layer up wearing thick clothes and snuggling up with a cozy blanket.
Thermostat Constantly on Aux Heat Setting
In the dead of winter, or even on a milder winter day, some people find their thermostat is stuck on aux heat. As we’ve talked about before, this is because their HVAC system simply can’t keep up with a reasonable demand to heat the home. This is a serious issue that will likely need to be seen by an experienced HVAC technician.
Schedule Seasonal Heating Tune-Up
Before the winter season really begins, it is best to schedule a time to have your heating system inspected and tuned up. Early fall is a great time to make sure that all of your moving components are in working order, air filters are changed, and that your heat source itself does not need any major repairs.
Look to Your Thermostat
If you got your heating maintenance performed back in the fall and aux heating is still staying on, your thermostat could be the culprit. Newer thermostats may cause the auxiliary heat to come on more quickly than older ones. If the temperature does not pick up within the first few minutes, many of the new models will prompt the aux heat setting.
If you have an older thermostat and are suddenly experiencing aux heat overload, it may have to do with the specific model or time to have an HVAC contractor look at your heat pump. The issue could lie with your heating system.
Frequently Asked Questions
Is It Bad If Auxiliary Heat Comes On?
Not usually. Aux heat coming on during cold weather or a defrost cycle is a normal part of how your heat pump works. It’s worth a closer look if it runs constantly, turns on in mild weather, or your home still can’t reach the set temperature. In those cases, schedule a heating system inspection.
How Long Should Auxiliary Heat Stay On?
Aux heat should shut off once your home reaches the thermostat set point or the defrost cycle ends. Short bursts on cold mornings are typical. If the aux heat indicator stays lit for long stretches or all day, your heat pump may not be keeping up, and an experienced HVAC technician can help find the cause.
Does Auxiliary Heat Run Up Your Energy Bill?
It can. The U.S. Department of Energy notes that the electric resistance elements used for backup heat are inefficient, so their use should be kept to a minimum. Avoiding large, sudden thermostat increases and keeping your heat pump maintained may help reduce how often aux heat runs and help keep heating costs in check.
Why Is My Aux Heat Running When It’s Not That Cold Outside?
If aux heat comes on in mild weather, check for a dirty air filter, ice or debris around the outdoor unit, and your thermostat settings. Raising the temperature several degrees at once can also trigger it. If none of these explain it, your heat pump may need repair, so it’s a good time to schedule service.
