Thermostat Says Auxiliary Heat On – What to Do?

Disclosure: We may get commissions for purchases made through links in this post.

In a majority of heating systems, thermostats are installed to control temperature. You may have noticed lately that your thermostat indicates that the auxiliary heat is on, and you are concerned about how this may affect your electricity bill or heating system. We asked HVAC experts what to do in this situation; here's their answer. 

Check that the thermostat is not set higher than usual, but you may need to call an expert if the auxiliary heat is on when the day is warmer. One of the reasons your auxiliary heat will come on is because your heat pump cannot draw in enough heat from the outside. As a result, the auxiliary heat is activated to assist the heat pump in reaching the thermostat-set temperature.

There are many other reasons why your auxiliary heat turns on. Keep reading as we explore more on auxiliary heat and what to do the next time it comes on.

Homeowner changing the thermostat level using an iPad, Thermostat Says Auxiliary Heat On - What to Do?

What is Auxiliary Heat? 

Auxiliary heat is your heat pump's backup heating source. Within the time range of 5 to 20 minutes, if your thermostat does not reach the target temperature, its auxiliary heat is automatically activated.

Setting the Nest thermostat to 74 degrees using the iPad

It is possible to manually or automatically set your thermostat. So, if your heat pump is having difficulty maintaining its set temperature, your auxiliary heat will kick in to boost it. 

Why is Auxiliary Heat 'On' in Thermostat? 

The word 'AUX' will light up on the thermostat when the auxiliary heat is turned on. Some factors that may cause your auxiliary heat to turn on are: 

Cold Weather

It should not come as a surprise if your auxiliary heat turns on during colder weather, especially if the temperature drops beneath 40 degrees Fahrenheit. At this temperature, heat pumps lose some efficiency causing problems due to their inability to extract warm air from outside to keep the house warm.

The auxiliary heat serves as a support to help the heat pump achieve the temperature set by the thermostat. When the set temperature is reached, they both go off.

Defrost Mode

It is common for heat pumps to freeze. However, if the ice is too much, it limits its ability to work effectively; as a result, your heat pump activates a defrost control that monitors ice formation on the outside unit.

If the defrost control detects that the pump has been cold for an extended period, it instructs the reverse valve to deliver hot refrigerant to thaw the coil in the outdoor unit. The auxiliary heat steps in when in defrost mode, thereby keeping the home warm.

Faulty Compressor

Part of the heat pump includes the compressor, which converts refrigerant in a gaseous form to liquid. When the compressor breaks down, the heat pump cannot transfer heat and will cause the auxiliary heat to come on.

Insufficient Refrigerant

When the refrigerant level is low, various issues can arise, such as insufficient heating or cooling. The heat pump may be unable to absorb heat if the refrigerant is low and have to rely on the auxiliary heat.

What to do When the Auxiliary Heat of the Thermostat is On

Hand turning a home thermostat knob to set temperature on energy saving mode

It is almost impossible to prevent your auxiliary heat from coming on when you live in a cold region. It plays an essential part in ensuring your comfort, especially when the temperature drops and the heat pump is not efficient. Here's what to do when it comes on. 

Lower The Temperature of the Thermostat 

When the temperature inside your home is too high, it causes the auxiliary heat to come on to compliment the heat pump. You can counteract this by setting the thermostat between 60 and 68 degrees Fahrenheit to lower the temperature.

Prevent Malfunction by Regular Maintenance

It is better to avoid cold nights and outrageous electric bills through constant maintenance. An HVAC professional will inspect and ensure that the heating parts are in good condition and functional.

They can go as far as cleaning and adjusting parts that require it to ensure optimal performance. Maintenance is the most effective way to reduce any need for extra or additional heat.

How Does Heat Pump Work? 

An AC unit mounted on a concrete slab

Heat pumps move heat from one point to another to heat or cool an environment. Extracting heat from outside during the cold season warms up a home. The heat pump transfers heat from inside your home to the outside to keep you cool during summer.  

As two units, the heat pump has one outside and one inside the house. The outside unit is a heat pump, while the inside unit is the auxiliary system. The outdoor unit will be too cold to quickly heat your home during cold weather so the auxiliary system will be activated.

Heat pumps are an efficient form of heating and cooling because they transfer heat rather than produce it, as a furnace would.

What is Emergency Heat? 

Emergency heat or EM heat is a setting responsible for switching on a secondary heat source when the heat pump is faulty or in defrost mode. In colder regions, a secondary heat source is required to ensure that you receive adequate heating.  

The heat pump shuts down when the emergency heat is turned on. This prevents damage caused to the central heater. It is a less efficient way to warm up the home, so it should only be used when necessary.

How Often Should AUX Come On?

Auxiliary heat should switch on for the duration it takes your home to reach the thermostat-set temperature. It should be evaluated if your system's auxiliary heat runs each time you turn it on or runs for an unusual time (more than 30 minutes daily).

On days where there is a significant amount of heat during the day and cold at night, the auxiliary heat will only run for a short while during the coldest hours of the morning.

However, if the region is such that it is cold both day and night, the auxiliary heat will be activated more frequently to meet demand. The frequency at which your aux comes on will also vary depending on system features and how they are configured. 

Are AUX Heat and EM Heat The Same?

Though not found in every thermostat, if you take a good look at your thermostat, you may notice two heating features: auxiliary heat and emergency heat. It is crucial to understand the difference as they are not the same but are sometimes mistaken to be.

While Auxiliary heat will automatically come on to support the heat pump, you will have to power on the Emergency heat manually to serve as a primary heat source if your heat pump fails. Both are intended to work in extremely cold conditions, and choosing the incorrect one could be detrimental to your home and heater.

Why is My Auxiliary Heat Blowing Cold Air? 

A thermostat set to 78 degrees mounted on a white wall

It can be disappointing to discover that your auxiliary heat is blowing cold air. In most models, the blower comes on first, and then cool air can blow in before it gets warmed up. But if it persists, something is wrong. Some potential reasons that can cause your auxiliary heat to blow cold air are:

Wrong Thermostat Setting

If your auxiliary heater is blowing cold air, you should double-check your thermostat settings. Modern thermostats are programmable and have a variety of settings that can be used to ensure that the system is set on heat and that the proper temperature is selected. If the settings have been confirmed, you may need to recalibrate or replace the thermostat. 

Overheating Due to Clogging

A dirty filter is one of the many things that can cause the furnace to overheat. A limit switch, a safety device installed in the furnace, shuts down the burner when this occurs. 

Even if you replace the filter with a new one, the dirt can still be within the system and eventually cause overheating.

Faulty Heat Pump

Heat pumps, by design, produce colder air when compared with electric or gas furnaces. The pumps use heat strips in the auxiliary unit in extremely cold conditions.

If there is a problem with these strips, the heat pump will begin to blow cold. In addition, if refrigerant levels are low in warmer seasons, the heat pump will not transfer heat properly.

Damaged Valve

A damaged reversing valve or a valve clogged with debris and dirt is unlikely to provide hot water for your heater during winter. If the reverse valves are faulty, your auxiliary heat may not come on at all or will blow cold air. It must be operating efficiently to supply hot water to the outdoor unit during the winter. 

In Conclusion

Homeowner changing the thermostat level using an iPad

Auxiliary heat is essential in keeping you and your home warm. However, when it runs for a long time, it may indicate a problem with your primary heat source. Do not overlook the importance of routine maintenance.

To read more related topics on thermostats, check out these posts: 

How Long Do Batteries Last in a Thermostat? 

Will a Bad Thermostat Cause AC to Freeze Up? 

Leave a Reply