If you use a heat pump for heating, your thermostat will usually have both an auxiliary heat and emergency heat mode. When your heating system runs in these modes, your thermostat will usually read “Aux” or “EmHeat.” If you’ve ever noticed that your heating system is running in one of these modes, you may be wondering exactly what it means. Both auxiliary and emergency heat indicate that your heat pump’s secondary or backup heat source is currently running, and this guide will explain why this happens and what purpose these heating modes serve.

How Backup Heating Works in a Heat Pump System

All heat pumps require a backup heat source, as there are some times when a heat pump will stop heating or not be able to heat quickly enough. Some homes have a gas or electric furnace as the backup heat source, but most heat pumps come with electric heat strips that are installed in the air handler compartment inside the home.

If a heat pump runs in temperatures below 40 degrees, frost will start to form on the outdoor coil and the unit will eventually freeze. Inside the heat pump is a sensor that will signal the heat pump to run in defrost mode when it detects that the coil is below a certain temperature. Whenever a heat pump needs to defrost, it reverses the refrigerant flow and temporarily runs in cooling mode. This means it pumps cold refrigerant inside and uses it to capture heat from the air in the home, and it then pumps hot refrigerant through the outdoor coil to melt the ice.

Without a backup heat source, the temperature in your home would quickly drop when your heat pump is defrosting since it is actively pulling heating out of your house. To prevent this, the thermostat will always automatically switch on the backup heat source any time the heat pump is running in defrost mode.

If the thermostat ever detects that your home is more than 3 to 5 degrees below the temperature it is set to, it will also switch on the heat strips or whatever other backup heat source you have. In this case, the heat pump will still keep running and pumping heat inside, but the backup source will help it to raise the temperature more quickly. Once the temperature gets to within 1 or 2 degrees of the thermostat setting, the backup heat source will turn off.

This situation normally only happens when the weather is below freezing, as this limits how much heat a heat pump can produce. If the backup heat didn’t switch on in this situation, it would often take well over an hour or two before the heat pump fully heated your home.

What Does It Mean When Your Thermostat Is in Auxiliary Heat Mode?

Any time your thermostat shows that it is in auxiliary heat mode, all it means is that the backup or auxiliary heat source is currently running. If the thermostat is in auxiliary mode and shows the temperature is more than a few degrees below the desired temperature, it means that both the backup heat source and the heat pump are running. In this case, the thermostat will automatically switch out of auxiliary mode and shut off the backup heat source as soon as your home is almost at the desired temperature.

If your thermostat shows “Aux” and registers that your home is at or near the desired temperature, it means that your heat pump is currently running in defrost mode. The thermostat will switch back to normal heating mode as soon as the defrost cycle completes. Depending on how cold it is outside, it typically only takes somewhere between five and 15 minutes for a heat pump to fully defrost and switch back to heating mode.

What Is the Purpose of Emergency Heat Mode?

While your thermostat will automatically switch to auxiliary heat mode as needed, you have to go into the settings and manually switch to emergency heat mode. If your thermostat ever shows it’s in emergency mode, this means that someone accidentally changed the settings.

When you switch to emergency heat mode, the thermostat will again automatically switch on the heat strips or furnace. The point of this emergency mode is to prevent your home from starting to get too cold if your heat pump isn’t working for any reason. The only time you should ever switch your thermostat to emergency heating is in an emergency when your heat pump stops working and you’re waiting to have it inspected and repaired.

The heat strips used as the backup in most heat pump systems are similar to the heating elements found in a toaster oven. While they produce quite a bit of heat, they also use more than three times as much energy as your heat pump. This is why you only ever want to switch to emergency mode in an actual emergency, as this will lead to your heating costs skyrocketing if the system runs in emergency mode for a long time.

What to Do If Your Thermostat Constantly Shows Aux Heat

Heat pumps generally only ever need to defrost when operating in temperatures under 40 degrees. Since winters in the Atlanta area tend to be mild, your heat pump won’t run in defrost mode very often and should usually defrost in less than 10 minutes. Similarly, the backup heat source and heat pump will typically only need to run at the same time on much colder days or if your home is much colder when you turn your heating system on. This means that your thermostat should only rarely show it’s in Aux mode and will usually only ever stay in this mode for a short time.

Your thermostat should never stay in Aux mode for more than 30 minutes to an hour except on extremely cold days. If it does, it indicates that there is some issue with your heat pump and you need to have it inspected. It may be that the heat pump either won’t turn on for some reason or isn’t working effectively enough to keep the temperature in your home from dropping. A thermostat can also sometimes malfunction and fail to turn off the auxiliary heat when it should.

Your thermostat will also constantly show Aux mode if your heat pump is frozen and can’t switch to defrost mode. In this case, you’ll usually see lots of ice built up on the heat pump. If your heat pump isn’t defrosting, it usually means that the reversing valve is broken or stuck, as this valve is what allows it to reverse the refrigerant flow and switch between modes.

Precision Heating & Air is a family-owned company, and we’ve been providing expert heating and cooling services to customers in Dallas and the Atlanta area for nearly 40 years. If you’re having any issues with your heat pump or thermostat, our experienced technicians can quickly get everything working again to ensure your home remains comfortable. We repair and service all models and brands of heat pumps as well as furnaces and air conditioners, and we specialize in HVAC installation. Contact us today to schedule a service call or if you have any questions.

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