If you’ve had a cold snap recently, you probably noticed a new symbol or the words “Aux Heat” or “Aux Heat Mode” flashing on your thermostat. Aux heat, short for auxiliary heat is a supplemental electric heating system built into many modern heat pump systems. Though it can indicate a maintenance issue, the aux heat indicator is usually a good thing. It means your system is working properly to ensure it maintains your comfort and doesn’t overtax the heat pump components.
Let’s dig into auxiliary heat a little more so you can understand its purpose and how it works.
What is Auxiliary Heat?
A typical auxiliary heating system includes a one or more electric heating elements built into your indoor air handler. These elements are controlled by your thermostat, which turns on the auxiliary heat when it detects a delay of five, ten, or twenty minutes, depending on your system’s settings in reaching your desired temperature.
A heat pump operates by pumping heat into or out of a home, transferring it from the outdoors to the indoors – or vice-versa when operating in cooling mode. This process is handled by coils, one inside and one out, and a compressor. These components work together to circulate refrigerant throughout the system, condensing it to a liquid for heat absorption and expanding it to a gas for heat dissipation. This process is called the refrigerant cycle, and it’s how all heat pumps do their job.
While heat pumps are very efficient machines, they are designed to work best in moderate climates where the winters are mild. When outdoor temperatures dip below about 40º, or thermostat settings are raised significantly, transferring enough heat to accommodate can become difficult, or even impossible for your heat pump to do on its own. That’s when the auxiliary heat system comes into play.
The supplemental heat added by the electric elements takes some of the burden off your heat pump allowing it to reach your comfort zones without over-stressing the equipment.
At this point, you’re probably asking yourself how efficient your heat pump is when the aux heat is active. Well, obviously electric heat is less energy efficient than the refrigerant-based process your heat pump relies on normally. But, in proper working order, your system should only engage the auxiliary heating system when it’s absolutely necessary – keeping electric bill bumps to a minimum.
My Heat Pump also has Emergency Heat. Is that the same as Auxiliary Heat?
The purpose of emergency heat is to temporarily take the place of your heat pump when there is a problem with the equipment. It should be engaged when your heat pump is unable to reach thermostat temperatures for an extended period. This usually indicates a mechanical issue, or low or leaking refrigerant. If you ever have to engage your emergency heat, you should ask a technician immediately to come inspect your system. Delays can cause further issues, and, if you run on your emergency heat for long, steep upticks in your electric bills.
The purpose of emergency heat is to temporarily take the place of your heat pump when there is a problem with the equipment. It should be engaged when your heat pump is unable to reach thermostat temperatures for an extended period. This usually indicates a mechanical issue, or low or leaking refrigerant. If you ever have to engage your emergency heat, you should ask a technician immediately to come inspect your system. Delays can cause further issues, and, if you run on your emergency heat for long, steep upticks in your electric bills.
Why Is Auxiliary Heat Necessary?
Auxiliary heat is built into most heat pumps as a precaution. As we mentioned, heat pumps operate best in moderate weather where temperature swings are minimal and winters mild. Having an aux heat unit as a part of your HVAC system gives your equipment the helping hand it needs to overcome unusual drops in temperature, and it gives you the peace of mind of knowing your family won’t have to weather any cold nights due to inadequate equipment.
When and Why Does Auxiliary Heat Turn On?
There are a few specific things that will cause your aux heat to come on.
Cold Weather
As we’ve already covered, the most common reason aux heat will be engaged is when outside temperatures drop below 40º. Heat pumps are superstars when it comes to cooling, and are quite efficient at handling moderately cold weather but extremely low temperatures can tax a heat pump causing it to take longer to reach desired temperatures and reducing its efficiency. The auxiliary heating elements work alongside the heat pump’s refrigerant cycle to keep up with those steep demands and make sure you and your family remain comfortable, no matter the outside temp.
Raising The Thermostat
All HVAC systems operate best when consistency and gradual shifts to thermostat settings are applied – such as the programmed transition from daytime temperatures to nighttime temperatures. When weather conditions are unpredictable or other factors push you to increase your ideal indoor temperature by more than 3 or 4 degrees, it can take a heat pump a while to compensate, leaving you uncomfortable longer. Auxiliary heat kicks on in these cases as well, reducing time-to-temperature, decreasing the workload put directly on your heat pump, and helping to maintain an optimum level of efficiency across the system.
Your System is in Defrost Mode
Outdoor HVAC equipment can accumulate snow and ice buildup during colder months – despite its name, a heat pump is no different. To help deal with this and prevent your outdoor coil from freezing, heat pumps are equipped with defrost control boards. These boards have sensors that monitor ice buildup on your condenser coil. When the coil has been too cold for too long, the computer board tells the system to engage the reversing valve, transferring hot refrigerant to the outside unit and thawing the coil. Doing this process, the auxiliary heat steps in to maintain indoor temps while the heat pump defrosts.
There is a Problem with Your Heat Pump
All of the reasons we’ve discussed so far represent normal processes and functions of the aux heat system that work to keep your heat pump in tip top shape. But the aux heat is also engaged when there is a mechanical malfunction or loss of refrigerant that is preventing your heat pump from doing its job. In these cases, a delay in reaching your set temperature will trigger the auxiliary heat as with any other reason. The difference here is that the underlying issue may prevent the heat pump from catching up, which is what allows the aux heat to go back offline. If you notice the auxiliary heat is engaged for an extended period of time – more than about 30 minutes – we advise you engage your emergency heat if you have the option, and reach out to a local HVAC professional to come have a look at your system.
Some common malfunctions that will cause your aux heat to stay on are:
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- A Faulty Condenser Fan Motor. Without consistent air flow across the coil, the condenser unit will have trouble absorbing enough heat to meet your thermostat’s requests. While this obviously applied to broken fan motors, it can also be caused by dirt and debris, or ice buildup that can slow the fan or make startup difficult. With these issues, you’ll see a drop in your system’s efficiency accompanied by a rise in energy costs.
- Low Refrigerant. The refrigerant flowing through your system is the key to how your heat pump does its job. Without the optimum amount of refrigerant, your heat pump will struggle to transfer heat. Low refrigerant is generally a result of a leak and can result in poor heat absorption, which can affect heating and cooling. If you‘ve noticed your heat pump working particularly hard, starting and stopping frequently, or any other reductions in efficiency or performance, have a technician inspect your system for leaks as soon as possible. They use special equipment designed to detect leaks and can get you repaired and recharged in no time.
- A Failing or Failed Compressor. The compressor is the component that transitions the refrigerant from a hot gas to a cold liquid allowing it to absorb heat, and is critical to the refrigerant cycle. Without an operating compressor, your system’s ability to transfer heat will stop completely.
- A Faulty Defrost Board. Without a properly operating defrost board, your outdoor coil will eventually freeze, restricting the flow of refrigerant and compromising the heart exchange process. A failed board could also cause the system to believe it’s in defrost mode more than necessary, or all the time, causing the Aux Heat to be engaged far more than normal.
- A Reversing Valve Issue. The reversing valve is the component that gives a heat pump the ability to both heat and cool. If your valve is having issues, your system may be having trouble switching between cooling and heating modes. With your thermostat calling for your house be warmed, and your heat pump stuck in cooling mode, the system has no alternative but to engage the auxiliary heat to meet the thermostat’s demands. This can tank your system’s efficiency and send your energy bills soaring.
Expert Tips: Ways to Keep Auxiliary Heat in Check
There are a few key things you can do to help avoid overuse of your heat pump’s Auxiliary Heat system.
Optimize Your Thermostat Settings in Cold Weather
One of the main causes of over-engagement of auxiliary heat is setting indoor temperatures too high. Setting winter temperatures higher than about 68º can put a lot of extra stress on your heat pump, and thus increase the need for Aux Heat assistance. If this sounds low, you can help to ensure everyone stays comfortable by doing things like opening window shade and allowing the sunlight to warm the space, and encouraging layering and warmer clothing inside the house.
When and if you do need to raise the temperature, only raise it one degree at a time. This will reduce the stress on your heat pump and except in extreme cold, likely eliminate the need for the Auxiliary Heat to engage.
Upgrading to a smart thermostat can also make managing and monitoring these things much easier. Having the information and the ability to adjust and correct at your fingertips will help increase system efficiency and help grow your knowledge of how your system is performing.
Shut Off Unused Rooms and Seal Windows and Doors
Your heat pump will heat or cool every room you leave open to it. If there are spaces within your home that are rarely used or can be closed during certain times of the day or week, close the vents and doors to those rooms. This lowers the number of cubic feet your heat pump must heat, and will reduce wear on your system while improving efficiency.
Leaky or drafty windows and doors can be a huge source of lost heat – and lost money. Checking and repairing these seals to minimize escaping warm air will help reduce the burden on your heat pump.
Keep Up with Regular Maintenance on Your Heat Pump
Maintenance, maintenance, maintenance. We can’t say it enough. Simple homeowner maintenance like changing filters regularly and making sure vents and equipment stay clean and free of debris will go a long way to keeping your heat pump heathy. Also be sure to schedule a professional check-up and/or tune-up twice a year during the spring and fall transition periods. Catching an issue before it becomes an emergency will save you not only money and frustration, it will also extend the life of your heat pump.
Signs Your Heat Pump May Need Attention
The auxiliary heat function built into your heat pump can be an excellent indicator of the health of your heat pump system overall. Understanding your system’s routines, and normal reactions to changes in demand can help you identify small issues before they become big ones.
Just keep in mind, an aux heat light that comes on frequently or stays on more than about 30 minutes at a time is an indicator that your heat pump can’t handle the workload it’s being handed. And if it’s above 40º and you see the aux heat light on your thermostat, there’s most likely an issue that needs professional attention.
When it comes to keeping on top of your heat pump’s performance during the colder months, paying attention to your thermostat and monitoring the frequency and length of auxiliary heat engagement tops the list of good practices.
If you run into a problem you can’t resolve, or have noticed something unusual happening with your heat pump, don’t hesitate to consult a local HVAC professional. Putting it off will only leave you in the cold, and cost you more money in the long run.
