Not all heat pumps have auxiliary heat, but most modern residential models do
Auxiliary heat — also called backup heat or supplemental heat — is a second heating system that turns on when the heat pump alone cannot keep your home warm enough. It is usually electric resistance heating (like a space heater built into your system) or a gas furnace paired with the heat pump. Whether your heat pump has it depends on the model, the installer's choice, and how the system was set up.
Single-stage heat pumps in mild climates sometimes run without auxiliary heat. Cold-climate heat pumps almost always include it because they need a backup when outdoor temperatures drop below the point where the heat pump works efficiently. If you already have a furnace, your installer may wire the heat pump to use it as auxiliary heat instead of adding a separate electric element.
Key Takeaways
- Auxiliary heat is a backup heating source that activates when the heat pump cannot meet your home's heating demand on its own.
- Most residential heat pumps installed in the last ten years include auxiliary heat, either as built-in electric resistance or a connected furnace.
- Cold-climate heat pumps rely on auxiliary heat because outdoor temperatures regularly drop below the efficiency threshold of the heat pump alone.
- If your heat pump has no auxiliary heat and the outdoor temperature falls below its minimum operating point, your home may not reach the set temperature.
How auxiliary heat activates and when it turns on
Auxiliary heat does not run all the time. Your thermostat monitors the difference between the set temperature and the actual indoor temperature — a gap called temperature differential. When the heat pump cannot close that gap fast enough, the thermostat signals auxiliary heat to turn on.
The trigger point varies by system and thermostat setting. On many units, auxiliary heat activates when the indoor temperature falls 1 to 3 degrees below the set point, or when the outdoor temperature drops below 35 to 40 degrees Fahrenheit. Some thermostats let you adjust this threshold. Auxiliary heat also turns on during defrost cycles, when the heat pump briefly stops heating to melt frost off the outdoor coil.
Once auxiliary heat is running, it stays on until the heat pump catches up and the indoor temperature rises back to the set point. In very cold weather, auxiliary heat may run continuously alongside the heat pump for hours or days.
Types of auxiliary heat and how they differ in cost
Electric resistance heating is the most common auxiliary heat in modern heat pump systems. It works like a toaster — electricity flows through a wire coil that heats up. It is straightforward, reliable, and requires no additional equipment beyond what the heat pump already has. The downside is cost: electric resistance is the most expensive way to generate heat, so running it for long periods will raise your utility bill noticeably.
Gas furnace backup is less common in new installations but still found in homes where a furnace already existed. When the heat pump cannot keep up, the thermostat switches to the furnace. Gas is cheaper per unit of heat than electricity in most regions, so this option costs less to run. However, it requires a gas line, regular maintenance on two systems instead of one, and venting for the furnace.
A few high-efficiency cold-climate heat pumps use a hybrid approach: they run the heat pump and auxiliary heat together at the same time, rather than switching between them. This can be more efficient in very cold weather but is not standard in most residential systems.
Why auxiliary heat matters in cold climates
Heat pumps extract warmth from outdoor air even when it is cold, but their efficiency drops sharply as temperature falls. Below about 35 degrees Fahrenheit, most standard heat pumps struggle to keep up with heating demand. In climates where winter temperatures regularly drop below freezing, a heat pump without auxiliary heat will not maintain your set temperature on the coldest days.
Cold-climate heat pumps are designed to work at lower temperatures than standard models — some operate down to 0 degrees or below — but even these eventually reach a limit. Auxiliary heat fills that gap. Without it, you would either have to lower your thermostat on cold days or accept that your home will be colder than you want it.
In mild climates where temperatures rarely drop below 40 degrees, auxiliary heat is less critical. Some heat pump systems in the South or Southwest are installed without it to save upfront cost, since the heat pump alone can usually handle the heating load.
Checking whether your heat pump has auxiliary heat
Look at your thermostat display. If you see a label or indicator for "Aux Heat" or "Emergency Heat," your system has auxiliary heat. Some thermostats show it only when it is actively running.
Check your system documentation or the nameplate on your indoor unit. It will list the heating sources — for example, "Heat Pump + Electric Auxiliary" or "Heat Pump + Gas Furnace." If you cannot find the paperwork, ask your installer or HVAC contractor. They can tell you what is installed and how it is configured.
If you have a smart thermostat, you can often see auxiliary heat activity in the app or on the device history. This shows you how often it runs and for how long, which helps you understand your heating pattern and energy use.
What happens if your heat pump has no auxiliary heat
If your heat pump has no auxiliary heat and outdoor temperatures drop below its minimum operating point, the system will run continuously but may not reach your set temperature. Your home will be warmer than it would be with no heating at all, but it will be colder than you want.
This is why heat pumps without auxiliary heat are risky in cold climates. If you live somewhere with regular freezing winters and your heat pump has no backup, you should talk to an HVAC contractor about adding auxiliary heat. The cost of retrofitting electric resistance heating is usually between $1,500 and $3,000, depending on your system and region.
In mild climates, the lack of auxiliary heat is less of a problem because the heat pump rarely reaches its limit. However, even in the South, an unusually cold winter can expose the weakness.
Auxiliary heat and your energy bill
Auxiliary heat is expensive to run because electric resistance is inefficient compared to the heat pump itself. A heat pump moves heat from outside to inside, so it produces more warmth per unit of electricity than resistance heating, which generates heat directly. Running auxiliary heat for hours will increase your monthly bill.
In cold climates, some auxiliary heat use is unavoidable — it is the price of staying warm. But you can reduce it by setting your thermostat lower, improving insulation, sealing air leaks, and using a programmable thermostat to lower the temperature when you are away or asleep. Some thermostats also let you disable auxiliary heat manually or set a higher threshold for when it activates, though this means accepting a colder home.
If your auxiliary heat is running constantly during winter, it may signal that your heat pump is undersized for your home or that your house is losing heat faster than the system can replace it. An HVAC contractor can assess whether the system is working as designed.
Frequently Asked Questions
Can I turn off auxiliary heat to save money?
You can disable it on most thermostats, but doing so means your heat pump alone must keep your home warm. In cold weather, this will result in a colder indoor temperature. Some thermostats let you raise the set up threshold instead, so auxiliary heat runs less often but still activates if needed.
Does auxiliary heat turn on automatically or do I have to switch it on?
It turns on automatically when the thermostat detects that the heat pump cannot meet the heating demand. You do not have to do anything. The only exception is "emergency heat" mode, which is a manual setting on some thermostats that forces auxiliary heat to run instead of the heat pump — this is used only if the heat pump breaks down.
Why is my auxiliary heat running so much in winter?
Frequent auxiliary heat use in winter is normal in cold climates, especially during the coldest weeks. If it is running almost constantly, your heat pump may be undersized, your home may have poor insulation or air leaks, or the outdoor temperature may be below the heat pump's efficient operating range. An HVAC contractor can diagnose the cause.
Is a heat pump with auxiliary heat more reliable than one without?
Auxiliary heat does not make the heat pump itself more reliable, but it does may support your home stays warm even if the heat pump cannot keep up. In cold climates, this is essential. In mild climates, it is a safety net for unusually cold weather.
What is the difference between auxiliary heat and emergency heat?
Auxiliary heat activates automatically when needed. Emergency heat is a manual mode you switch on if the heat pump fails — it runs only the backup heating system and skips the heat pump entirely. Emergency heat is a temporary fix while you wait for repairs.