Air source heat pumps need auxiliary heat because their heating capacity drops as outdoor temperatures fall, while your home’s heating demand increases. At some point, the heat pump simply can’t extract enough warmth from cold outdoor air to keep up. That crossover point, called the balance point, is where supplemental heating kicks in to cover the gap.
How Cold Air Limits a Heat Pump
A heat pump doesn’t generate heat the way a furnace does. It moves heat from outdoor air into your home using a refrigerant cycle. Even cold air contains thermal energy, but the colder it gets, the harder the system has to work to pull that energy out. This is a basic thermodynamic constraint: as the temperature difference between the outdoor air and your desired indoor temperature grows, the heat pump’s efficiency and output both decline.
You can see this clearly in real-world performance numbers. A high-quality multi-zone heat pump might achieve a coefficient of performance (COP) of about 3.95 at 47°F, meaning it delivers nearly four units of heat for every unit of electricity it consumes. At 17°F, that same system drops to a COP around 2.85. By 5°F, some systems fall to a COP between 1.8 and 2.8, and at -10°F, you’re looking at roughly 1.4. The lower the COP, the less heat the system produces per dollar of electricity, and the less total heat it can deliver at all.
This creates a frustrating mismatch. On the coldest days, when your home loses heat fastest through walls, windows, and the roof, your heat pump’s maximum output is at its lowest. The building needs more heat at exactly the moment the equipment can deliver less of it.
The Balance Point
Every heat pump installation has a balance point: the outdoor temperature where the heat pump’s maximum capacity equals the home’s heat loss. Above this temperature, the heat pump handles the load on its own. Below it, the home loses heat faster than the pump can replace it, and auxiliary heat is needed to make up the difference.
The balance point depends on both the size of the heat pump and how well insulated the home is. A tightly sealed, well-insulated house in a moderate climate might have a balance point well below freezing. A drafty older home with a modestly sized system could hit its balance point at 25°F or 30°F. One calculation example from HVAC documentation shows a balance point as low as 5°F for a properly matched system, meaning the heat pump alone covers the load down to that temperature before needing help.
Sizing a heat pump large enough to cover the absolute coldest day of the year sounds like a solution, but it creates its own problem. An oversized system runs inefficiently during milder weather, cycling on and off too frequently. The most cost-effective approach for most homes is to size the heat pump to handle the majority of heating hours and let auxiliary heat cover the coldest extremes.
What Auxiliary Heat Actually Does
Auxiliary heat is a backup heating source built into or paired with the heat pump system. Your thermostat activates it automatically when it detects the heat pump is falling behind. Depending on the system, the thermostat triggers auxiliary heat after a delay of five, ten, or twenty minutes if the indoor temperature isn’t rising toward your set point. You’ll typically see “AUX” on your thermostat display when this happens.
The most common form is electric resistance heating strips installed inside the air handler. These work like a large electric space heater, converting electricity directly into heat at a COP of 1.0 (one unit of heat per unit of electricity). That’s far less efficient than a heat pump running at a COP of 2 or 3, so your electricity consumption spikes when the strips are running. During extended cold snaps, this can cause a noticeable jump in your electric bill.
Some systems use a gas furnace as the backup instead, creating what’s called a dual-fuel system. Propane or natural gas backup tends to heat more efficiently during extreme cold than electric resistance strips, keeping energy costs lower during the hours when auxiliary heat is running. The tradeoff is higher installation complexity and the need for a fuel supply.
Defrost Cycles Need Backup Too
Cold weather isn’t the only trigger. When outdoor temperatures hover near freezing with high humidity, frost builds up on the heat pump’s outdoor coil. The system periodically reverses its refrigerant cycle to melt this frost, temporarily running in cooling mode for a few minutes. During defrost, the heat pump stops heating your home entirely.
Without auxiliary heat, your vents would blow cool air into the house during every defrost cycle. The thermostat automatically engages the backup strips or furnace to keep warm air flowing while the outdoor unit defrosts. This is normal and typically lasts only a few minutes at a time, but it’s another reason auxiliary heat is a standard part of heat pump systems rather than an optional add-on.
Auxiliary Heat vs. Emergency Heat
These two terms appear on many thermostats and are often confused. Auxiliary heat activates automatically alongside the heat pump when extra capacity is needed. The heat pump keeps running and the backup supplements it. Emergency heat is a manual setting you switch to when the heat pump itself has failed. In emergency mode, the system relies entirely on the backup heating source and the heat pump compressor stays off.
If you see “AUX” on your thermostat during a cold stretch, that’s expected behavior. If it stays on for more than about 30 minutes continuously during moderate weather, something may be wrong with the heat pump’s performance. Emergency heat should only be used as a stopgap until the heat pump can be repaired, since running solely on electric resistance strips is two to three times more expensive than normal heat pump operation.
Cold Climate Heat Pumps Are Closing the Gap
Newer cold climate heat pumps have pushed the balance point significantly lower, reducing how often auxiliary heat is needed. These systems use advanced compressor technology, including vapor injection, to maintain higher output and efficiency at temperatures that would cripple older models. The U.S. Department of Energy has supported development of cold climate systems that operate down to -13°F while maintaining a COP above 2.3, meaning they still deliver more than twice as much heat as the electricity they consume even in extreme cold.
For homeowners in northern climates, these systems can handle the vast majority of winter hours without backup. But even the best cold climate heat pumps have physical limits. At some temperature, the refrigerant cycle can’t extract enough heat to meet demand, and auxiliary heat remains a necessary safety net. The difference is that a modern cold climate system might only need that backup for a handful of hours per year rather than hundreds.