Introduction to Auxiliary Heat for Heat Pumps
When temperatures drop, a heat pump becomes an essential component for maintaining indoor comfort. Even so, during extremely cold weather, a heat pump may struggle to extract sufficient heat from the outside air. This is where auxiliary heat steps in as a vital backup heating source. Understanding how auxiliary heat works, when it activates, and how to manage it efficiently can save energy, reduce costs, and prolong the life of your HVAC system.
How a Heat Pump Works
A heat pump operates by transferring heat from the outdoors to the indoors during winter, and reversing the process in summer. On the flip side, it uses a refrigerant cycle to move heat rather than generating it directly. This makes heat pumps highly energy‑efficient in moderate climates. On the flip side, as outdoor temperatures fall below a certain point—typically around 35–40°F (2–4°C)—the heat pump’s capacity to extract heat diminishes. At this stage, the system requires additional heating to maintain the desired indoor temperature.
What Is Auxiliary Heat?
Auxiliary heat (often labeled as “aux heat” on thermostats) is a secondary heating source integrated into or paired with a heat pump system. It activates automatically when the heat pump alone cannot meet the heating demand. Auxiliary heat can come in several forms:
- Electric resistance heat (strip heat) – Common in many heat pump systems, using coils similar to a toaster.
- Gas furnace – Known as a dual‑fuel system, providing heat through combustion.
- Oil furnace – Less common, used in some older systems.
- Hot water (hydronic) coils – Using heated water from a boiler.
The term emergency heat is sometimes used interchangeably, but it typically refers to a manual setting that forces the system to rely solely on the backup heat source, bypassing the heat pump Which is the point..
When Does Auxiliary Heat Turn On?
Auxiliary heat engages automatically under specific conditions:
- Low outdoor temperatures – When the outside air is too cold for efficient heat extraction, the thermostat signals the need for auxiliary heat.
- Defrost cycle – During winter, the outdoor unit can ice up. The system periodically reverses to defrost, shutting off the heat pump temporarily. Auxiliary heat maintains indoor warmth during this brief period.
- Temperature setpoint increase – If you raise the thermostat setting by more than a few degrees, the system may call for auxiliary heat to quickly reach the new temperature.
- Heat pump malfunction – If the outdoor unit fails, the system may rely entirely on auxiliary heat until repairs are made.
Importance of Auxiliary Heat
Auxiliary heat ensures that your home stays warm even in the coldest weather. Without it, the heat pump would be unable to maintain comfort, potentially leading to:
- Uncomfortable indoor conditions.
- Increased strain on the heat pump, risking damage.
- Possible freezing of indoor pipes in extreme cases.
Efficiency Considerations
While auxiliary heat is essential, it is often less efficient than the heat pump itself. Electric resistance heat, for example, consumes significantly more electricity than the heat pump’s compressor operation. This can lead to higher energy bills, especially during prolonged cold snaps.
- Maintain a consistent thermostat setting – Avoid large temperature swings.
- Use a programmable thermostat – Set lower temperatures overnight or when away.
- Ensure proper home insulation – Reduce heat loss through windows, doors, and walls.
- Keep filters clean – Restricted airflow forces the system to work harder.
- Consider a dual‑fuel system – A gas furnace as auxiliary heat can be more cost‑effective in very cold climates.
Troubleshooting Auxiliary Heat Issues
If you notice auxiliary heat running constantly or more often than expected, it may indicate a problem:
- Dirty air filter – Replace or clean filters monthly during heavy use.
- Faulty thermostat – Calibrate or replace if it inaccurately reads temperatures.
- Refrigerant leak – Low refrigerant reduces heat pump efficiency, causing auxiliary heat to overcompensate.
- Outdoor unit obstruction – Clear leaves, snow, or debris from around the unit.
- Defrost control issues – If the unit ices over frequently, the defrost cycle may not be functioning correctly.
If simple checks don’t resolve the issue, contact a qualified HVAC technician for a thorough diagnosis It's one of those things that adds up..
Maintenance Tips for Optimal Performance
Regular maintenance keeps both the heat pump and auxiliary heat working efficiently:
- Schedule professional tune‑ups at least once a year, preferably before winter.
- Inspect and clean coils on both indoor and outdoor units.
- Check electrical connections and tighten any loose wires.
- Test the defrost cycle to ensure it operates correctly.
- Verify thermostat accuracy with a separate thermometer.
Cost Implications
Auxiliary heat can significantly impact your energy bills. Understanding its usage patterns helps manage costs:
- Monitor your energy bills for spikes during cold periods.
- Compare fuel costs if you have a dual‑fuel system; sometimes switching to the auxiliary heat source (e.g., gas) is cheaper than running the heat pump in very low temperatures.
- Invest in a smart thermostat that learns your schedule and optimizes heating to reduce auxiliary heat reliance.
Frequently Asked Questions
What’s the difference between auxiliary heat and emergency heat?
Auxiliary heat activates automatically when needed, while emergency heat is a manual setting that turns off the heat pump and relies solely on the backup source.
Is it normal for auxiliary heat to come on when it’s 30°F outside?
Yes, most heat pumps begin to need auxiliary heat around 35–40°F. At 30°F, it’s typical for auxiliary heat to assist It's one of those things that adds up. Turns out it matters..
Can I prevent auxiliary heat from turning on?
You can reduce its frequency by improving home insulation, sealing drafts, and using a programmable thermostat to avoid large temperature increases.
How can I tell if my auxiliary heat is electric or gas?
Check your system’s documentation or look at the indoor unit. Electric auxiliary heat will have visible coils; gas systems will have a small furnace with a burner and flue Turns out it matters..
**Why does my heat pump ice up in winter
Why Does My Heat Pump Ice Up in Winter?
It’s normal for a heat pump’s outdoor coil to develop a light coating of frost or ice during cold, humid weather. Day to day, the unit is designed to periodically enter a defrost cycle, reversing refrigerant flow to melt the accumulation. On the flip side, if you notice excessive, thick ice that doesn’t clear during defrost cycles, or if the unit is encased in ice for extended periods, it signals a problem Most people skip this — try not to..
Common causes include:
- Low refrigerant levels (often from a leak), which reduces the coil’s ability to transfer heat and can lead to prolonged operation in suboptimal conditions.
- Faulty defrost controls or sensors that fail to initiate or terminate the defrost cycle properly.
- Outdoor unit obstructions (like snowdrifts, leaves, or debris) that restrict airflow and prevent the coil from warming sufficiently.
- Malfunctioning fan motors that don’t spin at the correct speed, disrupting airflow over the coil.
Most guides skip this. Don't.
When a heat pump is iced over, it can’t effectively extract heat from the outdoor air, forcing the system to rely more heavily on auxiliary heat. This not only spikes energy consumption but can also lead to compressor damage if left unresolved. If basic troubleshooting (clearing debris, ensuring proper drainage) doesn’t resolve the icing, a professional technician should inspect the refrigerant charge, electrical components, and defrost system.
Conclusion
Understanding and managing your heat pump’s auxiliary heat is key to maintaining comfort, controlling energy costs, and extending the life of your system. While auxiliary heat is a necessary backup for extreme cold, frequent or unexpected activation often points to underlying issues—from simple fixes like a dirty filter to more complex problems like refrigerant leaks or defrost malfunctions. Regular professional maintenance, vigilant monitoring of your energy bills, and simple homeowner practices (like improving insulation and sealing drafts) can minimize reliance on this costly backup. When in doubt, consulting a qualified HVAC technician ensures your system operates safely and efficiently all winter long No workaround needed..
