Key Takeaways

• Auxiliary heat is an emergency source of heat that runs when the heat pump cannot satisfy demand, and it should run as little as possible to keep energy bills down.

• Auxiliary heat should sometimes briefly come on during defrost cycles or very cold weather, but it should not stay on constantly. That means there’s a problem that needs to be inspected.

• Check your thermostat settings first. Make sure it’s not set to emergency or aux mode, in a bad location, or programmed with big temperature jumps.

• Check for system problems such as low refrigerant, failed components, or restricted airflow by monitoring for ice on coils, strange noises, clogged filters and blocked vents.

• Monitor your monthly heating bills and schedule a system tune-up, additional insulation, or an upgrade if your bills or auxiliary heat usage continue to be elevated.

• If your troubleshooting does not halt constant auxiliary heat, call a licensed HVAC technician for a full diagnostic and repairs to restore efficient operation.

Heat pump auxiliary heat always on means the backup heater runs right along with the heat pump. This typically occurs during low outdoor temperatures, thermostat settings that prefer emergency heat, or when the system detects a lack of output.

Heat pump auxiliary heat always on denotes continuous auxiliary operation, which drives up energy use and costs and can indicate wiring, sensor, or capacity problems.

Homeowners can check thermostat mode, outdoor unit performance, and heat strip condition prior to consulting a qualified HVAC technician for a diagnosis.

Auxiliary Heat Function

Auxiliary heat is the back-up heat source in a heat pump system that kicks on when the primary heat pump cannot meet your home’s heating need. It is not designed for continuous operation, but is intended to maintain comfort when the heat pump cannot keep up. The thermostat senses both indoor and outdoor conditions and turns on auxiliary heat to keep temperatures from dropping below the desired set point.

Auxiliary heat is typically electric resistance heat strips or a separate gas furnace in conjunction with the heat pump. Electric strips heat fast by sending current through resistive cables. A gas furnace will fire to supplement warmth when the system requests. Both offer immediate, direct heat when outside temperatures are extremely low or when the system needs to increase indoor temperature rapidly.

If the thermostat is set 5 °F above the current room temperature on a chilly morning, the heat pump might not be able to provide heat quickly enough. In this case, the thermostat will switch on the auxiliary heat to accelerate the warming.

Thermostat logic controls when aux heat operates. They typically incorporate an outdoor trigger temperature of about 1 to 4 °C (35 to 40 °F). Below that, the heat pump is less efficient, and the thermostat will permit or require the backup heat. The thermostat tracks the indoor temperature change rate, and increasing the set point by more than 1 to 2 °C (3 to 4 °F) can trigger auxiliary heat.

This avoids waiting for comfort and clarifies why repeated big set-point jumps generate more aux. Auxiliary heat keeps you comfortable and stops the house from cooling down below the set temperature during winter cold snaps or fast drops.

In reality, auxiliary heat is useful during short blasts of extreme cold or when people come home to a freezing house and want it heated up fast. It serves as a safeguard in the event that the heat pump becomes capacity deficient due to frosting, low refrigerant, or compressor faults.

Auxiliary heat uses more energy and can drive up heating bills if it runs frequently. Electric resistance heat is expensive per kilowatt in comparison to the heat pump’s high efficiency. As temperatures plummet outside and heat pump capacity falters during winter months, auxiliary consumption naturally increases.

If the aux light comes on for extended periods when the outside temperature is greater than roughly 4 °C (40 °F), it might indicate a bad thermostat, a malfunctioning outdoor unit, or an incorrectly sized system. By monitoring runtimes and keeping the system serviced, you’re reducing unnecessary auxiliary use.

Normal Activation

Normal activation describes what is going on in common scenarios when that auxiliary heat kicks on and why it’s considered normal operation for a heat pump. Auxiliary heat is a fallback source that activates when the primary heat pump can’t keep up. It isn’t necessarily indicative of failure, though it’s frequently an intentional action to keep the interior comfortable and safeguard the system.

Common scenarios when auxiliary heat is activated:

• Outside temperature drops below roughly 2 to 4 degrees Celsius (35 to 40 degrees Fahrenheit), heat pump efficiency declines.

• Quick thermostat jumps of more than 1.5 to 2 degrees Celsius (3 to 4 degrees Fahrenheit) to get to setpoint fast.

• Hot gas defrost – Heat pump goes into defrost mode to melt ice on outdoor coil.

• Cold snaps when the heat pump cannot keep up with heat loss.

• Timed defrost cycles on a few units, like every 45 or 90 minutes.

• Cold starts after the system has been down for a while and the house requires swift warmth.

Auxiliary heat is essentially a safety net. When outdoor temperature falls close to or below 2 to 4 degrees Celsius, the heat pump’s efficiency coefficient of performance decreases and it transfers less heat indoors per kilowatt of electricity. In that band, the system can demand supplemental electric strips or a gas or propane furnace to make up the deficit.

This maintains indoor temperature without overworking the compressor. In a cold snap, the backup source could be on and off or on for long stretches until outside temperatures normalize.

Defrost mode is the signature moment for aux heat to make an appearance. When the outdoor unit detects frost buildup, it interrupts the heating cycle for a short while to operate in cooling mode and melt the ice. That would blow cool air if it weren’t for the aux heat.

Most systems allow the backup heat to run for approximately 15 to 30 minutes in a defrost cycle to prevent cold drafts. Some makers schedule defrosts at regular intervals, maybe every 45 or 90 minutes, depending. If auxiliary kicks in well beyond 30 minutes over and over again, it could be a defrost or coil problem.

For activation, it’s like a thermostat. Increasing the setpoint by more than approximately 1.5 to 2°C (3 to 4°F) tends to engage auxiliary heat in order to reach the new setpoint rapidly. Programmable and smart thermostats can eliminate wasteful backup by making incremental changes and learning system boundaries.

They can restrict secondary lockout to avoid overuse. If the auxiliary indicator stays on for extended periods, have a technician verify system efficiency, refrigerant charge, outdoor fan operation, and thermostat setting.

Why It Stays On

Auxiliary heat running non-stop is a guaranteed indication that either the heat pump system or thermostat settings aren’t allowing the primary heat pump to satisfy the home’s load. Constant aux heat use increases energy costs and decreases system efficiency because auxiliary strips or backup furnaces usually burn more energy per kilowatt-hour than the heat pump’s compressor-fueled heat.

1. Thermostat Settings

If the thermostat set point lingers significantly higher than the indoor temperature, the system will demand quick heat and maintain auxiliary heat to get there. Verify that the thermostat is not in emergency heat or a continuous auxiliary mode, as emergency heat makes the system override the heat pump.

Programmable or smart thermostats can cut aux use by staging heat and learning occupancy patterns to minimize big temperature jumps. Verify the thermostat’s location and calibration: a thermostat near a heat source or in a cold draft can misread the room temperature and keep aux heat running.

If the thermostat is defective or miscalibrated, replace or recalibrate it.

2. Outdoor Temperature

Auxiliary heat typically comes on more when it gets below roughly 1.7°C (35°F) outside, and most systems have a user-set auxiliary trigger at about 4°C (40°F). Air-source heat pumps lose capacity as outdoor air cools, so it’s expected to witness more aux during cold snaps.

Follow outdoor temperature tendencies to distinguish normal activation from helpless addiction. Better insulation, air sealing, and weatherstripping mean the main heat pump can make it longer without the backup source.

3. Refrigerant Leaks

A low refrigerant charge makes the unit less able to pull heat from the outside, so the control system switches to auxiliary heat. Search for symptoms similar to ice on the outside coil, decreased airflow, or reduced indoor temperatures lower than normal even after long run cycles.

Refrigerant problems keep the compressor working harder and auxiliary heat running longer. Prompt service to locate and repair leaks and recharge properly is necessary to get the system back to efficient heating.

4. Component Failure

Failed parts — a stuck or faulty reversing valve, bad compressor, or malfunctioning defrost control — can drive the system into aux-only mode. Symptoms include the heat pump not going into heat or the unit remaining in aux mode indefinitely.

Checking the air handler and outdoor unit and listening for weird noises catches problems. It’s about why it stays on.

5. Airflow Restriction

Clogged filters, blocked vents and dirty coils all reduce air flow and heat transfer causing controls to invoke auxiliary heat. Change filters promptly, clear vents, and clean indoor/outdoor coils.

Proper airflow allows the heat pump to meet load without resorting to the backup heater.

System Inefficiency

System inefficiency happens when a heat pump uses its auxiliary heat too frequently or for too extended a period. This can manifest itself in the form of elevated bills, constant AUX HEAT flashing on the thermostat, and discomfort. This is often the result of setting indoor temperatures too high.

Keeping winter setpoints above 20°C (68°F) causes your heat pump to work harder and backup heat to engage more frequently. Heat pumps are most efficient in temperate climates with mild temperature fluctuations. In colder climates, the unit may have difficulty transferring sufficient heat, causing auxiliary heat to activate more often and system inefficiency to become evident.

Here’s a simple table to help you identify patterns in performance with your monthly utility and heating bills. Seek consistent growth without other household disruptions.

When bills increase even though usage and rates remain unchanged, inefficiency is probably at fault. Track thermostat readouts: if the display shows AUX HEAT for more than an hour or two when outdoor temperature is above 4°C (40°F), that points to a problem.

Most systems have a 1.5 to 3-degree threshold that tells the unit when to call for aux heat, and if that threshold is set too tight or the thermostat logic is off, the aux will engage unnecessarily.

Test your existing unit’s capacity versus your home’s heat load. Undersized heat pumps will run their auxiliary strips frequently because they cannot meet the load. A load calculation (Manual J) can determine if the system is undersized.

If your heat pump is older or has lower SEER or HSPF ratings, it may be worth upgrading to a more efficient model with improved cold-climate capability. Make time for a professional tune up to optimize controls and refrigerant charge, check defrost cycles, and verify the thermostat’s settings.

Bad defrost lingering beyond one hour indicates a fault and decreases heat exchange, spurring aux usage. Technicians can recalibrate the auxiliary trigger point, fix defrost controls, and clean coils to get back up to speed.

When outside temps plummet below 4°C (40°F) or you increase setpoints substantially, anticipate increased aux usage. Control setpoints, check capacity, calculate monthly bills and schedule a tune up to minimize inefficiency.

The Human Factor

This is where the human factor comes in when aux heat is on. Easy thermostat decisions frequently trigger backup heat to run. If you turn the thermostat up by 3 to 4 degrees Celsius (roughly 3 to 4 degrees) or more at a time, you are telling the system to get there ASAP, and the control logic will supplement with electric heat to achieve the set point.

Lots of folks crank winter temperatures above around 20 degrees Celsius (68 degrees Fahrenheit) for cozy comfort. That higher set point causes the heat pump to labor and increases the likelihood that the aux strips will kick in, perhaps for hours at a stretch if it is very cold outside.

Family routines count. Dropping the thermostat at night and then turning it up aggressively in the morning makes the system play catch up. The heat pump by itself heats slowly. Sudden increases trigger secondary heat to fill the gap.

If the family aren’t on the same schedule, somebody will adjust the set point to a much warmer level, sending the aux heat switch on and left on while the home fights to catch up to the new setting. In others, residents think they’re freezing until the house arrives at the higher temperature. Then they conclude the system is breaking down, not understanding that the heat pump requires time and consistent input.

Education and clear rules assist. Educate family members to prevent big, quick temperature leaps. Recommend small shifts of 1 to 2 degrees Celsius at a time so the heat pump can take the brunt of it.

Employ some regular day and night goals. If residents desire cooler nights, schedule a slow morning increase rather than a huge immediate bump.

Technology can minimize human error. A digital or smart thermostat can take care of slow temperature ramps and lock out large manual jumps. Program schedules that align with daily schedules and maximum set point limits protect against an accidental high setting.

Smart thermostats can run “eco” modes that favor the heat pump over auxiliary strips and display alerts when the auxiliary heat is running too often, which can indicate the system is under strain or malfunctioning.

Tweaks for the extremes remain. During cold snaps, a small bump in auxiliary use is typical. During mild days, stable settings allow the heat pump to provide the bulk of the heat.

If the auxiliary light burns long in spite of slow ramps in set points, that implies the heat pump is unable to keep up and requires servicing. Lowering the indoor set temperature a few degrees would cause the auxiliary indicator to vanish while the heat pump stayed on run mode.

Professional Diagnosis

It needs a professional diagnosis to discover why the auxiliary heat remains on and to verify if the heat pump is functioning properly. Should the aux heat light stay on too long, a professional diagnosis is conducted following signals from the thermostat to the control board, the outdoor unit, and the aux stage. They will verify that the system really requires backup heat or if a control fault or thermostat wiring error is causing the auxiliary stage to be on.

If the aux heat light comes on a lot or is on for more than 30 minutes at a time, it could indicate that your heat pump is not running efficiently. A pro will take supply and return air temperatures, check the refrigerant charge, and record cycle times. Low refrigerant, frozen coils, or a failing compressor can diminish the heat pump’s capacity and cause the system to cycle auxiliary heat repeatedly.

The tech can verify that the heat pump is actually in defrost mode, which temporarily utilizes backup heat. Incorrect defrost control settings lead to longer auxiliary runs. A comprehensive system diagnosis should include sizing and load checks to determine if the heat pump is properly sized for the home.

Mini units or badly insulated homes will make the auxiliary heat run more frequently when you increase the internal set temperature. For instance, upping the setpoint a few degrees in a leaky housing stock causes long auxiliary cycles. A professional will do a heat-load calculation for test sizing and recommend insulation or window upgrades to minimize backup use.

Electrical issues and defective parts are common causes that should only be diagnosed by a licensed HVAC provider. Your technician will test relays, contactors, sensors, and the thermostat itself. A misconfigured or failing thermostat can tell the system to engage auxiliary heat even when the heat pump can manage the load.

They’re going to check safety limits and wiring for shorts or loose connections that might keep the aux buzzer going. Ask a professional for advice on thermostat settings and home envelope improvements to minimize the use of auxiliary heat. Request from the contractor professional advice, such as using smaller setpoint steps, activating smart setback schedules, and ensuring proper heat pump and auxiliary staging.

Frequent maintenance and tune-ups reduce the chance of failure, but lingering or erratic peripheral activity, particularly when ambient temperature is greater than 40 °C, should get a professional diagnosis to prevent expensive repairs and keep the system safe and efficient.

Conclusion

If the auxiliary heat is on all the time, it’s a waste and the bill is huge. Check the thermostat mode, setpoint, and fan settings first. Check for faults or low refrigerant that shut the compressor down. Check the outdoor unit for ice or debris or a broken contactor. Keep an eye out for extended defrost cycles and strange noises. See if you can track when the aux heat kicks in and what the outdoor temperature is at. Give those details to a tech to quicken diagnosis. A trained pro can test voltages, check sensors and swap parts if necessary. Small fixes, such as a stuck relay or a bad sensor, frequently cut off the extra heat. Ready to return your system to normal. Schedule a service call or request a troubleshooting checklist.

Frequently Asked Questions

Why does auxiliary heat turn on with a heat pump?

Auxiliary heat comes on when the heat pump can’t keep up with the thermostat setpoint. That gives you quick supplemental heat during really cold outside temperatures or defrosting.

Is it normal for auxiliary heat to run during cold weather?

Yes. Auxiliary heat should run when the outdoor temperature is low or if the heat pump cannot keep up. It makes sure the house is at temperature.

Can a stuck relay cause auxiliary heat to stay on?

Yes. A stuck or failed relay can keep auxiliary heat energized. That causes continuous operation and elevated energy consumption. Have an electrician take a look.

Could thermostat settings force auxiliary heat to stay on?

Yes. Faulty thermostat programming or a narrow temperature differential can prematurely trigger auxiliary heat. Check the settings and increase the balance point if necessary.

How does system inefficiency make auxiliary heat run more often?

Neglected maintenance, low refrigerant, or an aging compressor cut heat pump output. The system then depends more heavily on auxiliary heat to sustain a comfortable temperature, which increases energy expenses.

When should I call a professional about auxiliary heat?

Call a reputable HVAC technician if aux heat runs a lot, won’t turn off, or energy bills rise. A pro can troubleshoot controls, electrical components, refrigerant charge, and general system condition.

Can I reduce auxiliary heat usage myself?

You can use less by insulating, sealing drafts, slowly raising the thermostat, and having routine heat pump maintenance. For electrical or refrigerant problems, call a technician.