Why is my heat pump's aux heat always on?
Updated August 2026
Auxiliary heat running when it is genuinely cold is normal, but if your heat pump's aux or emergency heat stays on in mild weather, the most likely cause is low refrigerant leaving the compressor unable to keep up, so the thermostat calls in the electric strip heaters to close the gap. Because strip heat costs roughly two to three times as much to run as the heat pump itself, chronic aux use in 40-degree weather is a real money leak worth chasing down.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Low refrigerant charge | Most common | A leak drops capacity so the heat pump cannot hit the setpoint, and the thermostat locks in aux heat. A tech finds and repairs the leak and recharges. | $200–1,500 |
| Dirty coils or clogged filter | Common | Restricted airflow across a dirty outdoor or indoor coil cripples heat output, so aux carries the load. Clean the coils and change the filter. | Free DIY, $150–400 |
| Aggressive or miswired thermostat | Common | A thermostat set to switch to aux on a large swing, or one miswired to run aux and the heat pump together, burns strip heat needlessly. Reprogram or rewire it. | Free, $150–300 |
| Stuck defrost or reversing valve | Occasional | A defrost board that runs too long or a sticky reversing valve keeps kicking aux in. A tech tests and replaces the faulty part. | $200–600 |
| Setpoint pushed up too fast | Occasional | Jumping the thermostat several degrees at once triggers aux to catch up quickly. Nudge the setpoint in small steps or use a smart recovery mode. | Free |
Check these first
- Confirm the outdoor temperature. Below your system's balance point, roughly 30 to 40°F, aux heat running is expected, not a fault.
- Check the thermostat: is it in EM HEAT or emergency mode by accident? Switch it back to normal HEAT so the heat pump runs first.
- Pull the filter and replace it if it is dirty; restricted airflow forces aux to cover the shortfall.
- Go outside and look at the heat pump coil for ice, dirt, or leaves blocking airflow, and clear what you can.
- Feel the supply-register air: warm and comfortable means the heat pump is working; cool, weak air with aux on points to low charge.
Prevent it next time
Keeping aux heat in its lane is mostly about airflow and charge. Change the filter on schedule and keep the coils clean with a regular coil cleaning, then book an annual heat pump service where a tech verifies the refrigerant charge and tests the defrost controls that trigger aux. A correctly charged, clean system leans on cheap heat-pump heat and only calls the expensive strips when it is truly cold. If yours needs constant aux even after service, our heat pump lifespan guide covers when age is the real answer.
When to call a pro
Call an HVAC tech when aux heat runs in mild weather after you have changed the filter and cleared the coils, when the supply air is cool with aux engaged (a low-charge sign), or when you suspect a stuck reversing valve or defrost board. A diagnostic runs $100–200, a leak repair and recharge $200–1,500 depending on the leak, and a reversing-valve or defrost-board fix $200–600. Since chronic strip heat can add well over $100 to a monthly bill, the diagnostic usually pays for itself fast.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How much more does auxiliary heat cost to run?
Electric strip heat is straight resistance heating, so it converts electricity to heat at about a one-to-one ratio. A heat pump moves heat instead of making it and delivers roughly two to three units of heat per unit of electricity in mild weather. That means running aux costs two to three times as much as the heat pump for the same warmth. A system stuck on aux for weeks can add well over $100 to a monthly bill, which is exactly why chronic aux use is worth diagnosing.
Is it normal for aux heat to come on at all?
Yes. Below the balance point, usually somewhere between 30 and 40°F depending on your home and system, the heat pump alone cannot keep up and aux heat is designed to help. It also kicks in briefly during defrost cycles and when you raise the thermostat several degrees at once. What is not normal is aux running in 45 or 50-degree weather, or for hours when the temperature is mild. Occasional cold-weather aux is fine; constant aux in mild weather is a symptom.
What's the difference between aux heat and emergency heat?
Aux heat is automatic: the thermostat brings the electric strips online to supplement the heat pump when it cannot keep up, and the heat pump keeps running. Emergency heat, EM HEAT on the thermostat, is a manual mode that shuts the heat pump off entirely and runs only the strips, meant for when the outdoor unit is broken. Running EM HEAT by mistake, or leaving it on after a cold snap, is a common reason for a shocking bill, so make sure the thermostat is back on normal HEAT.
Why is my aux heat on when it's not even that cold?
Mild-weather aux almost always means the heat pump cannot make enough heat on its own, and the usual culprits are low refrigerant from a leak, a dirty coil or filter choking airflow, or a thermostat set to switch to aux too eagerly. Start with the free checks: change the filter, clear the outdoor coil, and confirm the thermostat is in HEAT, not EM HEAT. If aux still runs with warm weather and a clean system, get the refrigerant charge checked, since that is the most common hidden cause.