If you see “Aux Heat” lit up on your thermostat and it stays on for hours, that’s not automatically a problem. Auxiliary heat is a built-in backup that kicks in when your heat pump can’t keep up on its own. The issue is knowing when it’s doing its job versus when it’s masking a system that’s slowly failing.
How Auxiliary Heat Is Supposed to Work
A heat pump pulls warmth from outdoor air and moves it inside. Most standard systems are set to call for aux heat somewhere around 35-40°F, because that’s where the balance point lands for a typical home: the heat pump alone can’t add heat fast enough to offset what the house is losing. (Cold-climate and variable-speed units can push that balance point considerably lower, sometimes well below 25°F.)
Normal aux heat situations:
- Outdoor temps are at or below your system’s balance point (often 35-40°F for a standard unit, lower for a cold-climate model)
- You set the thermostat 3+ degrees higher than the current indoor temp and the system is ramping up fast
- Defrost cycle is running (the outdoor unit briefly switches to cooling mode to shed ice off the coil, indoor temps dip a degree or two, aux fires to compensate)
If aux runs during those conditions and shuts off once the house catches up, that’s correct operation.
When It Becomes a Problem
The heat pump should carry most of the load in first stage. If aux is running constantly at 45°F or above, or if the house still isn’t warming up with aux running, something’s wrong. Common causes, roughly in order of likelihood:
Low refrigerant. This is the most common culprit I see. A slow leak means the heat pump can’t transfer enough heat, so it falls short in first stage and aux has to compensate. The system will still run, just not effectively. You’ll often notice longer run times and higher-than-usual bills even before you spot the aux indicator.
Dirty or blocked outdoor coil. Leaves, cottonwood, or ice buildup choke airflow. The unit works harder, capacity drops, aux picks up the slack. A tech can clean the coil during the visit and check for anything else that’s off.
Thermostat misconfiguration. Some thermostats have an outdoor lockout setting that’s supposed to suppress aux above a certain temp. If that’s set wrong, or if someone accidentally enabled Emergency Heat (which locks out the heat pump entirely and runs only the resistance strips), aux runs all the time regardless of outdoor conditions. Check that you’re not in Em Heat mode unless you put it there on purpose.
Failed reversing valve. The reversing valve lets the system switch between heating and cooling. If it’s stuck or partially stuck, the heat pump may run but produce little to no heat. Aux compensates. This tends to show up more on older systems and usually needs a technician.
Undersized system or poor insulation. If a previous owner had the system installed and the load calculation was off, the heat pump may genuinely not be large enough to cover your house in cold weather. Aux runs a lot. Not a refrigerant problem, not a mechanical failure, just a mismatch. Worth knowing before you spend money on repairs that won’t change the outcome.
What a Tech Will Actually Check
When I send a tech out for this complaint, here’s the diagnostic sequence:
- Verify operating mode. Confirm the system is in heat pump mode, not Em Heat. Check thermostat wiring (particularly the O/B reversing-valve wire, which is easy to swap during installs).
- Measure supply-air temperature. In first-stage heat pump operation without aux, supply air typically lands in the 90-110°F range depending on outdoor temperature, unit size, and load. If it’s well below that, it’s a screening signal that the heat pump may not be producing properly, but pressure gauges (step 4) are the authoritative check, not supply-air temperature alone.
- Check static pressure and airflow. A clogged filter or blocked return can hurt system performance quickly.
- Refrigerant pressures. Suction and discharge pressures tell the story on refrigerant charge. This requires gauges and, for refrigerant handling, an EPA Section 608 certification.
- Inspect the outdoor coil. Look for debris, heavy frost outside of a normal defrost cycle, or physical damage.
What You Can Safely Check Yourself
A few things are worth checking before calling:
- Confirm you’re not in Emergency Heat. On most thermostats this is a separate switch or setting. Em Heat is for when the heat pump itself is broken and you’re waiting on a repair. It should not be a normal operating mode.
- Check and replace the air filter. A clogged filter reduces airflow, which drops heating capacity and can cause the heat pump to underperform in first stage. If you haven’t changed it in three or more months, change it.
- Look for obvious debris around the outdoor unit. Leaves, grass clippings, or cottonwood packed against the coil restrict airflow. Note what you see and tell the tech.
- Check your thermostat’s outdoor lockout setting in the manual or app, if applicable. Make sure it’s not set to an unusually high temperature that suppresses the heat pump when it should be running.
Refrigerant work, reversing valve replacement, and thermostat control board wiring all require a pro. Getting those wrong costs more to fix than the original repair.
When to Call a Pro
If you’ve checked the basics and aux heat is still running constantly, or if the house isn’t reaching setpoint with both stages running, get a tech out. Refrigerant issues and mechanical failures don’t self-correct, and running exclusively on aux heat costs significantly more than a properly functioning heat pump.
We handle this diagnosis regularly in the Tri-Valley and East Bay. Give us a call or schedule at adriumservice.com and we’ll get you on the schedule fast, often same or next day when we can.