Heat Pump Not Heating: Defrost, Reversing Valves, and Aux Heat.

A heat pump not heating may be in the wrong thermostat mode, low on airflow, stuck in defrost, short on refrigerant, or unable to shift the reversing valve. You can safely check settings, filter, vents, breaker, outdoor disconnect, and ice buildup. Refrigerant, capacitors, reversing valves, and compressor tests are pro work.

What’s actually happening

A heat pump is not a furnace and it is not simply an air conditioner running backward, even though that is the rough idea. In heating mode, it moves heat from outside air into the house. The outdoor coil gets cold because it is absorbing heat. The indoor coil gets warm because it is releasing heat. A reversing valve changes refrigerant flow so the same equipment can cool in summer and heat in winter.

That means heat-pump air often feels different from gas-furnace air. A gas furnace may send very warm air from the registers. A heat pump may send air that is warm enough to heat the house but not hot against your hand. People sometimes think the system is broken because the air does not feel dramatic. The better test is whether the indoor temperature is rising over time and whether the system can satisfy the thermostat.

Defrost adds another layer. When the outdoor coil gets cold and damp, frost can form. The heat pump periodically switches into defrost to melt that frost. During defrost, the outdoor unit may steam, the fan may stop, and the system may use auxiliary heat to keep indoor air from feeling cold. A short defrost cycle is normal. A unit stuck in defrost, covered in thick ice, or blowing cold for long periods is not.

Arizona systems have their own stress pattern. Cooling season does most of the heavy lifting. Heating may sit mostly unused, then get called on during a cold snap. Homes are generally slab-on-grade with no basements, and ducts often run through attics that hit 150°F+ in summer. That attic life is tough on duct seals, air handlers, wiring, and controls. When heat finally runs, airflow problems and weak electrical parts become obvious.

Likely causes, from common to less common

Start by deciding whether the heat pump is running at all, running but not warming, warming weakly, or cycling through defrost behavior. The same complaint can come from a thermostat setting, airflow restriction, refrigerant-side problem, or control failure.

Checks you can safely make yourself

1. Set the thermostat to heat. Raise the setpoint five degrees above the room temperature. Make sure it is not in cool, off, or a vacation schedule.
2. Use auto fan for troubleshooting. Fan on can move room-temperature air between heat cycles and make the system feel colder than it is.
3. Check emergency heat status. Emergency heat should not be used as a normal setting unless the heat pump side has failed or a technician tells you to use it temporarily. It costs more to run.
4. Replace the filter. Heat pumps need steady airflow across the indoor coil. A packed filter can make the system run long and heat poorly.
5. Open supply vents and clear returns. Closed rooms and blocked returns reduce airflow and can make the system behave like it is undersized or failing.
6. Look at the outdoor unit. It should be clear of leaves, weeds, stored items, and packed debris. Light frost during cold damp weather can be normal. Heavy ice that does not clear is not.
7. Check the outdoor disconnect visually. Make sure it is seated if accessible and safe. Do not open live electrical compartments.
8. Check breakers one time. Heat pumps may have indoor and outdoor breakers. Reset a tripped breaker once. If it trips again, leave it off.
9. Watch for defrost. Steam from the outdoor unit during defrost can look alarming. If it clears within several minutes and heat resumes, that can be normal.
10. Check condensate float switches. A wet pan or tripped float can interrupt operation on some systems.

Do not remove electrical covers, test capacitors, add refrigerant, force contactors closed, or tap on the reversing valve. Capacitors hold charge. Refrigerant is licensed work. Gas stays closed if your home has any gas appliance nearby; do not mix heat-pump checks with gas-line experimenting. If the outdoor unit is iced solid, do not chip the coil with tools. Turn the system off or to emergency heat only if needed for temporary warmth, then call.

When to stop and call

Call when the outdoor unit will not run, the breaker trips twice, the system blows cool air for more than a brief defrost period, ice builds heavily on the outdoor coil, the thermostat calls for heat but the house keeps dropping, or emergency heat does not work. Also call if the system cools in heat mode or changes modes unpredictably. That points toward reversing valve, control, or refrigerant-side trouble.

Continuing to run a heat pump with low refrigerant, poor airflow, or a failing outdoor electrical part can damage the compressor. Running on auxiliary heat for days can raise energy use fast. Running with heavy ice can bend fins, block airflow, and make the system work harder while producing less heat. Heat pumps reward early diagnosis because small operating problems can look like normal long run times until they are not.

A proper visit should measure what the system is doing, not guess from vent feel. The technician should check thermostat command, airflow, outdoor operation, defrost controls, temperature split, refrigerant indicators, reversing-valve operation, auxiliary heat, and electrical components. Yeti Pros is veteran-owned & operated, AZ ROC #363372 · ROC #363423. We are a service-area business with no street address; we come to you. Office hours are Mon–Fri 8–5 with 24/7 emergency service. The diagnostic or service-call fee is disclosed at booking and credited toward the repair. After an in-person look, you get a flat-rate written quote before work starts. The quoted number is what you pay unless we find something and stop to re-quote. For local repair scheduling, use heat pump repair.

What the fix usually involves

Heat-pump heating repairs range from simple control corrections to refrigerant-circuit diagnosis. A thermostat setup issue, clogged filter, blocked outdoor coil, loose low-voltage connection, or failed sensor is on the smaller end. A capacitor, contactor, defrost board, fan motor, auxiliary heat relay, heat-strip component, reversing-valve coil, or refrigerant leak is more involved. Compressor and reversing-valve body problems are major repairs because they touch the sealed system.

The technician should prove heating mode operation, confirm defrost behavior, test auxiliary heat safely, and make sure the system can maintain temperature under real conditions. A heat pump that appears to run can still be failing if the outdoor unit is not moving heat or if the indoor strip heat is doing all the work.

Repair parts carry a 1-YEAR manufacturer parts warranty. Yeti Pros provides a 2-YEAR written workmanship warranty. Financing is available, and there is a 10% military, veteran & first responder discount. If you are sorting no-heat symptoms before scheduling, use contact or call (520) 556-8929.

Straight answers

Why is my heat pump running but not heating?

The indoor blower can run even when the outdoor unit is not moving heat. Common causes include thermostat settings, dirty filters, a tripped outdoor breaker, failed capacitor, defrost problem, low refrigerant, or reversing-valve trouble. Check settings, filter, vents, outdoor clearance, disconnect, and one breaker reset. If it still will not heat, call.

Is it normal for a heat pump to blow cool air?

Briefly, sometimes. Heat-pump supply air can feel cooler than furnace air, and defrost can make air feel cooler for a short period. It is not normal for the house temperature to keep dropping, for cool air to continue through long cycles, or for the outdoor unit to sit iced over while the system runs.

Should I switch to emergency heat?

Use emergency heat as a temporary setting when the heat pump side is not working and you need heat until service. Do not use it as a normal comfort setting. It usually costs more to run and can hide the real failure. If emergency heat does not warm the house, the auxiliary heat side may also need repair.

What does a bad reversing valve look like?

From inside the house, it may look like the system is stuck cooling when it should heat, or it may refuse to change modes. Sometimes the issue is the valve coil or control signal, not the valve body. Either way, diagnosis needs electrical testing and refrigerant-side understanding. It is not a homeowner adjustment.