A heat pump not heating is usually caused by an incorrect thermostat setting, dirty air filter, iced or blocked outdoor unit, stuck reversing valve, or another system problem, although brief cool air during a defrost cycle is normal. If the cool air continues or the house cannot reach the thermostat setting, the system needs attention.
Is It Just the Heat Pump’s Defrost Cycle?
A heat pump periodically enters a defrost cycle in cold weather to melt frost from the outdoor coil, and brief cool air indoors or steam outside during this cycle is normal.
The outdoor unit collects frost as it pulls heat from cold outdoor air. Defrost mode temporarily reverses operation to warm the outdoor coil and remove that frost.
Steam rising from the outdoor unit / condenser during defrost is water vapor, not smoke. Normal defrost lasts only a short time before the system returns to heating.
What Are the Common Reasons a Heat Pump Is Not Heating?
A heat pump can stop heating because of thermostat settings, restricted airflow, outdoor-unit icing, blocked airflow, a reversing-valve problem, low refrigerant, electrical trouble, or failed backup heat.
| Likely cause | What’s happening | What to do |
|---|---|---|
| Thermostat set wrong | Not on HEAT, set too low, or fan on ON | Set to HEAT, above room temp, fan on AUTO |
| Dirty air filter | Restricted airflow reduces heat | Replace the filter (DIY) |
| Iced-over outdoor unit | Frost buildup blocks heat transfer | Switch to EM heat; call a technician |
| Blocked outdoor unit | Snow, leaves, or debris restrict airflow | Gently clear the area (DIY) |
| Stuck reversing valve | Unit stuck in cooling and blows cold air | Call an HVAC technician |
| Low refrigerant | Not enough charge to move heat | Call an HVAC technician |
A dirty air filter or blocked outdoor unit is easy to check. A reversing valve, refrigerant, sensor, or internal electrical problem requires professional diagnosis.
What Do “Aux Heat” and “Emergency Heat” Mean?
Auxiliary heat automatically helps the heat pump when additional heating is needed, while Emergency Heat is a manual setting that relies on backup heat instead of normal heat-pump operation.
| Setting | What it does | When to use |
|---|---|---|
| Heat | Heat pump provides efficient heating | Normal, everyday setting |
| Aux (auxiliary) heat | Backup heat strips assist automatically | Kicks in on its own in cold weather – normal |
| Emergency (EM) heat | Runs backup heat only, no heat pump | Only if the heat pump fails; less efficient |
Seeing auxiliary (AUX) heat on the thermostat during colder weather does not automatically indicate a problem. The system can activate its heat strips when the heat pump alone cannot satisfy the current heating demand.
Emergency (EM) heat is different. It should be used when the heat pump itself has failed and backup heat is needed until the system can be repaired.
What Can You Check Yourself When a Heat Pump Isn’t Heating?
Homeowners can safely check the thermostat, filter, breakers, and outdoor-unit airflow before calling an HVAC technician for a heat pump that will not heat.
- Check the thermostat. Make sure the thermostat is set to HEAT, the temperature is above the current room temperature, and the fan is set to AUTO.
- Inspect the air filter. Replace a dirty filter that is restricting airflow.
- Check the breakers serving the indoor and outdoor equipment. If one has tripped, reset it once. If it trips again, leave it off and call a technician.
- Inspect the outdoor unit. Gently remove leaves, loose snow, and other debris restricting airflow.
- Look for excessive ice. A little frost can be normal, but a unit encased in ice or repeatedly freezing needs service.
Do not chip ice from the coil, force hot water over the equipment, open the unit, or handle refrigerant lines. If the outdoor unit is heavily frozen, switch to Emergency Heat and call a technician.
When Should You Call an HVAC Technician?
Call an HVAC technician if the heat pump keeps blowing cold air, repeatedly freezes, trips a breaker, or cannot warm the house after the basic thermostat, filter, and outdoor-unit checks.
Persistent problems can indicate low refrigerant, a stuck reversing valve, failed sensor, electrical fault, or backup heat that is not engaging. These problems require testing inside the equipment.
GAC provides heat pump repair and 24/7 availability for no-heat situations. Routine maintenance and a heat pump tune-up can help identify developing problems before they lead to larger heating issues.
ENERGY STAR recommends regular heat-pump maintenance, including filter care and professional service, to support efficient operation and help prevent equipment problems. ENERGY STAR heat pump maintenance guidance
When Should You Call GAC Services?
GAC Services provides heat pump diagnosis and repair throughout Gaithersburg and Montgomery, Frederick, and Howard Counties with 24/7 availability.
GAC’s HVAC technicians serve Gaithersburg, Rockville, Silver Spring, Germantown, Frederick, Bethesda, Potomac, and surrounding Maryland communities. The team offers upfront pricing and free estimates for heating work.
If an existing system can no longer meet the home’s needs, GAC offers heat pump installation & replacement. Additional heating services, including furnace repair, are available for other heating-system problems.
Schedule Online (240) 224-0192
Whether you require installation, repair, or maintenance, our technicians will assist you with top-quality service at any time of the day or night. Take comfort in knowing your indoor air quality is the best it can be with MOE heating & cooling services Ontario's solution for heating, air conditioning, and ventilation that’s cooler than the rest.
Contact us to schedule a visit. Our qualified team of technicians, are always ready to help you and guide you for heating and cooling issues. Weather you want to replace an old furnace or install a brand new air conditioner, we are here to help you. Our main office is at Kitchener but we can service most of Ontario's cities
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