Heat Pump Blowing Cold Air in Winter? Causes and Fixes

A heat pump blowing cold air in winter may be in its normal defrost cycle, or it may be set wrong, starved of airflow or low on refrigerant. Check that the thermostat is on Heat, replace a dirty filter, look at the outdoor unit for ice, and wait 10 to 15 minutes. If the air stays cold after defrost, call an HVAC technician.

Heat pumps are not furnaces. They feel different, and some cold air is normal. The question is whether what you feel is normal or a fault.

Is a heat pump blowing cold air normal?

Sometimes. Here is the difference:

• Normal: brief cool air for a few minutes, steam or a whooshing sound from the outdoor unit, then warm air again. This is the defrost cycle.
• Normal: air that feels cooler than a furnace's air, but is above room temperature. A heat pump typically delivers air in roughly the 90s in degrees Fahrenheit, which feels cool on your hand but still heats the house.
• Not normal: air that stays at room temperature or colder, or that never warms up after defrost.

If you are not sure, hold a thermometer near a vent. Air noticeably warmer than the room, even if it feels mild, means it is heating.

Why is my heat pump blowing cold air?

• The defrost cycle. In cold, damp weather the outdoor coil ices up. The heat pump briefly runs in reverse to melt it, which sends cool air indoors. Backup electric heat should come on to help.
• The wrong thermostat setting. Cool mode, or the fan set to On, can blow unheated air.
• A dirty air filter. Restricted airflow makes the system less effective and can ice up the coil.
• Ice on the outdoor unit. A thick coating means the defrost system is not keeping up.
• Very cold weather. As outdoor temperatures drop toward freezing and lower, many heat pumps lose output and lean more on backup heat.
• Low refrigerant. A leak reduces heating, and no amount of resetting fixes it.
• A stuck reversing valve. The valve switches between heating and cooling. If it sticks, the unit can stay in cooling mode.
• The outdoor unit is not running. A tripped breaker, a failed capacitor or a failed fan motor means the indoor blower runs with no heat made.
• Leaky ducts. Warm air escapes before it reaches the rooms.

What the pattern tells you

How to fix it

1. Check the thermostat. Set it to Heat, with a temperature at least five degrees above the room. Set the fan to Auto. If the screen is blank, see what a blank thermostat means.
2. Wait 10 to 15 minutes. If the outdoor unit is steaming or making a whooshing noise, it is probably in defrost.
3. Replace the air filter. A clogged filter is one of the most common causes of poor performance.
4. Open all vents. Make sure supply registers and the return grille are clear.
5. Look at the outdoor unit. A thin frost that clears during defrost is fine. Thick ice, or ice on the fan blades, is not. Clear leaves, snow and debris from around it, leaving at least a couple of feet of space.
6. Do not chip the ice. Switch the thermostat to Off or Fan so the ice can melt. Never use a tool on the coil, because the fins are fragile.
7. Check the breakers. The outdoor unit and the indoor air handler may be on separate breakers. Look for a trip.
8. Use backup heat if you must. If the house is cold and the heat pump is struggling, the emergency heat setting runs electric strips. It works, but costs more to run, so use it only until a technician comes.

What is emergency heat?

Heat pumps usually have auxiliary heat, electric strips that help when the heat pump cannot keep up, and emergency heat, a setting that runs only the strips. Aux heat should come on by itself during defrost and on very cold days. Emergency heat is for when the outdoor unit is not working. Leaving it on all winter can raise your electric bill noticeably, because electric resistance heat uses far more electricity than a working heat pump. If the aux light is on all the time, and the house is not warm, the heat pump may have a fault.

How cold is too cold for a heat pump?

Modern heat pumps work at low temperatures, and cold-climate models are built for it, but output and efficiency fall as it gets colder. Many standard units lean heavily on backup heat once outdoor temperatures drop well below freezing. If your system seems to lose its edge only on the coldest days, that may be a limit of the equipment, not a fault. A technician can check whether the heat pump is sized right and whether the backup heat is working.

Is it worth repairing?

A dirty filter, a thermostat setting or a breaker are free fixes. A capacitor or a fan motor is a moderate repair. A refrigerant leak, a compressor or a reversing valve can be a large expense. If your heat pump is 12 to 15 years old, or the repair quote is a big share of a new unit, compare the two. Ask about energy savings and any rebates, since many areas offer incentives for heat pumps.

When to call a professional

Call an HVAC technician if:

• Air stays cold after a full defrost cycle.
• The outdoor unit is covered in thick ice, or ices up again right after melting.
• The outdoor fan or compressor does not run.
• You hear hissing, banging or grinding, or smell burning.
• The aux heat runs constantly and the house is still cold.
• You suspect a refrigerant leak, such as oily residue on the lines.

If your home still feels cold, which company to call for no heat can help you choose the right pro, and finding air leaks around the house can help it hold the heat you make.

What not to do

• Do not chip or pour hot water on ice.
• Do not run emergency heat all winter by default.
• Do not cover the outdoor unit.
• Do not add refrigerant yourself.

Bottom line

A short burst of cool air during defrost is normal, and heat pump air always feels cooler than furnace air. Cold air that lasts is a fault: check the thermostat, filter and outdoor unit, then call a technician. If you have a gas furnace instead, read gas furnace cold air troubleshooting.