A heat pump blowing cool or lukewarm air in heating mode is one of the most common service calls we get in Upstate SC during winter. Sometimes it's completely normal. Sometimes it's a real problem. Here's how to tell the difference.
Normal: Defrost Mode
Heat pumps periodically enter a defrost cycle in cold weather. When the outdoor coil gets cold enough, frost can form on it — which reduces efficiency. The system automatically reverses to briefly heat the outdoor coil and melt the frost.
During defrost mode, the system temporarily blows cool or room-temperature air inside. This is normal and lasts 5–15 minutes. You may also see steam rising from the outdoor unit — that's the frost melting, not smoke.
If the cool air lasts less than 20 minutes and then returns to normal heating, defrost mode is the likely explanation.
Normal: Heat Pump Air Feels Cooler Than Furnace Air
Heat pumps deliver air at roughly 90–100°F. Gas furnaces deliver air at 120–140°F. If you're used to a gas furnace, heat pump air can feel cool by comparison — even when the system is working correctly.
The heat pump is still heating your home; it just does it more gradually and at a lower supply air temperature. If your home is reaching the set temperature, the system is working fine.
Problem: Low Refrigerant
Low refrigerant is one of the most common causes of a heat pump that won't heat properly. Refrigerant doesn't deplete on its own — if it's low, there's a leak. Signs include the system running constantly without reaching the set temperature, ice on the refrigerant lines, and a hissing or bubbling sound.
This requires a technician to locate the leak, repair it, and recharge the system.
Problem: Reversing Valve Failure
The reversing valve is what allows a heat pump to switch between heating and cooling mode. If it fails or gets stuck, the system may stay in cooling mode even when set to heat — blowing cold air in winter.
A stuck reversing valve is a relatively common heat pump repair. A technician can diagnose it with a temperature measurement across the valve and replace it if needed.
Problem: Outdoor Unit Not Running
If the outdoor unit isn't running, the heat pump can't move heat. The indoor blower may still run, pushing unconditioned air through the vents. Check that the outdoor unit is running when the system is in heating mode. If it's not, check the breaker for the outdoor unit.
Problem: Backup Heat Strips Not Working
On very cold days, heat pumps rely on backup electric heat strips to supplement heating. If the heat strips fail, the system may struggle to heat the home adequately when temperatures drop below 35–40°F. A technician can test the heat strips and replace them if needed.
Frequently Asked Questions
How long should defrost mode last?
Typically 5–15 minutes. If your heat pump is in defrost mode for more than 20–30 minutes, or cycles in and out of defrost frequently, there may be an underlying issue.
My heat pump is running but the house won't get warm. What's wrong?
Common causes include low refrigerant, a failing reversing valve, or backup heat strips that aren't working. If the outdoor temperature is below 35°F and the system is running constantly without reaching the set temperature, call a technician.
Can I run my heat pump in emergency heat mode?
Emergency heat mode bypasses the heat pump and runs only the backup heat strips. It will keep your home warm but is much less efficient. Use it only if the heat pump itself has failed and you're waiting for repair.