A heat pump that blows cool air is not always broken. Heat pump air comes out at 90–100°F, which feels cooler than a furnace’s 130°F blast, and several times an hour in cold weather the unit runs a defrost cycle that briefly sends cool air through the vents. But if the house keeps getting colder, something needs attention. Work through these causes in order; the first five you can check yourself.
First: is it actually a problem?
- Air feels lukewarm but the house holds temperature: normal.
- Cool air for 2–10 minutes, steam rising from the outdoor unit: defrost cycle. Normal.
- Cool air for more than 15 minutes and the house is cooling down: keep reading.
1. Thermostat set to cool or fan “On”
Check that the thermostat is on Heat, the setpoint is above room temperature, and the fan is on Auto. With the fan on “On”, the blower keeps running between heating cycles and pushes room-temperature air that feels cold.
2. Dirty air filter
A clogged filter cuts airflow across the indoor coil. The system overheats, trips a safety switch and stops heating. Replace the filter and restart the system.
3. Outdoor unit blocked by snow or ice
Some frost on the outdoor coil is normal and the defrost cycle clears it. A solid block of ice that stays for hours means defrost is failing or water is refreezing from a roof drip or poor drainage. Clear snow away by hand, turn the system to Emergency Heat to protect the compressor, and call for service if the ice returns. Never chip at the coil.
4. Tripped breaker or outdoor disconnect off
The outdoor unit and the indoor air handler are often on separate breakers. If the indoor fan runs but the outdoor unit is silent, check the breaker panel and the disconnect box next to the outdoor unit. If a breaker trips again right away, stop and call a technician.
5. Closed vents or blocked returns
Closing supply vents in unused rooms does not save energy with a heat pump. It lowers airflow and can cause the problems in cause 2. Keep at least 80% of vents open and return grilles clear of furniture.
6. Low refrigerant
A heat pump is a sealed system, so low refrigerant always means a leak. Symptoms include weak heat, ice on the refrigerant lines and a hissing sound. A technician needs to find and repair the leak, then recharge; that usually costs $100 to $600 or more.
7. Stuck reversing valve
If the heat pump blows cold air in heat mode and warm air in cool mode, the reversing valve or its solenoid may have failed. Replacing the valve is a major repair, typically $1,000 to $2,000, because the refrigerant has to be recovered and the valve brazed in.
8. Failed capacitor or contactor
If the outdoor fan or compressor hums but does not start, the start capacitor is a likely cause. It is one of the most common heat pump repairs and usually costs $100 to $250. Capacitors hold a dangerous charge even with the power off, so leave this to a technician.
9. It is simply too cold outside
Standard heat pumps lose capacity as the temperature drops. Below the “balance point”, often 25–35°F for older models, the heat pump alone cannot keep up and backup heat should switch on. If the backup heat strips have failed, the house slowly cools on very cold days. Modern cold-climate heat pumps keep most of their output down to 5°F.
Emergency heat vs auxiliary heat
Auxiliary heat turns on automatically when the heat pump needs help: during defrost, when you raise the setpoint by several degrees, or on very cold days. It works alongside the heat pump.
Emergency heat is a setting you choose. It shuts the heat pump off completely and heats the house with backup heat only. Use it when the outdoor unit is damaged, iced solid or making alarming noises, until it can be repaired. Emergency heat usually runs on electric resistance strips, which cost two to three times as much per hour as the heat pump, so don’t leave it on longer than needed.
When to call a technician
- A breaker trips repeatedly
- Ice covers the whole outdoor unit after a defrost cycle
- You hear grinding, screeching or loud buzzing
- Cool air lasts more than 15 minutes with the thermostat set correctly
Regular heat pump maintenance prevents most of these problems.