Heating Emergency
Heat Pump Not Keeping Up?
Heat pumps are efficient in mild weather but struggle when Houston gets a hard freeze. Here's what's normal and what isn't.
What's Going On
Understanding the Problem
Your heat pump system isn't producing enough warm air, or the air from the vents feels lukewarm instead of hot. Heat pumps work differently from gas furnaces — they extract heat from outdoor air and move it inside. When outdoor temperatures drop below 35–40°F, heat pumps lose efficiency and may need supplemental (auxiliary) heat to keep up.
Diagnosis
Common Causes
1Temperature too low for the heat pump
Heat pumps are very efficient above 40°F but lose capacity as outdoor temps drop. Below 30°F, most heat pumps can't keep up alone and rely on auxiliary electric heat strips (AUX or EM HEAT on your thermostat) to supplement.
2Auxiliary heat not working
If the outdoor temperature is very low and your system has auxiliary heat strips that have failed, the heat pump alone won't be enough. Your thermostat should show AUX or EM HEAT when these are active.
3Outdoor unit frozen over
A light coating of frost on the outdoor unit is normal in cold weather — the defrost cycle handles it. But if the entire unit is encased in ice, the defrost cycle has failed. The system can't exchange heat through a block of ice.
4Defrost board failure
The defrost control board tells the system when to reverse the cycle and melt ice off the outdoor coil. If it fails, ice builds up unchecked and the system loses heating capacity.
5Reversing valve stuck in cooling mode
The reversing valve switches the system between heating and cooling. If it's stuck or the solenoid has failed, the system may be running in cooling mode even though the thermostat is set to heat.
6Low refrigerant
Heat pumps use refrigerant for both heating and cooling. A leak reduces the system's ability to move heat, and heating performance drops significantly — especially in cold weather when the system is already working hard.
DIY Troubleshooting
What You Can Try
Check if AUX heat is engaging
Look at your thermostat. When it's very cold outside, you should see AUX or AUX HEAT on the display. If you don't, the auxiliary heat strips may not be working. Try switching to EM HEAT (emergency heat) to force them on.
Check the outdoor unit for ice
Go outside and look at the unit. Light frost is normal. If the entire coil is covered in thick ice, turn the system to EM HEAT (this stops the outdoor unit) and call for service. Don't try to chip the ice off.
Make sure the outdoor unit fan is spinning
In heating mode, the outdoor fan should be running. If it's not, the system can't pull heat from the outdoor air. Check the breaker for the outdoor unit.
Switch to EM HEAT as a temporary fix
Emergency heat runs only the auxiliary electric heat strips, bypassing the heat pump entirely. It's more expensive to operate, but it will keep your house warm until a tech can look at the heat pump.
Know When to Call
When to Call a Pro
If the outdoor unit is iced over, the auxiliary heat isn't working, or the system is blowing cold air in heating mode, call a technician. Heat pump diagnostics require checking refrigerant pressures, the reversing valve, and the defrost cycle — all professional-level work.
Pro Tip
Don't panic if your heat pump air doesn't feel as hot as a gas furnace. Heat pumps produce air around 90–100°F (compared to 130–140°F from a gas furnace). It feels lukewarm at the vent but still heats the house — just more gradually. If the house is maintaining temperature, the system is working correctly.
More Troubleshooting