Does this match your problem?
- Heat pump blows lukewarm air in heating mode
- Outdoor unit is coated in ice or steam
- Electric bill spiked in winter
- System runs constantly but temperature won't rise
- Reversing valve makes a whooshing sound
Quick check first
In heating mode, heat pumps deliver air around 90–95°F, noticeably cooler than a gas furnace's 120°F. This is normal. If air temp is below 85°F, keep troubleshooting.
Step-by-step troubleshooting
1. Confirm the outdoor unit is running
Unlike an AC, the outdoor unit runs in winter too. If it's silent while the thermostat calls for heat, check the outdoor disconnect and breakers. A tripped outdoor breaker is a top cause of 'my heat pump won't heat' calls.
2. Look for a normal defrost cycle
In cold, damp weather the outdoor coil frosts up. Every 30–90 minutes the system reverses briefly, blowing warm air outdoors to melt the frost, you'll see steam rising from the unit. This is normal and lasts 5–15 minutes.
3. Check for a fully iced outdoor coil
If the entire coil is buried in ice (not just frost), defrost has failed. Common causes: bad defrost board, stuck reversing valve, or the coil is buried in snow/leaves. Kill power, clear any physical blockage, and call a pro.
4. Verify emergency heat isn't stuck ON
If the thermostat is set to EMERGENCY HEAT or EM HEAT, the heat pump is off and you're running expensive electric strips. This explains huge winter bills. Switch to HEAT (auto mode) unless you have a known compressor fault.
5. Inspect the air filter
Pull the filter and hold it to a light. A loaded filter starves a heat pump the same way it starves any system: capacity drops, the indoor coil can freeze in cooling mode, and the electric strips run hotter than designed in heating mode. If light barely passes through, you've found a likely cause. Sizing, filter grade, and the replacement routine are in the DIY Quick-Fix Playbook, or a tech can handle it on a visit.
6. Check the auxiliary heat function
When outdoor temps drop below ~35°F, the electric strips should assist the heat pump. If the thermostat shows AUX or EM without you setting it, and stays there for hours, either the heat pump can't keep up (needs service) or a control board issue is forcing aux heat.
7. Listen for the reversing valve
A single whoosh or clunk when switching between heating and cooling is normal. Constant clunking, or the system stuck in one mode, points to a failed reversing valve solenoid.
Stop and call a licensed pro if:
- •Outdoor coil is fully iced over for hours
- •System stays in emergency heat even after you switch to HEAT
- •Air temperature at the vents is below 80°F in heating mode
- •Reversing valve makes repeated clunking, or system won't switch modes
- •Electric bill doubled and you're not on emergency heat