Heat Pump Not Working

By HVAC Load Calculate Team, licensed HVAC professionals · Updated July 2026

The most likely cause is stuck or failed reversing valve, or thermostat set to emergency heat. Heat pump fails to heat or cool, won't switch modes, or runs but produces no temperature change. Most cases take diy: 1 hour | professional diagnosis: 1-2 hours | reversing valve: 3-5 hours to resolve once you know which cause you are dealing with, and the checks below start with the ones you can do yourself in a few minutes.

How to Confirm This Is Your Problem

You are probably dealing with this exact issue if you notice one or more of these signs:

No heating or cooling produced
Won't switch between heating and cooling
Stuck in one mode
Ice buildup in winter heating mode
Blowing cold air in heat mode
Running constantly without results
Strange sounds from outdoor unit
Emergency heat running constantly

Step-by-Step Diagnosis

Follow these steps to identify the exact cause of your problem:

Verify thermostat is set correctly for season
Check if in emergency heat mode
Inspect outdoor unit for ice buildup
Listen for reversing valve click when switching modes
Check both outdoor and indoor units running
Verify sufficient airflow from vents
Look for error codes on thermostat
Check circuit breakers for both units
Note if problem is heating only or both modes
Verify outdoor temperature (may affect efficiency)
Check for blocked outdoor coils
Note if auxiliary heat is running constantly

What Causes It, Ranked by Likelihood

1. Stuck or failed reversing valve

The reversing valve decides whether the system heats or cools. Stuck in between, you get neither, just fan air. A tech confirms by checking pressures and energizing the solenoid. Replacement runs $500 to $1,500, worth weighing against system age.

One of the most frequent culprits. This one needs a professional, typical cost $400-1,200.

2. Low refrigerant from leak

Heat pumps live and die on charge because refrigerant does the work in both directions. Weak heating and weak cooling together strongly suggest low charge. Needs an EPA certified tech; recharging without fixing the leak just rents you a few months.

Worth checking after the common causes. This one needs a professional, typical cost $500-1,500.

3. Ice buildup on outdoor coils

Light frost in winter is normal and defrost cycles clear it. A solid ice block encasing the unit is not; it blocks all heat transfer and can bend fan blades. Shut the unit off, clear what you safely can with water (never a sharp tool), and find out why defrost is not keeping up.

Worth checking after the common causes. You can usually fix this yourself, typical cost $0-100.

4. Defrost control failure

When the defrost board or its sensor fails, the outdoor coil ices over in heating mode until output collapses. If ice rebuilds within hours of clearing, the defrost cycle is not firing. Board replacement runs $300 to $600.

Worth checking after the common causes. This one needs a professional, typical cost $200-500.

5. Failed compressor

A silent outdoor unit with the fan running, or a hum-click-stop pattern, points to the compressor or its start gear. Capacitor first, $150 to $400. A dead compressor on a unit past 10 years usually means replacement math.

Less common, but it happens. This one needs a professional, typical cost $1,500-3,000.

6. Thermostat set to emergency heat

EM heat runs the electric strips only, bypassing the heat pump entirely. The house stays warm but bills double or triple. If someone flipped it during a cold snap and forgot, switch back to HEAT and confirm the outdoor unit starts.

One of the most frequent culprits. You can usually fix this yourself at no cost.

7. Dirty air filter or coils

Heat pumps run more hours than any other system type, so filters and coils foul faster. Restricted airflow cuts capacity in both modes and forces long expensive runtimes. Monthly filter checks are the cheapest heat pump insurance there is.

Worth checking after the common causes. You can usually fix this yourself, typical cost $15-50.

DIY Solutions

You will need replacement air filter, garden hose, shovel (for snow), flashlight. Plan on 30-60 minutes + defrost time.

Check thermostat mode - ensure not on emergency heat
Replace air filter
Inspect outdoor unit for ice - turn off if iced
Clean outdoor coils with garden hose
Clear snow and debris from around unit
Raise outdoor unit if sitting in standing water
Switch between heat and cool modes to test
Listen for reversing valve click when switching
Check both breakers (indoor and outdoor)
Verify vents are open and unobstructed
Let unit defrost completely if iced (2-4 hours)
Test both heating and cooling modes after repairs

Before you start, know these safety limits:

Turn off power before cleaning outdoor unit
Never pour hot water on frozen unit - thermal shock can crack components
Don't use emergency heat continuously - it's expensive and for backup only

When to Call a Professional

Stop and call a licensed technician if you run into any of these situations:

Unit won't switch between modes after troubleshooting
Ice forms repeatedly on outdoor unit
No click from reversing valve when switching modes
Refrigerant leak suspected (oil spots on lines)
Emergency heat running all the time
Loud grinding from outdoor unit
Significant performance loss in cold weather
Unit is more than 10 years old with recurring issues

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How to Keep It From Happening Again

Schedule maintenance twice yearly (spring and fall)
Keep outdoor unit clear of snow and debris
Replace filters monthly during heavy use
Don't block outdoor airflow with shrubs
Install outdoor unit on raised platform in flood-prone areas
Test both heating and cooling modes seasonally
Install programmable thermostat with heat pump settings

Frequently Asked Questions

Why is my heat pump blowing cold air in winter?

Heat pumps blow cool air during defrost cycles (normal) or when in emergency heat mode. If it's constant, causes include: reversing valve stuck in cooling, low refrigerant, outdoor unit iced over, or failed defrost control. Temperatures below 25°F also reduce heat pump efficiency significantly.

How do I know if my heat pump reversing valve is bad?

Signs include: heat pump stuck in one mode, no click when switching modes, works in cooling but not heating (or vice versa), or lukewarm air in both modes. Listen for the reversing valve click when switching between heat and cool - no click indicates valve failure.

Should my heat pump have ice on it in winter?

Light frost is normal. The heat pump should automatically defrost every 30-90 minutes. Heavy ice buildup, ice that doesn't melt, or icing in temperatures above 35°F indicates problems with the defrost cycle, refrigerant levels, or airflow.