Ever stared at a humming heat pump and wondered why your home feels like a sauna instead of a cool retreat? If your unit is running nonstop yet the air stays warm, you’re dealing with a classic heat pump not cooling scenario. Below we break down how a heat pump should work, why it sometimes quits cooling, and what you can do before calling a technician.
Quick refresher: how a heat pump cools your home
At its core, a Heat Pump moves heat from inside your house to the outdoors using the refrigeration cycle. The cycle relies on several key parts:
- Compressor pressurizes the refrigerant, raising its temperature
- Refrigerant a fluid that absorbs and releases heat as it circulates
- Thermostat tells the system when to switch to cooling mode
- Outdoor Coil rejects heat to the outside air
- Indoor Coil absorbs heat from the indoor air
- Expansion Valve drops the refrigerant pressure, cooling it further
- Fan Motor pushes air across both coils
- Capacitor gives the fan motor that extra start‑up boost
When everything’s healthy, the indoor coil gets cold, the indoor fan blows chilled air, and the outdoor coil dumps the unwanted heat outside. If any link in that chain breaks, the pump may keep running but fail to cool.
Top reasons your heat pump runs without cooling
- Thermostat set to the wrong mode
It sounds basic, but a thermostat still stuck in “heat” or “auto” can keep the outdoor fan spinning while the cooling circuit stays idle. Verify the display shows “cool” and the temperature set point is lower than the room temperature. - Low refrigerant charge or a leak
The refrigerant is the coolant. If the system has lost charge-common after a pipe or coil puncture-the compressor still runs, but there isn’t enough fluid to absorb indoor heat. Look for ice buildup on the outdoor coil; that’s a tell‑tale sign. - Dirty air filter or coil
Clogged filters reduce airflow, causing the indoor coil to frost over. A frosted coil can’t exchange heat, so the unit runs idle while the fan struggles. - Frozen outdoor coil (defrost cycle problem)
During very cold nights, the outdoor coil can ice up. If the defrost timer or sensor fails, the coil stays frozen and can’t release heat, leaving the indoor side warm. - Faulty Compressor
A dying compressor may spin but not create the pressure needed for the refrigerant to circulate properly. You might hear unusual rattling or feel the unit vibrating more than usual. - Bad fan motor or capacitor
If the Fan Motor stalls, the indoor coil never gets cold air passed over it. A failed Capacitor can prevent the motor from starting at all, leaving the unit humming without airflow. - Electrical issues (circuit breaker, wiring)
A tripped breaker or loose connection can cut power to the cooling components while leaving the fan circuit alive. - Stuck expansion valve
The valve controls refrigerant pressure drop. If it jams, the refrigerant stays too warm to cool the indoor coil.
Diagnostic checklist - step‑by‑step
- Check thermostat settings. Confirm it’s on “cool” and the set temperature is at least 4 °F below the room temperature.
- Inspect the indoor air filter. If it’s dirty, replace it. A clean filter restores airflow instantly.
- Look at the outdoor unit. Is the coil covered in ice or snow? If yes, turn the system off for 30 minutes to allow it to melt, then reset the thermostat.
- Listen for the compressor. A steady humming sound means it’s running; any clicking or grinding suggests internal failure.
- Feel the indoor coil (the part behind the return grille). If it’s warm, the refrigerant isn’t circulating.
- Check the circuit breaker panel for any tripped switches. Reset any that are off.
- If you have a multimeter, test the voltage at the fan motor terminals. No voltage means an electrical fault upstream.
- When in doubt, call a licensed HVAC tech. Refrigerant handling requires certification.
Quick fixes you can try today
- Reset the thermostat. Remove it from the wall for a minute, then reinstall. Sometimes a simple reboot clears hidden mode glitches.
- Replace the air filter. A $10 filter can solve airflow problems that mimic a cooling loss.
- Clear debris around the outdoor unit. Trim bushes, sweep leaves, and gently rinse the coil with lukewarm water. Better airflow = better heat rejection.
- Power‑cycle the system. Turn off the breaker for the heat pump, wait 2 minutes, then turn it back on. This resets the compressor’s internal control board.
- Inspect the thermostat batteries. Some models still rely on AA cells; weak batteries can cause mode‑selection errors.
When to call a professional
If after the above steps the unit still runs without cooling, it’s time to get an expert. Typical professional interventions include:
- Recharging or replacing refrigerant (requires EPA‑certified handling).
- Replacing a failed compressor, fan motor, or capacitor.
- Diagnosing and fixing electrical wiring or control board issues.
- Testing and swapping the expansion valve.
- Running a full system pressure test to locate hidden leaks.
Professional service usually starts around $150 for diagnostics, with parts adding $100‑$800 depending on what's needed.
Maintenance tips to keep your heat pump cooling year‑round
- Seasonal filter change. Replace every 1‑3 months during heavy use.
- Annual coil cleaning. Hire a tech to clean both indoor and outdoor coils before summer.
- Schedule a refrigerant leak check. Most leaks are caught during the yearly service visit.
- Inspect the condensate drain. Clogged drains can cause the system to shut down the cooling cycle.
- Keep the outdoor unit clear. A 2‑foot clearance on all sides ensures proper airflow.
Consistent upkeep not only improves cooling performance but also extends the lifespan of expensive components like the compressor.
Comparison table - common causes, symptoms, and fixes
| Cause | Typical Symptom | DIY Fix | When to Call Pro |
|---|---|---|---|
| Thermostat set to heat | Unit runs, air stays warm | Switch to cool, lower set point | None |
| Low refrigerant | Ice on outdoor coil, warm air | Check for visible leaks, clear ice | Recharge or repair leak |
| Clogged air filter | Reduced airflow, warm output | Replace filter | None |
| Frozen outdoor coil | Unit runs but no cooling, coil covered in frost | Turn off, let melt, check defrost timer | Defrost sensor/timer replacement |
| Failed compressor | Humming sound, no temperature drop | None | Compressor replacement |
| Bad fan motor or capacitor | Fan doesn’t run, coil stays warm | Inspect motor, test capacitor | Motor or capacitor swap |
Frequently Asked Questions
Why does my heat pump run continuously but never reaches the set temperature?
The most common culprits are a low refrigerant charge, a dirty air filter, or a thermostat that’s still in heating mode. Start by checking the thermostat, swapping the filter, and looking for ice on the outdoor coil.
Is it safe to add refrigerant myself?
No. Refrigerants are regulated substances. Only EPA‑certified technicians can charge or recover them. DIY attempts can cause legal issues and damage the system.
My outdoor unit is making a clicking sound. What does that mean?
Clicking often indicates the compressor’s start‑up relay or overload protector is tripping. It could be a sign of low voltage, an over‑heated compressor, or a failing relay. Turn the unit off and call a technician.
How often should I clean the outdoor coils?
At least once a year, preferably before the cooling season begins. If you live near trees, a semi‑annual gentle rinse helps keep performance high.
Can a weak capacitor cause the unit to run without cooling?
Yes. A failing capacitor may let the fan motor start weakly or not at all, so the indoor coil never gets cold air while the compressor still spins.
