System runs but air is barely cool
The indoor fan runs and air comes out, but the house temperature does not drop much.
Start here: Start with thermostat settings, filter condition, and blocked vents or returns.
HVAC Troubleshooting
Heat Pump Not Cooling
Direct answer: If your heat pump is not cooling, the most common causes are thermostat settings, a clogged heat pump air filter, blocked airflow, a tripped breaker, or an outdoor unit that is not running. If the system is iced over, short on airflow, or the outdoor unit hums but will not start, stop there and move carefully.
Most likely: Start with the thermostat mode and setpoint, then check the heat pump air filter, supply and return vents, and whether the outdoor unit fan is running.
A heat pump that runs but does not cool can look like a refrigerant problem when it is really a setting, airflow, or power issue. Separate those simple checks first. If you find ice, burning smells, loud electrical buzzing, or a dead outdoor unit with power present, this moves out of basic DIY quickly.
Don’t start with: Do not start by opening electrical panels, replacing capacitors, adding refrigerant, or guessing at hidden parts.
Warm air from ventsCheck thermostat mode, setpoint, and filter before assuming the heat pump is low on refrigerant.
Outdoor unit not runningLook for a tripped breaker or disconnect issue first, then stop if the unit hums, clicks, or trips power again.
Last reviewed: 2026-03-31
What kind of cooling failure do you have?
Indoor unit runs but outdoor unit is off
You hear airflow inside, but the outdoor section is silent or only clicks.
Start here: Check breakers and the outdoor disconnect, then stop if power keeps tripping or the unit only hums.
Airflow is weak and cooling fades
Some rooms get little air, the system runs a long time, or the coil may start icing.
Start here: Check the heat pump air filter, return grilles, supply registers, and any visible ice.
It cools for a while, then stops keeping up
The system may start normally but loses cooling after running, especially in hot weather.
Start here: Look for a dirty filter, iced refrigerant lines, a dirty outdoor coil, or an outdoor fan that slows or stops.
Most likely causes
1. Wrong thermostat mode, setpoint, or fan setting
A heat pump can appear to run normally while never calling for real cooling if the mode or setpoint is off.
Quick check: Set the thermostat to Cool, lower the setpoint at least 3 to 5 degrees below room temperature, and leave the fan on Auto.
2. Restricted airflow from a clogged heat pump air filter or blocked vents
Low airflow makes cooling weak and can ice the indoor coil, which makes the air feel even warmer.
Quick check: Inspect the filter, open supply registers, and make sure return grilles are not blocked by furniture or rugs.
3. Outdoor unit not operating correctly
If the outdoor fan or compressor is not running, the indoor unit may still blow air but it will not remove much heat.
Quick check: Listen and look outside after the thermostat calls for cooling. The fan should run and the unit should sound steady, not just click or hum.
4. Iced coil, dirty outdoor coil, or refrigerant-related fault
Ice on the lines or coil, or a system that cools briefly then fades, points to airflow trouble first and refrigerant or sealed-system trouble if airflow is normal.
Quick check: Look for frost or ice on the refrigerant line or indoor coil area, and check whether the outdoor coil is packed with lint, cottonwood, or debris.
Step-by-step fix
Step 1: Confirm the thermostat is actually calling for cooling
A wrong mode, fan setting, or schedule problem is common and safe to rule out first.
- Set the thermostat to Cool, not Heat, Emergency Heat, or Off.
- Lower the setpoint at least 3 to 5 degrees below the current room temperature.
- Set the fan to Auto so you can tell whether the system is cycling normally.
- If the thermostat uses batteries and the display is weak or blank, replace the batteries if your model has them.
- Wait several minutes and listen for both the indoor unit and the outdoor unit to start.
Next move: Cooling starts and the air from the vents turns noticeably cooler. Leave it running and monitor the temperature drop over the next 15 to 30 minutes. Go to airflow checks next. If the indoor unit runs but the outdoor unit does not, move to the power and outdoor-unit step after that.
What to conclude: This rules out a simple control setting problem first.
Stop if:
- The thermostat is blank and does not come back after simple battery replacement.
- You smell burning plastic or see sparking near the thermostat or air handler.
- The system starts and immediately trips a breaker.
Step 2: Check the heat pump air filter and basic airflow
Restricted airflow is one of the most common reasons a heat pump stops cooling well or starts icing.
- Turn the system off at the thermostat before removing the filter.
- Pull out the heat pump air filter and inspect it against a light. If it is packed with dust, replace it with the same size and airflow direction.
- Make sure return grilles are open and not blocked by furniture, curtains, or rugs.
- Open supply registers in the rooms you want cooled.
- If your indoor unit has a visible coil access area and you see ice, leave the system off and let it thaw before restarting cooling.
Next move: Airflow improves and the system begins cooling normally again after restart. The clogged filter or blocked airflow was likely the main problem. If airflow is still weak or you found ice, keep the system off and continue with the icing and outdoor-unit checks.
What to conclude: Low airflow can cause weak cooling and coil freeze-up.
Stop if:
- The filter slot or indoor cabinet is wet, heavily iced, or leaking around electrical parts.
- You cannot restore airflow without opening sealed panels or disturbing wiring.
- The system ices up again soon after a clean filter and open vents.
Step 3: Look for icing and clean only what is safely accessible
Ice changes the diagnosis. It usually points to airflow trouble first, but it can also signal a refrigerant or metering problem that needs a pro.
- With power off at the thermostat, inspect the larger insulated refrigerant line near the outdoor unit and any visible indoor coil area for frost or ice.
- If you find ice, switch the thermostat to Off and the fan to On only if the indoor blower still works, so the coil can thaw.
- Once thawed, gently clear leaves, grass, and debris from around the outdoor unit and keep at least a clear working space around it.
- If the outdoor coil fins are dusty, rinse the exterior gently with plain water from the inside out only if you can do it without opening electrical compartments or bending fins.
- Do not chip ice off coils or lines and do not use chemicals or high pressure water.
Next move: After thawing and restoring airflow, the heat pump cools normally again. The issue was likely airflow restriction or a dirty outdoor coil. If ice returns, cooling stays weak, or the blower will not run in fan-only mode, stop DIY and schedule service.
Stop if:
- Ice is heavy inside the cabinet or around wiring.
- The indoor blower does not run during fan-only thawing.
- You suspect a refrigerant leak or hear hissing from the system.
Step 4: Check whether the outdoor unit has power and is running normally
A heat pump can blow indoor air without cooling if the outdoor section has lost power or a start component has failed.
- Listen for the outdoor unit after the thermostat calls for cooling. A normal unit should start and run with a steady fan and compressor sound.
- Check the HVAC breakers in the main panel for a tripped breaker. Reset a tripped breaker once only.
- If your outdoor unit has an external disconnect and it is safely accessible, make sure it is fully inserted or switched on.
- Watch for clear symptoms: totally dead, repeated clicking, loud humming, fan not spinning, or breaker tripping again.
- If the outdoor fan runs but the air is still not cooling after airflow checks, note that for the service call rather than opening the unit.
Next move: The outdoor unit starts and stays running, and cooling improves. A power interruption or disconnect issue was likely the cause. If the breaker trips again, the unit only hums, or the outdoor section stays dead with power available, stop here and call for service.
Stop if:
- A breaker trips more than once.
- The outdoor unit hums, buzzes loudly, or smells burnt.
- You would need to remove electrical covers to continue.
Step 5: Decide between a maintenance fix and a service call
By this point you have ruled out the safe homeowner checks and can avoid guessing at expensive parts.
- If cooling returned after correcting settings, replacing the heat pump air filter, opening vents, or thawing the coil, keep using the system and watch for repeat icing or long run times.
- If the system still runs but does not cool well, write down what you observed: weak airflow, ice, outdoor unit dead, outdoor fan running without good cooling, or breaker trips.
- If the unit cools poorly only in one mode and has trouble switching seasons, mention that to the technician because reversing-valve or control issues are possible but not DIY-friendly.
- If your heat pump is now failing in heating mode instead of cooling mode, use the related heat pump not heating guide for the right next checks.
- Schedule professional service for refrigerant, compressor, capacitor, contactor, control, or reversing-valve faults rather than replacing hidden electrical parts by guesswork.
A good result: You have either restored cooling with a safe maintenance fix or narrowed the problem enough for a focused service visit.
If not: Do not keep forcing the system to run hot, iced, or on a tripping breaker. Leave it off and book service.
What to conclude: The remaining faults are usually not safe or reliable DIY repairs.
Stop if:
- The house is getting hotter and the system is icing or tripping power.
- You hear grinding, hard buzzing, or metal-on-metal noise from the outdoor unit.
- Any step would require refrigerant work or live electrical testing.
Replacement Parts
Repair Riot may earn a commission from qualifying purchases, at no extra cost to you.
FAQ
Why is my heat pump running but not cooling the house?
The usual causes are wrong thermostat settings, a clogged heat pump air filter, blocked airflow, an iced coil, or an outdoor unit that is not running. Start with settings and airflow before assuming a refrigerant problem.
Can a dirty filter really make a heat pump stop cooling?
Yes. A clogged heat pump air filter can cut airflow enough to weaken cooling and even freeze the indoor coil. That is one of the first things to check because it is common and easy to fix.
What does it mean if the outdoor unit is not running?
If the indoor unit blows air but the outdoor unit stays off, check for a tripped breaker or disconnect issue first. If power is present and the unit only hums, clicks, or trips the breaker again, stop DIY and call for service.
Should I turn the system off if I see ice on the lines or coil?
Yes. Turn cooling off and let the system thaw. If the indoor blower still works, fan-only mode can help melt the ice faster. Do not chip ice off the coil or lines. If icing returns after airflow is corrected, schedule service.
Does a heat pump that is not cooling always need refrigerant?
No. Low refrigerant is only one possibility, and homeowners often misread airflow or power problems as a refrigerant issue. Check thermostat settings, filter condition, vents, icing, and outdoor unit operation first.
Can I replace a capacitor or contactor myself to fix cooling?
Those parts are inside electrical compartments and can hold dangerous voltage. On a heat pump, that is not a good guess-and-replace repair for most homeowners. Use the safe checks on this page, then hand off the rest to a technician.