Symptoms checklist
If your heat pump is blowing warm air when it's set to cool, that's a clear sign something's wrong. You should feel air that's 15–20°F cooler than what's coming back through the return vent 1. Unusual noises like grinding or screeching point to mechanical issues that usually need a pro's touch 2.
Most likely causes (ranked)
If your heat pump is blowing warm air or not cooling at all, start with the simplest fixes first. Most cooling problems come down to a handful of common issues you can check yourself before calling a pro.
First, double‑check your thermostat settings. Incorrect settings are the most common cause of a heat pump not cooling 1. Make sure it’s set to “Cool” mode and the target temperature is lower than the current room temperature. The fan should be on “Auto,” not “On” — running the fan continuously can blow warm air when the compressor isn’t cycling 1.
Next, look at your air filter. A dirty filter chokes airflow across the indoor coil, and that can make the heat pump blow lukewarm air that feels cold 3. Replace it every 1–3 months, more often if you have pets or dusty conditions. A severely clogged filter can even cause the coil to ice up, making the problem worse.
Then head outside and check the outdoor unit. Leaves, grass clippings, snow, or debris piled against the cabinet block airflow and prevent proper heat exchange 2. Clear away anything within two feet of the unit, and gently rinse the fins with a garden hose if they’re caked with dirt. Just don’t use a pressure washer — it’ll bend the fins and make things worse.
If these three checks don’t solve it, you’re likely dealing with a refrigerant leak, a stuck reversing valve, or a failed compressor — all of which need a licensed tech with gauges and recovery equipment. But in most cases, the fix is one of the simple ones above.
Diagnostic steps
Start with the thermostat — it's the brain of your system and the easiest thing to check. Make sure it's set to "Cool" mode. It's surprisingly common for the setting to get bumped to "Heat" or "Fan Only" by accident, and that'll stop cooling dead in its tracks.4
Next, head to your electrical panel. A tripped circuit breaker can kill power to the indoor or outdoor unit, so the heat pump won't turn on at all. If you find a tripped breaker, don't just flip it back on — a trip often signals an underlying electrical fault, and resetting it could cause damage. Call a pro if you see this.1
While you're moving around, pop open the air handler and take a look at the air filter. A clogged filter chokes airflow, and that's one of the most common reasons for lukewarm air or poor cooling. Replace it if it's dirty — you should be swapping it out every 1–3 months anyway.3
Finally, step outside and inspect the outdoor unit. Leaves, grass clippings, snow, or other debris can pile up against the coil and block airflow. Clear away anything that's built up around the unit so it can breathe.2
Fix steps
Start with the simplest checks before you call for service. Many heat pump cooling problems have a straightforward fix you can handle yourself.
If your system seems completely unresponsive, try resetting it first. Turn off power to the unit at the breaker or disconnect switch, wait five minutes, then turn it back on. This often clears temporary glitches that can stop cooling 2.
A dirty air filter is one of the most common reasons a heat pump blows lukewarm air. Check your filter and replace it if it's clogged. You'll want to swap in a fresh filter every one to three months—severe airflow restrictions can keep your home from cooling properly 3.
Head outside and look at the outdoor unit. Leaves, grass clippings, snow, or other debris piled against the coil block airflow and hurt heat exchange. Clear away anything within a couple feet of the unit, then gently rinse the fins with a garden hose if they're caked with dirt 2.
Still not cooling after these steps? You might have a refrigerant leak, a faulty reversing valve, or a compressor that's struggling. Those repairs need a pro with the right tools and training—don't try to open the refrigerant lines yourself.
When parts need replacing
Sometimes the problem isn't a dirty filter or a tripped breaker—it's a failed component. If you've checked the easy stuff and your heat pump still won't cool, one of these parts may be the culprit.
A faulty reversing valve can prevent the heat pump from switching to cooling mode 5. If the unit blows cold air continuously in heating mode, it may indicate a reversing valve problem 5. This valve is what lets your heat pump flip between heating and cooling, so when it sticks, you're stuck with whatever mode it's locked into.
A refrigerant leak prevents the system from cooling effectively 3. Low suction pressure and low head pressure with high superheat indicate an undercharged system (refrigerant leak) 3. You might notice ice on the outdoor coil or hear a hissing sound—both are red flags. Leaks need a pro to find and fix, then recharge the system to the right level.
A malfunctioning compressor can cause the heat pump to not cool 5. Continuous cold air in heating mode or no cooling in summer may indicate a compressor issue 5. The compressor is the heart of the system—if it's not pumping refrigerant, you won't get any cooling. This is a major repair, so you'll want a qualified technician to diagnose and replace it if needed.
⚠️ Warning: Refrigerant work and compressor replacement involve high-voltage electricity and pressurized gas. Don't open the sealed system yourself—call a licensed HVAC pro.
When to Call a Professional
Some heat pump problems are straightforward, but others need a trained eye—and the right tools. If you've checked the thermostat, changed the filter, cleared the outdoor unit, and the system still isn't cooling, it's time to step back. Messing with refrigerant, electrical components, or the compressor without proper training can make things worse or even be dangerous.
Call an HVAC technician if you notice any of these:
- Ice on the indoor or outdoor coils when the system is in cooling mode. This often points to low refrigerant or airflow issues that require gauges and leak detection.
- Buzzing or humming from the outdoor unit but the fan isn't spinning. That could be a failed capacitor or a seized compressor—both need electrical troubleshooting.
- The breaker for the heat pump trips repeatedly. Don't keep resetting it; there's likely a short or an overload that needs diagnosis.
- You hear grinding, squealing, or rattling that doesn't go away after cleaning and basic checks. These sounds often mean failing motor bearings or a damaged compressor.
- The system blows warm air even after you've verified the thermostat settings and clean filter. This can be a reversing valve stuck in heating mode or a refrigerant leak—both are sealed-system repairs.
Experienced plumbers and HVAC pros also recommend calling if your heat pump is over 10 years old and needs a major repair. The cost of fixing an older unit can approach half the price of a new, more efficient system. A pro can help you weigh repair vs. replacement.
⚠️ Warning: Never attempt to open refrigerant lines or access sealed electrical panels unless you're EPA-certified and trained. High-voltage capacitors can hold a charge even when the power is off, and refrigerant can cause frostbite or environmental harm if mishandled.
If you're unsure, most reputable companies offer a diagnostic visit. The fee is usually reasonable, and you'll get a clear answer instead of guessing—and potentially causing more damage.
Frequently Asked Questions
Why does my heat pump go into defrost mode when it's cooling?
Heat pumps can enter a defrost cycle even in cooling mode if the outdoor coil gets cold enough to frost over—usually when outdoor temperatures dip below about 40°F and humidity is high. The unit briefly switches to heating mode to melt the frost, then resumes cooling. This is normal operation, but if you see frost that doesn't melt within a few minutes or the unit stays in defrost for more than 10–15 minutes, you might have a faulty defrost sensor or control board. 1
I hear a loud buzzing or humming noise from the outdoor unit—what does that mean?
A buzzing sound often points to an electrical issue like a failing contactor, loose wiring, or a struggling compressor. If the noise is accompanied by the fan not spinning, the capacitor might be bad. A humming compressor that won't start could be seized or have a failed start capacitor. Turn off the unit at the breaker and call a pro—continuing to run it can cause more damage. 3
How do I reset my heat pump after it stops cooling?
Start with the simplest reset: turn your thermostat to "off" and the fan to "auto," then flip the breaker for the heat pump to "off" for at least 30 seconds before turning it back on. Wait a few minutes for the system to reboot, then set the thermostat back to cooling. If the unit still doesn't start, check your outdoor disconnect box—it might have tripped. Some units also have a reset button on the outdoor cabinet, but not all. If a reset doesn't work, you likely have a tripped internal safety switch or a control board fault that needs a technician. 2
Can a dirty air filter really stop my heat pump from cooling?
Absolutely. A clogged filter restricts airflow across the indoor coil, which can cause the coil to freeze up and the system to shut down or blow warm air. Check your filter monthly during heavy-use seasons and replace it if it's dirty. Most 1-inch filters need changing every 1–3 months, while thicker media filters can go 6–12 months. 5
My heat pump is cooling but not as well as it used to—what should I check first?
Start with the basics: make sure your air filter is clean, all supply vents are open, and the outdoor unit isn't blocked by leaves, grass, or debris. Next, check your thermostat settings—make sure it's set to "cool" and the temperature is lower than the room reading. If those are fine, look at the outdoor coil; if it's dirty, gently rinse it with a garden hose (power off first). If performance doesn't improve, you could have a refrigerant leak or a failing compressor, which requires a pro. 4
Understanding the defrost cycle
If your heat pump suddenly blows cool air on a cold day, don't panic—it's likely just the defrost cycle kicking in. During defrost mode, the system briefly reverses operation to melt frost or ice from the outdoor coil. While this happens, you might feel cooler air from the vents, but that's completely normal 3. Backup heat usually engages to keep you comfortable until the cycle finishes, so the temperature drop is temporary.