Symptoms checklist
If your heat pump isn't performing like it should, start here. Match what you're seeing to the common symptoms below—each one links to the troubleshooting steps that follow.
- No heat or cool air: The system runs but the air coming from the vents isn't warm in winter or cool in summer. This often points to a refrigerant issue, a stuck reversing valve, or a thermostat set incorrectly.
- Heat pump won't turn on at all: You adjust the thermostat and nothing happens—no fan, no compressor, no response. Check the circuit breaker, thermostat batteries, and the emergency shut-off switch first.
- Short cycling: The heat pump starts and stops frequently, never completing a full heating or cooling cycle. A dirty air filter, an oversized unit, or a refrigerant leak can cause this.
- Unit runs constantly: The system never seems to shut off, even when the house reaches the set temperature. This could be due to extreme outdoor temperatures, a failing thermostat, or low refrigerant.
- Ice buildup on the outdoor unit: A thin layer of frost is normal in winter, but thick ice covering the coils or the entire unit indicates a defrost cycle failure, low refrigerant, or blocked airflow.
- Unusual noises: Grinding, squealing, rattling, or banging sounds coming from the indoor or outdoor unit. These can signal a failing motor, loose components, or debris in the fan.
- Water leaks around the indoor unit: Puddles or moisture near the air handler often mean a clogged condensate drain line or a frozen evaporator coil that's thawing.
- Higher than normal energy bills: A sudden spike in your electric bill without a change in usage patterns suggests the heat pump is working harder than it should—possibly due to dirty coils, duct leaks, or a failing component.
Once you've identified your symptom, jump to the matching section for step-by-step DIY checks and fixes.
Most likely causes (ranked)
When your heat pump acts up, start with the simplest fixes before calling a pro. Most problems come from easy-to-miss issues you can check yourself.
- Thermostat settings or dead batteries. A surprising number of no-heat or no-cool calls trace back to the thermostat being set wrong or its batteries dying. Make sure it’s on “heat” or “cool” and the temperature is set correctly. If the display is blank, swap in fresh batteries.
- Tripped breaker or blown fuse. Heat pumps pull a lot of current, and a power surge or overload can trip the breaker. Check your electrical panel and reset any tripped breakers. If it trips again right away, stop — there’s a short or overload that needs a pro.
- Dirty air filter. A clogged filter chokes airflow, causing the system to work harder, freeze up, or even shut down on safety. Check it monthly and replace it if it’s gray and matted.
- Outdoor unit blocked or dirty. Leaves, grass clippings, snow, or ice piled against the outdoor coil cut efficiency and can cause the unit to overheat or freeze. Clear debris at least two feet around the unit and gently rinse the coil with a garden hose (power off first).
- Refrigerant leak or low charge. If the system runs but doesn’t heat or cool well, and you see ice on the outdoor coil in summer or on the indoor coil in winter, you may have a refrigerant leak. This isn’t a DIY fix — it takes an EPA-certified tech to find the leak, repair it, and recharge the system.
- Failed capacitor or contactor. These electrical components in the outdoor unit give the compressor and fan the kick they need to start. A bad capacitor often shows a humming sound with no start; a pitted contactor may chatter or not pull in. Both require a multimeter and safe handling of high voltage, so most homeowners call a pro.
- Reversing valve stuck. If the heat pump blows cold air in heat mode (or hot air in cool mode), the reversing valve may be stuck. Sometimes tapping it gently with a rubber mallet frees it, but if it’s stuck internally, a tech will need to replace it.
- Compressor failure. The most expensive and least common cause. A compressor that won’t start, runs noisily, or trips the breaker instantly usually means end of life for the unit. This is always a pro job.
If you work through these and the problem persists, the next step is to check for error codes on your unit’s control board — many modern heat pumps flash LED patterns or show digital codes that point straight to the fault.
Diagnostic steps
When your heat pump won't start, you can check a few simple things before calling a pro. First, make sure the thermostat is set to the right mode—heating or cooling—and the temperature is set correctly. Next, check your home's circuit breaker panel. A tripped breaker can cut power to the outdoor unit or the air handler. Reset any tripped breakers, but if it trips again right away, you've got a bigger electrical issue that needs a pro.
Take a look at the air filter, too. A clogged filter chokes airflow and can cause the system to overheat and shut down. If it's dirty, swap in a clean one. These three checks—thermostat settings, power supply, and air filter—are the most common DIY fixes when a heat pump won't turn on 1.
If those don't solve it, listen for any humming or buzzing from the outdoor unit. A hum with no fan or compressor start could point to a bad capacitor or contactor. These are high-voltage parts, so if you're not comfortable with a multimeter, it's time to call a licensed tech.
Fix steps
When your heat pump acts up, you can often get it running right again with a few simple checks. Start with the easy stuff before calling a pro.
Check the air filter
A dirty filter chokes airflow, making the system work harder and ice up the indoor coil. Slide the filter out and hold it up to a light — if you can't see light through it, it's time for a new one. Most systems need a swap every 1–3 months, more often if you have pets or allergies 2.
Reset a tripped breaker
If the outdoor unit won't start, head to your electrical panel. Look for a breaker that's flipped to the middle or "off" position. Flip it fully off, then back on. If it trips again right away, don't keep resetting it — that's a sign of a short or overload, and you'll need an electrician or HVAC tech 1.
Clear the condensate drain
A clogged drain can trigger a float switch that shuts the system down. Find the drain line near the indoor unit and check for standing water. You can try flushing it with a cup of vinegar or using a wet/dry vac on the outside exit. If the switch is wet, dry it off and see if the unit restarts.
Understand the defrost cycle
On cold days, you might see steam rising from the outdoor unit or hear a whoosh — that's the defrost cycle melting frost off the coil. It's normal and lasts 5–15 minutes 2. Don't panic and shut the system off; let it finish. If the unit stays in defrost too long or frost builds up unevenly, you could have a bad defrost sensor or low refrigerant — time to call a pro.
Check the thermostat
Make sure it's set to "heat" or "cool" and the temperature is right. Pop in fresh batteries if the screen is blank. If you have a smart thermostat, verify the schedule isn't overriding your settings.
⚠️ Warning: Never try to add refrigerant or open the sealed refrigeration system yourself — it's illegal without EPA certification and can cause serious injury 3.
Still no luck? If you've gone through these steps and the heat pump still won't cooperate, the next move is usually a service call. Write down what you've tried — it'll help the tech diagnose faster.
When parts need replacing
Some heat pump problems go beyond a quick reset or a simple cleaning. If you've already checked the basics and the unit still isn't working right, a failed component may need professional replacement. Knowing what's involved helps you talk with a technician and avoid unnecessary work.
One part that can cause confusion is the protection control module (PCM). On some systems, the PCM controls three solenoids—the PEV, RVS, and LLS—plus two electronic expansion valves (EXV‑H and EXV‑VI) and a variable‑frequency drive (VFD) control interface 4. When the PCM fails, you might see multiple error codes or erratic behavior from the compressor and fans. Because it ties into so many circuits, swapping a PCM isn't a DIY job—a qualified tech needs to verify the diagnosis and handle the replacement safely.
Other components that commonly need replacement include the reversing valve, contactor, defrost control board, and the outdoor fan motor. If your heat pump blows only cold air in heating mode, the reversing valve may be stuck. A contactor that's pitted or burned can prevent the compressor from starting. And a dead outdoor fan motor will cause the unit to overheat and shut down on its high‑pressure safety.
⚠️ Warning: Capacitors store a dangerous charge even after power is disconnected. Only a licensed HVAC technician should discharge and replace them—improper handling can cause severe shock.
When you call for service, describe the symptoms clearly and mention any error codes you've seen. A good tech will test each suspect part before quoting a replacement, so you're not paying for guesswork.
When to Call a Professional
Some heat pump problems go beyond a DIY fix. If you've checked the basics and the system still won't run, or you smell something burning, it's time to step back and call a licensed technician.
Refrigerant work is a hard stop for homeowners. Federal law requires EPA Section 608 certification to handle refrigerants, and Type III is specifically for low-pressure appliances like large chillers and industrial cooling systems 3. Messing with refrigerant without the right training isn't just illegal — it can damage the compressor or create a safety hazard.
If your heat pump uses a remote machinery room, ASHRAE Standard 15 requires refrigerant detectors that alarm at concentrations no higher than the refrigerant's TLV-TWA (Threshold Limit Value–Time-Weighted Average) 5. That's a life-safety system you don't want to bypass or ignore. If an alarm goes off, evacuate and call a pro.
Water-source heat pumps or hydronic systems bring their own risks. Exterior water supply piping must be buried at least 12 inches below grade and 6 inches below the frost line in freezing climates, per IPC Section 305.4 6. A shallow or uninsulated line can freeze and burst, and repairing that usually means digging — not a weekend project.
Other signs you need a pro: electrical burning smells, repeated breaker trips, ice buildup on the outdoor coil in summer, or error codes that point to a failed compressor or control board. Trust your gut. If you're in over your head, a qualified technician will have the tools and training to diagnose the problem safely.
Frequently Asked Questions
Why does my heat pump go into defrost mode even when it's not that cold outside?
Defrost cycles are triggered by frost buildup on the outdoor coil, not just by outdoor temperature. When the coil temperature drops below freezing and moisture in the air condenses and freezes on it, the unit will switch to defrost to melt the ice. This can happen even when it's above freezing outside if humidity is high. You'll see steam rising from the outdoor unit during defrost — that's normal. If defrost cycles seem too frequent or too long, a faulty defrost sensor or control board could be the culprit, and you'll want a pro to check it out 2.
What do the blinking lights or error codes on my heat pump mean?
Most modern heat pumps have an LED on the control board or thermostat that flashes a code when something's wrong. The pattern tells you what the unit thinks is failing — things like high-pressure lockouts, sensor faults, or communication errors. You'll need your model's service manual to decode the flashes because they vary by brand. For example, a Trane unit might flash a certain number of times for a low-pressure fault, while a Carrier uses a different sequence 7. Don't guess — look up the code for your specific model. If the code points to a refrigerant or electrical issue, call a licensed tech. Handling refrigerant without EPA certification isn't just dangerous, it's illegal 3.
Can I reset my heat pump by turning it off and on again?
Sometimes. If the unit locked out due to a temporary glitch — like a brief power flicker or a one-time sensor misread — cycling power at the breaker can clear the fault and get it running again. Turn the breaker off, wait a full minute, then turn it back on. But if the error comes back right away, you've got a real problem that needs fixing, not just resetting. Repeatedly resetting without addressing the cause can damage the compressor 1.
Is it normal for my heat pump to blow cool air in heating mode?
Yes, especially when it's very cold outside or when the unit first starts up. Heat pump air feels cooler than furnace air because it's typically around 85–95°F, while your body temp is 98.6°F — so it can feel cool even though it's heating the house. During defrost, the unit may also blow cool air for a few minutes because it temporarily switches to cooling mode to melt outdoor coil ice. If the air never warms up or your house isn't reaching the set temperature, you could have a refrigerant leak, a stuck reversing valve, or a failing compressor 2.
Why is my heat pump running constantly?
Heat pumps are designed for longer run times than furnaces — they move heat rather than create it, so they work more steadily. But if it's running nonstop and your home isn't comfortable, check the obvious first: a dirty air filter, blocked return vents, or a thermostat set too high in winter or too low in summer. If those are fine, low refrigerant charge, a failing outdoor fan motor, or undersized ductwork could be forcing the system to run continuously. An HVAC pro can measure refrigerant pressures and airflow to pinpoint the issue 2.
What's that gurgling or whooshing sound from my indoor unit?
Gurgling noises often mean air is trapped in the refrigerant lines or the condensate drain line is partially clogged. A whooshing sound is usually just refrigerant flowing through the expansion valve — it's normal in most units. But if the noise is new or loud, check the condensate drain for clogs first. If that's clear and the noise persists, you might have a refrigerant leak that's letting air into the system, which requires a pro to fix 2.
How do I know if my heat pump's reversing valve is bad?
The reversing valve switches the unit between heating and cooling. If it's stuck, your heat pump might blow hot air in summer or cold air in winter, or it might not switch modes at all. You can sometimes hear a stuck valve — it'll chatter or fail to make the usual "whoosh" when changing modes. But diagnosing it requires checking refrigerant pressures and the solenoid coil, so this isn't a DIY fix. Call a tech if you suspect a valve problem 2.
Should I cover my outdoor heat pump unit in winter?
Generally, no. Modern heat pumps are built to handle snow and ice, and a cover can trap moisture, encourage rust, and provide a cozy home for rodents. If you're worried about leaves or debris, clear them away by hand and keep the area around the unit free of snow buildup. The only exception is if you have an older unit that's not designed to shed ice well, but even then, a cover that blocks airflow can cause more harm than good. When in doubt, check your owner's manual 2.
Why does my heat pump smell musty or like dirty socks?
That "dirty sock" smell is usually caused by microbial growth on the indoor coil or in the drain pan. It's more common in humid climates and when the system switches between heating and cooling frequently. Cleaning the coil and treating the drain pan with an antimicrobial product can help, but it's a temporary fix. A pro can apply a long-lasting coating to the coil to prevent regrowth. If the smell is more like burning or electrical, shut the system off immediately and call for service — that could be a motor overheating or wiring issue 2.
Can I add refrigerant to my heat pump myself?
No. Adding refrigerant requires EPA Section 608 certification because refrigerants are regulated substances that can harm the environment if mishandled 3. Plus, a heat pump is a sealed system — if it's low on refrigerant, there's a leak that needs to be found and repaired before recharging. Guessing at the charge without measuring superheat and subcooling can destroy the compressor. This is absolutely a job for a licensed HVAC technician.
Understanding the Defrost Cycle
If you see frost or a thin layer of ice on your outdoor unit during cold weather, don't panic. It's normal. The heat pump's defrost cycle clears it automatically 2. The defrost control uses two sensors—one for outdoor temperature and one for coil temperature—to decide when to run a cycle 2. When the coil temperature drops too low and the outdoor temperature is cold enough, the system briefly switches to cooling mode. It sends hot refrigerant through the outdoor coil, melting the ice. You'll hear a whoosh or see steam rising from the unit while this happens—that's just the frost melting away.
Error Code Diagnostics
When your heat pump flashes an error code, it's telling you exactly where to look. Different brands use different code systems, so you'll need to match what you see to your unit's manual. Here's what working plumbers and experienced homeowners have seen on common systems.
Trane rooftop units use LED flash codes to diagnose heating failures. One flash means a communication failure, two flashes signal a system lockout, three flashes point to a pressure switch failure, four flashes indicate TC01 or TC02 open, five flashes mean flame without gas valve, and six flashes warn of a flame rollout open 7. If you're seeing these on a residential Trane heat pump, the pattern might differ—always check your specific model's documentation.
Carrier heat pumps with PCM control support 120 diagnostic codes covering line voltage, communication, compressor, fan, sensors, and actuators 4. That's a lot of codes, but you don't need to memorize them. For Carrier furnace control boards, you'll often see codes like 1-1 (no previous code) and 2-1 (gas heating lockout, no auto-reset) 8. If you get a 2-1, you'll need to manually reset the system after fixing the underlying issue.
Still stumped after decoding? A common follow-up is that the code doesn't match any symptom you can see. In that case, cycle power to the unit and watch the startup sequence—sometimes the fault only appears under load. If it recurs, write down the exact code and timing before calling a pro; that detail saves a service call.