The simplest signs your heat pump is running
A working heat pump will blow warm or cool air from your vents within a few minutes of turning it on. Put your hand near a supply vent (the one air comes out of) and feel for steady airflow. In heating mode during winter, that air should feel noticeably warmer than the room. In cooling mode during summer, it should feel cool to the touch. If you feel weak airflow or air that matches room temperature, something is wrong.
You should also hear the outdoor unit running. Walk outside and listen near the condenser (the large metal box). During heating, you'll hear a fan and compressor humming. During cooling, the sound is similar but the unit may cycle on and off every 10 to 20 minutes. If the outdoor unit is completely silent when the thermostat calls for heating or cooling, the system likely isn't operating.
Check your thermostat display. It should show the mode you selected (heat, cool, or auto) and the temperature it's trying to reach. Most thermostats display the current room temperature as well. If the display is blank or unresponsive, the system won't run at all.
Key Takeaways
- Warm or cool air flowing from your vents within minutes of turning on the system is the most reliable sign the heat pump is working.
- The outdoor condenser unit should hum and run when the thermostat calls for heating or cooling; complete silence means the compressor isn't starting.
- A blank thermostat display or one that doesn't respond to button presses indicates an electrical problem that prevents the system from running.
- Weak airflow, uneven heating or cooling between rooms, or a unit that runs constantly without reaching the set temperature all suggest the heat pump needs service.
- Call a technician if the system won't start, produces ice on the outdoor unit during heating, or makes grinding or squealing noises.
What to check before calling a technician
Start with the thermostat. Make sure it's set to the mode you want—heat in winter, cool in summer, or auto if your system supports it. Check that the temperature setting is actually different from the current room temperature. If you set it to 72°F and the room is already 72°F, the system won't run because it has nothing to do.
Look at the thermostat batteries if it's a wireless model. A low or dead battery will prevent the thermostat from communicating with the heat pump. Replace the batteries and wait a minute for the system to respond. If the display comes back to life and the heat pump starts, that was your problem.
Check your electrical panel for a tripped breaker labeled for the heat pump or HVAC system. A tripped breaker will be in the middle position or clearly different from the others. Switch it fully off, then back on. Wait a few minutes and listen for the outdoor unit to start. If the breaker trips again when ready, stop and call a technician—that's a sign of an electrical fault.
Look at the outdoor condenser unit for visible damage: bent fins, ice buildup, or debris blocking the fan. Light frost on the coils during cold weather is normal, but thick ice means the system needs service. Clear away leaves, branches, or snow around the unit, but don't try to remove ice yourself—you can damage the coils.
Temperature differences that mean the system is struggling
If the heat pump is running but your home isn't reaching the set temperature, measure the difference between what you set and what the thermostat actually shows. A gap of 2 to 3 degrees is normal while the system catches up. A gap of 5 degrees or more that doesn't close after an hour suggests the heat pump isn't producing enough heating or cooling power.
Notice whether some rooms stay cold or hot while others reach the set temperature. This usually points to a blocked duct, a closed damper, or an imbalance in airflow rather than a failed heat pump. Walk through your home and feel the air coming from each vent. If one room's vents are much weaker than others, check whether the damper in that duct is partially closed. Dampers are small levers on the ductwork itself.
Pay attention to how long the system runs. A heat pump that cycles on for 5 minutes, off for 5 minutes, and repeats all day without the house reaching temperature is working harder than it should. A system that runs constantly without stopping is also a warning sign. Both patterns suggest the heat pump is undersized for your home or losing efficiency and should be inspected.
Sounds that mean the heat pump needs service
Normal heat pump noise is a steady hum from the outdoor unit and a gentle whoosh of air from the vents. You might hear the compressor click on or off as it cycles, and the fan speed may change. These sounds are not a problem.
Grinding, squealing, or rattling noises are not normal. A grinding sound from the outdoor unit usually means the compressor is failing. A high-pitched squeal often points to a refrigerant leak or a failing fan motor. Rattling can be loose parts, but it can also be a sign of internal damage. Stop using the system and call a technician. Running a damaged heat pump can make the repair more expensive.
A hissing sound near the outdoor unit or the indoor air handler may indicate a refrigerant leak. Refrigerant is essential for the heat pump to work, and a leak will cause the system to lose heating or cooling power over days or weeks. This is not an emergency, but it does need professional attention soon.
When the heat pump runs but produces weak airflow
Weak airflow from your vents usually means the filter is clogged. Most heat pumps have a 1-inch or 4-inch filter somewhere in the return ductwork or inside the air handler unit. Check your system's manual or look for a large rectangular box with a slot on the side. If the filter looks gray or brown instead of white, it's time to replace it. A clogged filter reduces airflow and makes the system work harder.
If the filter is clean and airflow is still weak, the problem may be deeper in the ductwork. Ducts can sag, disconnect, or become blocked by debris or pest nests. You can visually inspect any exposed ductwork in the attic or basement, but sealed ducts inside walls require a technician to diagnose.
Another cause of weak airflow is a failing blower motor—the fan that pushes air through the ducts. If you hear the outdoor unit running but feel almost no air from the vents, the blower motor may not be spinning at full speed. This requires professional service.
Ice on the outdoor unit and what it means
During heating mode in cold weather, a thin layer of frost on the outdoor coils is normal. The heat pump pulls heat from the outside air, and moisture in that air condenses and freezes on the coils. The system has a defrost cycle that periodically reverses operation to melt the frost. You might hear the outdoor unit switch to cooling mode for a few minutes, and the frost will disappear.
Thick ice buildup that doesn't melt during the defrost cycle is a problem. This usually means the defrost sensor is broken, the reversing valve is stuck, or there's a refrigerant leak. A heat pump with heavy ice will lose heating power because the coils are blocked. Stop using heating mode and call a technician. Trying to chip away the ice yourself can damage the coils and make the repair worse.
Frequently Asked Questions
How long does it take for a heat pump to warm up a cold house?
A heat pump typically raises the room temperature by 1 to 2 degrees per hour when starting from a much colder state. If your house is 60°F and you set the thermostat to 72°F, expect 6 to 12 hours for the system to catch up, depending on insulation and outdoor temperature. If it's taking much longer, the system may be undersized or losing efficiency.
Why does my heat pump keep running without stopping?
Continuous running usually means the heat pump can't reach the set temperature. This happens when the outdoor temperature is very cold (below 30°F), the system is undersized, the filter is clogged, or refrigerant is low. Check the filter first. If it's clean and the problem persists, call a technician to check refrigerant levels and system capacity.
Is it normal for the heat pump to switch between heating and cooling?
If you have the thermostat set to auto mode, the system will switch between heating and cooling to maintain the set temperature. This is normal. If it's switching back and forth rapidly (every few minutes), the set temperature may be very close to the current room temperature. Adjust the set point up or down by a few degrees to stop the cycling.
What should I do if the outdoor unit is making a loud noise?
Loud grinding, squealing, or rattling means the heat pump needs service soon. Don't ignore it or run the system longer than necessary. Turn it off if the noise is very loud, and call a technician. Continuing to run a damaged heat pump can turn a small repair into a major one.
Can I clean the outdoor unit myself?
You can gently rinse the outdoor coils with a garden hose to remove dust and pollen, but use low pressure and never spray directly at the fins. Don't try to straighten bent fins or remove ice. Avoid using pressure washers, which can damage the coils. If the unit needs more than a light cleaning, let a technician handle it.