Testing a heat pump comes down to checking a handful of things: whether it’s producing the right temperature difference between supply and return air, whether it’s switching between heating and cooling modes correctly, whether the airflow is adequate, and whether the electrical components are drawing normal power. You can do several of these checks with basic tools like a thermometer, a multimeter, and a manometer, though some tests are better left to a technician with refrigerant gauges.
Check the Temperature Split First
The simplest and most telling test is measuring the temperature difference (called delta T) between the air going into your system and the air coming out. Place a thermometer at a supply register close to the air handler and another at the return grille. In cooling mode, the supply air should be roughly 15 to 20 degrees Fahrenheit cooler than the return air. In heating mode, the supply air should be about 20 to 30 degrees warmer, though this varies depending on outdoor temperature and the specific unit.
If the split is too narrow, something is reducing your system’s ability to transfer heat. Common culprits include low refrigerant charge, a dirty coil, restricted airflow, or a compressor that isn’t running at full capacity. If the split is too wide, airflow is likely too low, meaning air is spending too long in contact with the coil. That can cause the evaporator to freeze in cooling mode.
Verify the Reversing Valve Switches Correctly
A heat pump’s reversing valve is what lets it both heat and cool. When it fails or gets stuck, you’ll get heating when you want cooling or vice versa. You can test this from the thermostat by switching between modes and listening for a distinct “click” or “whoosh” at the outdoor unit as the valve shifts.
At the thermostat wiring level, the reversing valve is controlled by either the O terminal or the B terminal. Most systems use the O wire, which energizes the reversing valve when the thermostat calls for cooling. The valve’s unpowered (default) position is heating. Some brands, notably Rheem, use the B wire instead: it energizes the valve when the thermostat calls for heat, and the default unpowered position is cooling. If your heat pump heats fine but won’t cool (or the reverse), the reversing valve solenoid or its wiring is a prime suspect.
Measure Airflow With Static Pressure
Restricted airflow is one of the most common reasons a heat pump underperforms, and it’s easy to miss because the system still runs. The proper way to check it is by measuring total external static pressure, which tells you how hard the blower is working to push and pull air through the ductwork.
You’ll need a manometer that reads in inches of water column. Take one pressure reading where air enters the air handler (the return side) and another where air leaves (the supply side). Add the two readings together. Then compare that number to the maximum static pressure listed on your equipment’s data plate. If your measured pressure exceeds the manufacturer’s rating, the ductwork is too restrictive. Dirty filters, undersized ducts, closed dampers, and clogged coils all drive static pressure up. For heat pumps specifically, if the system doesn’t have a separate remote cooling coil, you won’t need to account for an internal coil pressure drop in your measurement.
Before you break out any instruments, check the basics: make sure the filter isn’t clogged, all registers are open, and nothing is blocking the return grille. A dirty filter alone can cut airflow enough to make a heat pump struggle.
Test the Compressor’s Amp Draw
If you own a clamp meter (a multimeter with a current clamp), you can check whether the compressor is drawing the right amount of power. Every compressor has a data plate, usually on the outdoor unit, listing its Rated Load Amps (RLA). With the system running in either heating or cooling mode, clamp the meter around one of the compressor’s power leads and compare the reading to the RLA.
An amp draw significantly below RLA can indicate low refrigerant charge or a valve issue inside the compressor. An amp draw at or above the RLA suggests the compressor is working too hard, possibly due to high head pressure from a dirty outdoor coil, an overcharged system, or failing electrical components. If the draw exceeds the nameplate’s Locked Rotor Amps (LRA), the compressor is stalling and will trip its protection. Check the outdoor fan motor the same way: clamp the meter on its wire and compare to the motor’s rated amps on the data plate.
Test the Defrost Cycle
In heating mode during cold weather, frost builds on the outdoor coil. A working defrost cycle periodically reverses the system to melt that frost. If the defrost fails, ice accumulates on the coil, airflow drops, and heating capacity plummets.
Most control boards have test terminals or jumper points specifically for triggering a manual defrost. Jumping these terminals forces the system into defrost mode so you can observe the sequence. When defrost activates, the reversing valve should switch (you’ll hear it), the outdoor fan should stop, and you may feel cool air from the indoor vents briefly since the system is temporarily in cooling mode. The auxiliary heat strips often kick on during defrost to keep indoor air warm. If nothing happens when you jump the test terminals, the defrost board, the defrost sensor (mounted on the outdoor coil), or the reversing valve solenoid could be the problem.
Confirm Auxiliary and Emergency Heat
Most heat pumps have electric resistance heat strips in the air handler as backup. These supplement the heat pump when outdoor temperatures drop too low for the heat pump to keep up on its own. The temperature at which the system switches over to backup heat varies by manufacturer; some units allow adjustment down to 0°F, while others switch in the 15 to 25°F range. Below the switchover point, running the heat pump becomes less efficient than resistance heat because the heat pump’s efficiency (its coefficient of performance) drops below 1.0.
To test whether your backup heat works, switch the thermostat to “emergency heat” mode. This locks out the outdoor unit entirely and runs only the heat strips. You should feel warm air from the vents within a minute or two, and your amp draw at the air handler will jump noticeably. If the air stays cool, the heat strips, their sequencers (the relays that stage them on), or the W2 wire from the thermostat may be faulty. The W2 terminal on your thermostat is what activates emergency or second-stage heat. With a multimeter, you can check for 24 volts between the R and W2 terminals at the air handler when the thermostat calls for emergency heat.
Run Through the Thermostat Wiring
If a specific mode isn’t working, testing the thermostat signals can isolate whether the problem is the thermostat, the wiring, or the equipment. The key terminals for a heat pump are:
- R: The 24-volt power supply from the transformer.
- Y: Energizes the compressor contactor and outdoor fan. Active in both heating and cooling.
- G: Energizes the indoor blower.
- O or B: Controls the reversing valve. O energizes it in cooling (most brands). B energizes it in heating (some brands).
- W2: Activates emergency or auxiliary heat strips.
Set your multimeter to AC volts. At the air handler’s control board, check between R and each terminal while the thermostat calls for that mode. You should see roughly 24 volts on the active terminals. If the thermostat is calling for cooling but you don’t see 24 volts between R and O, either the thermostat isn’t sending the signal or there’s a break in the O wire. This method helps you trace exactly where a signal gets lost.
Know What “Good” Looks Like
It helps to have a performance baseline. Current ENERGY STAR minimums for split-system heat pumps are a SEER2 of 15.2 for cooling efficiency and an HSPF2 of 7.8 for heating efficiency. Cold climate models are held to a higher heating standard: HSPF2 of 8.1 for ducted systems and 8.5 for ductless. These ratings describe lab-tested seasonal efficiency, so you can’t measure them directly in the field. But if your energy bills have crept up while your usage hasn’t changed, or if the system runs constantly without reaching the set temperature, those are signs it’s fallen well below its rated efficiency.
A heat pump in good working order will cycle on and off regularly, maintain the set temperature within a degree or two, produce a consistent temperature split at the registers, and keep the outdoor coil free of heavy ice buildup (light frost in heating mode is normal). Any persistent deviation from that pattern is worth investigating with the tests above.