Heat Pump Troubleshooting Made Easy!

If you are reading this article regarding heat pump troubleshooting, you are probably like me when something in the house isn't working:


We all lead busy lives and these problems get in the way.

Let's Troubleshoot!

Okay...let's get to it! Click on this link for help in diagnosing general heat pump problems.

For more detailed, specific problems click on the links below that may apply:


After following the links above, their may be more details that you are looking for. Below are discussions on each component.

If there is no information attached to each item, please be patient as that information is coming soon!

As a final note, only qualified service technicians should perform the heat pump troubleshooting techniques described below. Many of the actions described below could result in equipment damage or physical harm or injury if not performed with care and with proper training. Do not take the actions below if it could cause harm to any person or equipment.

Heat Pump Troubleshooting:
Compressor Mechanical Problems

Bad Compressor Suction and Discharge Valves

If a bad suction valve is suspected (higher than expected suction pressure), connect your gauge manifold hoses to the connection port on each valve (suction and discharge). Ensure both the suction and discharge valves are open. Close the suction valve only, while the compressor is on. The compressor should pull down to a vacuum of at least 28 inHg within 2 minutes. If this does not happen, turn the compressor off for about 3 minutes. Then energize the compressor and wait for a vacuum of at least 28 inHg within 2 minutes. If the 28 inHg is not reached, the suction valve needs to be replaced.

If a bad discharge valve is suspected (lower than expected discharge pressure), connect your gauge manifold hoses to the connection port on each valve (suction and discharge). Ensure both the suction and discharge valves are open. Close the suction valve only and start the compressor. Pump the compressor into as deep of a vacuum as possible. Shut off the compressor and watch for the pressure to rise (on the compound gauge closest to the suction connection port). If it rises, pump the compressor into a vacuum again, shut of the compressor, and watch for another rise in pressure. If after continually repeating this process, the compound gauge stops rising in pressure, the discharge valve must be replaced.

Liquid Slugging

If the compressor is receiving slugs of liquid, this problem must be resolved before the compressor goes bad. It is quite obvious when a compressor receives a slug of liquid. You will hear it. It could sound like somebody through rocks into the refrigeration system or like a metal against metal clattering.

Liquid can be separated into two very different heat pump troubleshooting problems; oil slugging and refrigerant slugging.

Oil Slugging

If oil slugging is occurring the most likely reason is too much oil in the system. There are other potential reasons for oil slugging, however this brings the design of the refrigeration system into question. That is beyond the scope of this webpage.

Evacuate all of the refrigerant out of the system and remove oil from the compressor. Be careful not to removed too much oil. Too much oil is usually a better problem than not enough. When the compressor is returned to operation (after charging the system with refrigerant), ensure that the oil level operates at the manufacturer's recommendation.

Refrigerant Slugging

There are many different potential reason for liquid slugging. There are as follows:

  • Overcharge of refrigerant (non-thermostatic expansion valve systems)
  • Low super-heat setting
  • Low load on the evaporator

If you cannot resolve the problem of liquid slugging, a suction accumulator can be installed in the suction line leading to the compressor. You could also heat trace areas that may be trapping liquid during off periods. Ensure that the crankcase heater is operating correctly.

Compressor Bearings

Over time, the compressor bearings can become worn and result in poor refrigerant performance. Also, if the compressor is not getting sufficient lubrication, this can prematurely wear the bearings. In many cases, the entire compressor will need to be replaced. Sometime, the compressor can be rebuilt.

Worn bearings can be determined by:

  • Excessive noise or vibration
  • Low oil pressure
  • Low amp draw
  • High suction pressure and low discharge pressure

Please note that poor lubrication can cause the compressor bearings to seize, which can result in a locked rotor condition. If the compressor is replaced without resolving the lubrication issue, the new compressor will fail prematurely.

Heat Pump Troubleshooting: Checking for a bad compressor motor

Open Windings

Check for open motor windings by using an ohmmeter. First remove all wires connected to the motor externally. Check for continuity from one terminal to the other. There should be no continuity. Continuity is indicated by an infinite resistance reading on the ohmmeter.

Great instruction video on check for open motor windings:

Shorted Windings

Perform the same procedure described above to check for open windings. Record the resistance readings of all terminals to each other. Consult the manufacturer for proper resistance.

Grounded Windings

Perform the same procedure described above to determine if there are open windings. The difference here: check for continuity between each terminal and the motor housing.

Checking for bad overloads

Place and ammeter on the compressor common electrical line. As you energize the compressor, the motor should draw approximately 5-6 times the rated running amps. After a very short time, the amp draw should settle near the rated running amps of the motor. If the amp draw occurs as expected, but the overload shuts down the compressor, then the overload either needs adjustment or replacement.

How do you determine if it was the overload that caused a shutdown?

Do your own heat pump troubleshooting by safely using a voltmeter, check for an open circuit across the overload. If it is open after the mysterious shutdown, then it was the overload that shut down the compressor.

Heat Pump Troubleshooting: Motor starters and contactors

Burned Contacts

Energize the contactor and using a voltmeter check the corresponding contact points. If the energized contactor should have no voltage across it (as the contact should be closed when energized). Many times a burned contact can be determined visually.

Burned Coil

Remove power from the contactor and disconnect all external wiring from it. Using an ommeter, check for continuity. If the coil is open, there should be no continuity. A burned coil can often be seen visually by discoloration.

Sticking Contactors

It is usually quite obvious if a contactor is sticking open or closed. You may hear it buzzing. Or the activated equipment will not turn off. Whatever the case, it is best to replace the contactor in lieu of lubricating sprays. The lubricating spray may be a good temporary solution.

Heat Pump Troubleshooting: High and Low Pressure Controls

When the high or low pressure control shuts down the system, there are a number of possible reasons. Please follow the "heat pump problems" link above for a table of reasons and solutions.

What if the controller itself is faulty?

To test a low pressure controller, slowly close the suction shutoff valve, and using your gauges, watch the suction pressure fall. The low pressure controller should shut down the system at the cutout setting shown on the controller. If the controller has a reset button, use it as needed. Exercise caution when closing the suction valve so as to avoid equipment damage or physical injury.

To test a high pressure controller, block airflow or water flow (whichever applies) to the condenser side of the system. This will raise the discharge pressure. Using your gauges, monitor the discharge pressure and compare it to the cutout setting on the high pressure controller. Use the reset button as needed. Exercise caution when raising the discharge pressure. Do not allow the pressure to rise above the maximum operating pressure of the system.

Heat Pump Troubleshooting: Start and Run Capacitors

Start and run capacitors are commonly replaced when the system is not starting or running correctly.

If you suspect the start or run capacitor to be faulty, disconnect all external wiring from the start capacitor. Ensure power is off and proper safety precautions are taken. Ensure that there is no charge in the start capacitor. This can be accomplished by using an touching both terminals with the same metal object at the same time (some like to use a screwdriver with a an insulated handle). Make sure that your hands are insulated from the metal so that you don't get a shock!

Using an ohmmeter, test both terminals. If the reading is infinite, the capacitor is good. If not, the capacitor should be replaced.

Thermostatic Expansion Valves

A technician that is capable of servicing thermostatic expansion valves must be fully trained in calculating suction gas superheat. Here are few notes when troubleshooting a thermostatic expansion valve:

  • If you are wondering whether or not the expansion valve is operated at all, remove the sensing bulb and, while the system is running, GRADUALLY warm the bulb. An increase in pressure may indicate that the valve needs adjustment.
  • Ensure that the sensing bulb is mounted horizontally at the 4 o'clock position on the piping.
  • A sweating suction line or compressor crankcase could indicate liquid refrigerant slugging. This means that the system is receiving too much refrigerant through the expansion valve. Check to see if the expansion is operating. If it is stuck open, replace it.
  • A hunting expansion valve can be indicated by a suction pressure swing of 10-15 psi. If this is happening, the valve may need to be replace with a smaller size.

Heat Pump Troubleshooting: Check Valves

Check valves that are sticking open or closed are a common problem for heat pumps. Low/high suction pressures and low/high superheats could be an indication of a bad check valve. Follow the flow of refrigerant through the system in heating or cooling modes. Using an infrared temperature gun, check the temperature of the line on both sides of each check valve in the system. This may indicated whether or not the check valve is stuck open or closed.

Heat Pump Troubleshooting: Reversing Valves

In diagnosing a faulty reversing valve, it is usually sufficient to perform a temperature test on each of the four lines going to/from the reversing valve.

The discharge gas (hot) should go through the port which leads to:

  • The outdoor coil (cooling mode)
  • The indoor coil (heating mode)

The suction gas (cold) should be coming from:

  • The outdoor coil (heating mode)
  • The indoor coil (cooling mode)

I really hope that the information above, as well as the text links on this page (especially heat pump problems!) gave you sufficient information to keep you exploring your heat pump troubleshooting journey. 

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Low suction pressure on a heat pump water cooled condenser 
Low suction pressure on a heat pump water cooled condenser. What would be potential causes?


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My old 120V heat pump heater was retired when the compressor failed engage after a number of tries. I decided to troubleshoot by connecting it through …

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