Heat Pump Troubleshooting Tips

(photo: Darkdiamond67/iStock/Getty Images)

In the previous issue of HPAC (October 2022) I outlined some the of the basic principles of heat pump technology. For the purposes of a quick review, you may remember, like an air conditioner, a heat pump transfers heat from the conditioned space, but in a heat pump application the conditioned space can be the outdoors. So, the evaporator is now located outdoors.

The heat transferred to the refrigerant in that process, plus the heat added to the refrigerant during the compression process is transferred to air in the conditioned space via the condenser.

So, the heat pump is nothing more than the basic vapour compression cycle utilized in an air conditioning system, with added controls and valving to allow the system to either remove heat from the conditioned space (and transfer it to the outdoors), or remove heat from the outdoors (and transfer it to the conditioned space). As such, rather than a distinct evaporator and condenser, in a heat pump we now have two dual purpose coils, an “indoor” coil and an “outdoor” coil.

Like any mechanical system, the heat pump will experience periodic performance issues, some due to lack of maintenance, and some due to mechanical/electrical components failing.

When called upon for service, there are several things that can cause a heat pump’s inability to provide adequate heat to the conditioned space:

• Voltage issues (low voltage, tripped breaker, blown fuses): This type of failure is typically the manifestation of some other issue: a seized compressor motor or grounded compressor motor will draw excessive amperage and cause a breaker to trip (or fuses to blow). Finding the source of the excessive amperage draw will solve this issue.
• Thermostat issues: A defective thermostat, or a thermostat that is wired incorrectly, can prevent the unit from starting when heat is required. Checking the thermostat for proper operation, checking the thermostat settings and wiring will confirm whether any of these are the problem.
• Plugged air filters or dirty indoor coil restricting the air supply: This is a simple maintenance issue, but it will certainly result in the inability to heat the conditioned space. Simple remedy here, replace the filters and/or clean the indoor coil.
• Dirty outdoor coil: This is preventing the ability to transfer heat from the ambient to the refrigerant, resulting in a loss of heating capacity. Again, a simple remedy here, clean the outdoor coil.
• A system leak resulting in the loss of refrigerant: Since the refrigerant is the medium which facilitates the transfer of heat from one place to another, a loss of refrigerant results in a loss of heating capacity. Another simple remedy, locate the leak, repair it, evacuate the system and properly recharge.
• Plugged flow controls (solenoid valves, thermostatic expansion valve, filter drier): This will result in the reduced flow of refrigerant, again reducing the unit’s capacity for heat. Since plugged flow controls are typically the result of system contamination, simply replacing controls, or cleaning (in the case of a solenoid valve or thermostatic expansion valve) doesn’t solve the contaminant problem. Contaminants are typically the result of excessive discharge temperature (which might be caused by a dirty outdoor coil). If the contaminated system isn’t cleaned up (filter-drier replacement along with oil replacement if necessary), and if the issue which caused the contamination has not been resolved, it will likely manifest itself again.
• Contactor, relay and capacitor failures: More often than not, the failure of these components is simply due to age.

All of the above-mentioned potential problems are not unique to heat pumps only. These are common problems that could be experienced in any system using a standard vapour compression air conditioning (AC) system.

So, what would qualify as a system malfunction that is completely unique to the heat pump? This question can be best answered by quantifying the component differences between a standard AC system and a heat pump.

Reversing Valve

Again, you may recall from the previous article that in a heat pump, the reversing valve is located in the discharge line between the compressor outlet and the outdoor coil inlet. A solenoid coil, when energized, allows the valve to “shift” from one position to another.

In the de-energized mode, the refrigerant flows from the compressor discharge port to the inlet of the outdoor coil. The other two ports allow the refrigerant vapour from the indoor coil to flow to the compressor suction port.

Figure 1 is a photo of an outdoor condensing unit, showing the various flow components (including the four-way reversing valve). There are two common ports on the four-way reversing valve: (1) common discharge port, which is connected to the compressor discharge port, and (2) common suction port, which is connected to the compressor suction port.

Figure 1. Outdoor heat pump condensing unit.

The energizing/de-energizing of the pilot solenoid coil will cause the other two ports to interchange between supply to the outdoor coil/return from the indoor coil, and supply to the indoor coil/return from the outdoor coil, thus allowing the compressor to supply its discharge gas to either the outdoor coil or indoor coil.

The photos in Figure 2 show a close-up of the four-way reversing valve, and a cut-away photo of the reversing valve. This shows the pilot solenoid valve (less coil) and the various pilot lines.

Figure 2. Close up photos of a reversing valve and a cutaway of a reversing valve.

In the de-energized mode, the pilot solenoid will vent any high pressure from the left end of the valve body while simultaneously supplying high pressure vapour to right end of the valve body, forcing the slide piston to move to the right.

This will cause the two ports at the bottom left to align, allowing flow from the indoor coil outlet (bottom left port) to continue to the compressor suction (center port), and cause the top port and bottom right port to align, allowing flow from the compressor outlet to the outdoor coil inlet.

When the pilot solenoid coil is energized, the pilot valve shifts and vents any high pressure from the right end of the valve body while simultaneously supplying high pressure to the left end of the valve, forcing the slide piston to move to the left. This reverses the indoor and outdoor connections, and allowing the discharge vapour to flow to the inlet of the indoor coil, and connects the outlet of the outdoor coil to the compressor suction.

Four Way Stop

There are a number of scenarios that can develop in relation to the four-way reversing valve which will result in system malfunctions.

Four-way valve not shifting to the heat mode:

1. Defective thermostat, not supplying voltage to the pilot solenoid valve coil.
2. Wiring issue, resulting in no voltage to the pilot solenoid valve coil.
3. Contaminants plugging the tiny passageways in the pilot solenoid valve, which will result in the valve not properly shifting to either the heating or cooling position.
4. Heavy contamination resulting in the slide valve sticking.

Four-way reversing valves are high production and relatively low cost. As such, they are not built for accessibility. In either of the two failure modes described in 3 and 4, a valve replacement will be required.

Four-way valve partially stuck in one mode or the other:

1. When a heat pump system shifts from the cooling mode to the heating mode, you will hear a pressure surge from the four-way reversion valve. If this sound is not heard, it might be indicative of a valve that is stuck. But further analysis should be done.
2. Manufacturers recommend that the typical temperature difference between the compressor discharge and the outlet port connected to the outdoor coil inlet (in cooling mode), or the indoor coil inlet (in heating mode) should be approximately 3F to 6F. This would suggest that if the temperature drop is in this range, the entire discharge mass flow is flowing from the compressor discharge port to inlet of the respective coil that is serving as the condenser. If the temperature difference is higher than the range above, it would suggest that valve’s slide piston is stuck in between the cooling position and heating position, resulting in only a portion of the discharge mass flow from the compressor is flowing through valve’s port connected to the inlet of the coil serving as the condenser in whichever mode the unit is operating in.
3. In addition to the above temperature condition, an abnormally high discharge pressure combined with an abnormally high suction temperature would also suggest that the valve is stuck somewhere between the cooling position and heating position.
4. Finally, when the compressor is powered off, there should only be a gradual equalizing between the high and low side system pressures. A rapid equalization would also suggest that the four-way reversing valve is stuck in between the heating and cooling position.

Auto Defrost

Given that the heat pump produces heat by “cooling” the outdoor ambient condition, it is reasonable to understand that there will be times when the outdoor coil will be operating at a saturated temperature below 32F (0C).

When this happens, frost will build up on the fin and tube surfaces of the coil, eventually restricting airflow through the coil. This will not only result in a reduction in heat capacity, if the frost buildup is severe enough it will allow liquid refrigerant to flow to the compressor inlet resulting in compressor damage.

As such, a defrost cycle is initiated when the coil temperature falls to 32F (0C). Rather than an elaborate set of defrost controls, when defrost is required the unit simply reverses to the cooling mode.

This might seem a little counter intuitive, but this allows the outdoor coil to function as a condenser for several minutes, melting the frost buildup.

In some scenarios, the indoor fan motor will continue, and the addition of electric strip heat will negate the effect of the system being in cooling mode.

Outdoor coils that become severely frosted/iced-up are likely the result of a defective defrost temperature sensor.

Quick Study

Like most systems that a technician may be unfamiliar with, heat pumps might be a source of mystery and confusion. But a quick study of the system, its components, and the design behind the inclusion of those components, should result in the heat pump being just “another” easy system to troubleshoot.

Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor groups. Contact Dave at ddemma@uri.com.