Diagnosing Heat Pump systems is similar but not the same as straight cooling systems. First off there are a few different types of heat pump systems that will affect the diagnostic procedure.
One of the first things to consider is the common diagnostic practice of listening to the client’s description of what’s going on. They are “living with” the issue and while they may not be technical, they can provide good information to the diagnostic process. To review there are a few items unique to the heat pump: the reversing valve, defrost control, a second metering device (TXV) on the outdoor unit, bi-flow drier and a heat pump specific thermostat. The heat pump compressor is designed to accommodate some liquid slugging; however, most systems will have an accumulator ahead of the compressor.
As discussed in the previous article, Heat Pumps come in a few different varieties. Air to Air, looks like a straight cool, uses ambient air for the heat exchange. We will see these used in a typical residential application. Next, we have water source which uses water for the heat exchange medium, we can expect to see this commonly used in condominiums and large custom homes utilizing a cooling tower set up. We can tell a water source unit by the condensing coil; they are typically loop type coils with counterflowing water and refrigerant. Lastly there is ground source which uses the earth’s mean temperature to remove heat. These coils look like water source; however, the main difference is the ground source has tubing in the ground while the water source typically is connected to a cooling tower or heat exchanger. Ground source systems are very energy efficient since they do not require a cooling tower or as many pumps. Each will have unique features to diagnose. The best recommendation is to follow the manufacturer’s diagnostic flow charts for each. Below we will discuss some common diagnostics across all 3 types.
Heat pump compressors are like compressors in air-conditioning units. However, the heat pump version is different in that they can operate at lower ambient temperatures vs. the strait cool compressors. A heat pump compressor must be designed to handle some liquid slugging without damage. This is done by using slow or soft starts, stronger valves, as well as additional strengthened internal components. In addition, suction line accumulators and bi-flow filter-driers are used in heat pump systems to protect the compressor from liquid slugging and contamination.
Restrictions can occur in various places throughout the heat pump system; some unique locations are the outdoor TXV. This specifically occurs in the heating mode. During the cooling cycle the outdoor TXV is bypassed and a non-factor in the refrigeration cycle. When switched over to the heat mode the TXV can become stuck due to lack of exercise – wait, have they been talking to my cardiologist? No, by lack of exercise I mean they are not used often, especially here in south Florida. When the heat is used for the first time in several months or even years the TXV can be stuck. It is a best practice to exercise the heat operation during the PM visit.
Defrost Controls can cause a malfunction, typically this is the defrost control board (DFC) or the sensors providing coil temperature input to the DFC. These sensors are commonly 10k ohm resistors, but you’ll want to verify the manufacturer’s service literature to know for sure. Most will provide a temperature/resistance chart. i.e. if it’s 68°F ambient, the [10k ohm] sensor should read 12,491 Ω resistance. Knowing how to “ohm” a sensor is a critical skill. First disconnect power, then remove the sensor where it lands on the DFC, with your multimeter set to resistance (Ω) using your leads measure the resistance.
Reversing Valves can bypass (internal leak) hot gas to the “cold” side of the valve. Additionally, it can become stuck like the heat side TXV that hasn’t been used for a long time. You can measure the leakage with a thermometer or even a laser thermometer. No matter which mode the system is in, the center tube of the reversing valve is the suction line, sometimes call “true suction.”
Referencing the diagram here you can use your thermometer to identify a reversing valve that is bypassing or leaking. This will cause unusual pressures.
While discussing the reversing valve (sometimes called the switchover valve SOV), I want to clarify the low voltage terminal O/B. For those of us just getting familiar with Heat Pump systems we will see this terminal on the thermostat, AHU and outdoor unit. The “O” terminal is most widely used, it sends the 24v signal to the reversing valve. When the heat pump thermostat calls for cooling then this terminal closes and energizes the reversing valve causing it to switch from a default position of heating to a position of cooling. This is done as a failsafe for heating, which for most of the country is considered more important than cooling. We think differently here in sunny south Florida! If the reversing valve or its solenoid would fail for whatever reason, you could still have heat. The “B” Terminal is also for the reversing valve except in this case it reverses from a default position of cooling to a position of heating inside the heat pump. To further this discussion, Trane products label their common terminal “B” with their reversing valve being “O” making it important to review the specific manufacturer’s service literature. This can be confusing when landing low voltage on a thermostat with the O/B terminal.
Dirty sock syndrome is a name applied to complaints regarding a foul odor in heat pump systems, specifically the air handler portion. This is due to the description of an odor of gym socks emanating from the room grills. The cause of this problem is the growth of biologics and/or bacteria on the indoor coil. Heat pumps are susceptible to biological growth because their heating cycles are not hot enough to kill the microbes that thrive on the indoor coil. This biologic growth creates a maintenance problem and results in dirty coils, decreased efficiency, dirty drain pans, and excessive energy use. The most successful way to eliminate this growth is by using germicidal ultraviolet lights after a thorough cleaning.
I hope this introductory information about heat pump systems creates a desire to learn more about them, we’ve just scratched the surface here. I believe we as an industry are moving away from strait cooling units with electric resistance heaters to heat pumps. Either way, understanding these systems is a great skillset to have. Please stay safe in all that you do!
