Troubleshooting Heat Pumps

I'm on record many times for saying that the basis for troubleshooting any type of equipment is rooted in a simple question, "How are you ever going to know what wrong is if you don't know what right is in the first place?", and that the process involves the systematic elimination of the possibilities.

With that thought in mind, we'll take a look at a heat pump and follow up on the complaint by the customer that "it's not heating enough."

When we arrive at the site, what we find is a standard package unit heat pump and all components are operating....the compressor, indoor fan motor,  and outdoor fan motor (remember, we never refer to a "condenser fan motor" on a heat pump because the outdoor coil doesn't act only as a condenser in a heat pump the heating mode, the coil actually acts as an evaporator to absorb heat and transfer it into the conditioned space) are all operating normally. We also determine that the ambient temperature is high enough that supplemental heat is not a factor here. It's the refrigeration and air flow system alone that is supposed to be providing enough heat to keep the conditioned space at the desired comfort level.

And, when we check the indoor area, what we find is that it is, indeed, not as warm as it should be. Performing a simple dry bulb temperature differential test at the return and supply registers, we note that the temperature rise is not in accordance with the manufacturer's specifications.

Since we've made these observations, the next thing in the troubleshooting process would be to confirm our understanding of the operation of the refrigeration system in the heating mode. And the key to this is understanding the operation of the reversing valve itself, shown in the illustration below.

Figure One

Figure One

In this illustration, we see the path of refrigerant from the compressor discharge line as it connects to the body of the valve, and on through valve, discharging to the indoor coil. We can also see the refrigerant return from the outdoor coil, and its passing through the valve body before being routed back to the compressor via what is know as the True Suction Line.

This is where the process of elimination can begin in this situation, and the reason I'm making this recommendation is because, while it's not a common occurrence, it's possible that the reversing valve is leaking when the conditions we've outlined so far are encountered. So, the next step to take in dealing with this situation is to test the reversing valve via a couple of simple temperature tests before we would go ahead with any refrigeration system pressure tests or more time-consuming tests of the air flow system.

In the illustration below, two simple temperature tests are shown.

Figure Two

Figure Two

When you see the results of our temperature tests on this reversing valve, you can understand that we've proven that it is the source of the problem.

On the line coming from the outdoor coil, our temperature reading is 30-degrees. And, when making a second test at the tubing that exits the body of the reversing valve, we see a temperature of 40-degrees. Since we should never see a temperature difference of more than 3-degrees at any two points when testing the tubing that enters and exits a heat pump reversing valve, we've discovered that hot vapor is leaking into the suction line. And, this condition is keeping the heat pump from performing properly, with the end result that the customer is complaining that it just isn't heating enough.

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Troubleshooting HVAC Pages

Technical Training Associates
PO Box 2259
Green Valley, AZ 85622-2259
Phone: 520-625-6847

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