Diagnosing a Faulty HVAC Compressor Valve

Diagnosing a faulty compressor valve is something HVAC technicians have to deal with on a semi-regular basis, and while a bad valve is often not the main reason a compressor fails, it is an integral part of any condensing unit.

Members of The NEWS’ trainer panel examined compressor valves and determined the best methods for maintaining valves, diagnosing faulty ones, and implementing proper troubleshooting techniques when they go bad.

NOTICING THE ISSUE

Joseph Adeszko, program coordinator, HAC department, Moraine Valley Community College, Palos Hills, Illinois, does not see bad compressor valves as much as he used to, mainly because current technicians are doing a better job of properly reading subcooling and superheat during service visits. Still, “technicians need to know the inner workings of a compressor to properly diagnose it,” he said.

Reciprocating compressors themselves have been around for decades.

“They are reliable, dependable, and still looked at as the industry standard by many people,” said Dennis Silvestri, lead instructor, MRS Educational Training, New Haven, Connecticut. “Still available in the semi-hermetic and fully welded hermetic designs, more reciprocating compressors fail due to various electrical related issues than from compressor valve problems. However, when you take a close look at reciprocating compressor valve design, especially the ‘reed type,’ then we can see what’s looked at as the ‘weak spot.’ The valves will wear, leak, and even break. Floodback and slugging, even flooded starts can cause issues with the valves.”

According to Nicholas Griewahn, associate professor of HVACR, Northern Michigan University, Marquette, Michigan, it is very important to be knowledgeable about diagnosing any capacity loss in a compressor, mainly for the sake of the customer.

“A lot of time and money can be wasted on misdiagnosis, and it usually ends with a lost customer,” said Griewahn. “Not only is it important to be able to diagnose when capacity is lost due to damaged valves but also what caused the damage. Valves that are leaking through due to overheating call for very different diagnostic and corrective actions than valves that are snapped off because of liquid slugging. I’m a big believer that every compressor fails for a reason, and you should always find the cause and correct it before or when you install a replacement.”

Being able to recognize the symptoms of a faulty valve are essential, and Adeszko said problems can stem from a number of things.

“The symptoms could be a loss of cooling or refrigeration, higher-than-normal suction pressures with low discharge pressures, the compressor being very quiet, or low amp draw,” he said.

Griewahn said he normally hears complaints of the system not maintaining the temperature setting during a mild load.

“In refrigeration, the conditioned space will never make the control setting, or the customer will notice the compressor running a lot more than usual,” he said. “In air conditioning, the room temperature will rarely, or never, make the thermostat setting. Really savvy customers may notice an increase in their electric bills without a correlation with the outdoor temperature. Valves that are damaged enough may cause the compressor to overheat and overload, which leads to a no cooling call.”

THE NECESSARY STEPS

Once a technician has recognized the issue, there are a few different steps needed to properly address it.

“An in-depth procedure of checking the compressor valves requires the compressor to have a suction service valve and the technician use their low-side manifold pressure gauge,” said Silvestri. “With the compressor off and the low-side pressure gauge installed onto the suction service valve, front seat the suction service valve and turn on the compressor. The low-side gauge should read at least a 20-inch vacuum within a minute or so. If the gauge cannot read at least a 20-inch vacuum, then the compressor’s suction valves are leaking, which means they are not fully closing shut. The compressor’s discharge valves can also be checked to see if they are leaking.”

Griewahn said it’s important to note that the first hint of trouble usually comes from the pressure readings.

“If you’re familiar with estimating high- and low-side pressure, you’ll notice the high-side pressure will be lower than normal and the low side will be operating higher than normal,” he said. “Pressures being too close together are the usual suspects for lost pumping capacity. I’ll check superheat to confirm the metering device isn’t grossly overfeeding and make sure there aren’t any hot gas valves bleeding through or sticking open and rule them out. To confirm low pumping capacity, I usually check for abnormal line temperatures, low compressor amp draw, and an overheated compressor shell. Sometimes, depending on how badly the valves or compressor are damaged, you’ll hear the valves bypass when the compressor is turned off. If a whistling or ‘whooshing’ sound is heard — not to be confused with the check valve seating in a scroll compressor — when the compressor is turned off, it is likely that the valves or other internal components are bypassing and faulty.”

Adeszko added that there are no shortcuts to take when troubleshooting compressor valves. “A good tech must gather the data necessary to troubleshoot the system before analyzing the data to see what the issue may be,” he said.

A FULL REPLACEMENT

While the panel members agreed on most parts of the diagnostic process, they had differing opinions on when exactly it is time to replace an entire condensing unit.

In Adeszko’s opinion, if the compressor is more than five years old on a residential condensing unit, it’s better to replace the condensing unit completely. “This way the end user gets a new unit with a full five- to 10-year parts warranty,” he said. “The cost is not much more than a compressor-only change out.”

When faced with leaking or broken valves with the fully welded hermetic compressor design, the discussion needs to revolve around replacing the compressor or the entire condensing unit, said Silvestri.

“With the semi-hermetic type, there is the option of replacing the compressor’s complete valve plate(s),” he said. “This is not that difficult of a compressor tear down, especially if the compressor has both a suction service valve and a discharge service valve. This allows for simple isolation of the compressor from the rest of the system. Many factors need to be looked at when considering to replace/rebuild the compressor or just replace the entire condensing unit, such as age of the system, past problems, and how many refrigerant retrofits the system has gone through.”

Griewahn recommends a full replacement of a condensing unit if it meets one or more of the following conditions:

• The cost of purchasing and installing a new condensing unit is the same or lower than a new compressor;
• The condensing unit is available sooner when the equipment is mission critical;
• There is substantial or unrepairable damage or corrosion on the old condenser or unit base; or
• There is a condensing unit replacement option that will substantially increase efficiency for the owner.

“With these conditions in mind, smaller/simpler system condensing units don’t really have much on them that goes bad,” said Griewahn. “Fan motors, service valves, and a few controls don’t usually carry much cost to replace. So, unless one of the above mentioned conditions is present, or I’m unable for some reason to retrofit to a newer (and less expensive) generation of refrigerant, I tend to leave the old unit in place and just replace the compressor.”