Many times when servicing a piece of equipment, a service technician will omit reading the discharge pressure. This might be due to the system not having an access valve on the high side, or maybe the technician is trying to save steps or cut time on the job.

In any case, failure to read the discharge pressure could lead to misdiagnosing a problem or not finding the real problem with the system.

By reading only the suction pressure, a technician diagnoses only half of the system.

Incorrect Charge, or Something Else?

Let’s say that a tech is working on a system with a capillary tube as its metering device and he records a lower-than-normal suction pressure. His first diagnosis would be a system with a low refrigerant charge. This may be true. However, the capillary tube could be plugged, and this would also give a lower-than-normal suction pressure reading.

If the technician measured the discharge pressure, he would see that a system with a low charge would show both low suction and discharge pressures, and a system with a plugged capillary tube would show a low suction pressure and a higher-than-normal discharge pressure.

If a tech is working on a system and he finds that the suction pressure is higher than normal, this could mean either a compressor that is not pumping efficiently, or a system with an overcharge of refrigerant.

By measuring the discharge pressure, the tech should be able to tell the difference. A system with an overcharge of refrigerant will have both high suction pressure and high discharge pressure. A system with a defective compressor will have a high suction pressure and lower-than-normal discharge pressure.

Another risk of not measuring the discharge pressure is the possibility of overlooking other problems. These problems could appear in relation to the current problem, or they may cause new problems with the system in the future.

For example, on systems with remote air-cooled condensers, it isn’t always easy to get to the condenser to examine its condition. If a technician skips measuring the discharge pressure, his diagnosis may be unrelated to the actual problem. The condenser may simply be dirty and in need of cleaning. The tech could have easily determined this by looking at the discharge pressure.

Measuring the discharge pressure should be a normal part of any service call. Unfortunately, sometimes it does get overlooked, which leads to needless return calls and unhappy customers.

At times it may seem like an insignificant part of a service call, but it is truly necessary to completely troubleshoot any system properly.

Marchese is owner of Arctic-Air Refrigeration, Pittsburgh, PA.

Publication Date: 10/02/2000