Diagnosing problems in refrigeration or air conditioning systems equipped with expansion valves is like solving a puzzle. The temperature and pressure readings you gather at various points in the system are the pieces. When you have all the pieces your puzzle is easy to solve. Leave a piece out, and you may misdiagnose the problem.
The piece of the puzzle that’s most often skipped is the superheat measurement. This may be because while most pieces of the puzzle are easy to obtain, superheat requires a little more effort. Installing a temperature probe near the bulb of the expansion valve at the outlet of a small reach in cooler, or in a walk-in freezer with an evaporator surrounded by product, definitely presents a challenge. When taking your temperature readings keep a couple things in mind. First, make sure you install your temperature probe at the 5 o’clock or 7 o’clock position on a horizontal pipe. On vertical pipes, try to keep the probe as close to the bulb as possible. Good thermal contact is a must. Strap the probe tightly and insulate it. Sliding the probe under the insulation without strapping it tight isn’t good enough. An incorrect temperature reading will lead to the wrong diagnosis.
If you can’t get take your temperature readings at the bulb, and if the line set is short (less than 10-ft.) you can take the reading at the compressor. It’s not as accurate, but it’ll work. Just be sure there isn’t a liquid line heat exchanger before the evaporator. The heat exchanger, by its nature, will elevate the suction gas temperature and give you a faulty reading at the compressor.
On long line sets, the preferred method of checking superheat is obtaining the temperature and pressures at the evaporator. Because of the suction pressure drop between the evaporator and the compressor, a suction tap on the suction line or equalizer line will give you a true suction pressure reading, and eliminate the guesswork. It’ll also eliminate running back and forth to the compressor to read your gauges.
The superheat measurement is a very important piece of the diagnostic puzzle. Don’t overlook it.
Dave Galbreath is the service manager at Seaman’s Heating/Air Conditioning/Refrigeration, Grand Rapids, MI. He can be reached at 616/458-1544, or by e-mail at firstname.lastname@example.org.