If you were to ever write an article about HVACR servicing, what tool would you use to represent our business?

I think for most, the best tool to represent our industry is the refrigerant manifold and its gauges because that tool is unique to our type of work. Yet, it is surprising to see (at least here where I live) how few technicians really know what they’re reading.

If you ever want to know how qualified an HVACR service tech is, ask him/her what readings they are looking for on their refrigerant gauges when they are connected to a running system. If the person starts giving you pressure readings (and many will), they really don’t know what the gauges are for. Yes, the gauges do show the internal high-side and low-side pressures, but the pressures only are used when trying to determine compression ratios, which few ever see the need to do.

However, if you take a good look at the gauge faceplate you’ll see there also are readings that show temperatures. That’s usually the most important thing a technician needs to know about a cooling system.

The temperature reading on the high-side gauge shows the middle temperature of the outside coil when there are no contaminants in the system and it is properly charged.

A properly-operating system should read somewhere around 20° to 30° F warmer than the outdoor ambient temperature. The same is true of the low-side temperature. It should read 20° to 25° cooler than the indoor temperature after the system has been running a while if the system is properly charged and there are no contaminants.

You may have noticed in a previous column I wrote that it is better for technicians not to connect the manifold gauges to an operating system, as in a start up or when preforming maintenance, because that often results in refrigerant leaks, contamination of the system (the internal parts of the connecting hoses and manifold often are contaminated) and refrigerant overcharging, all of which reduce compressor life.

Although it may not look as professional, most cooling systems can be checked for proper operation with only a good thermometer.

Indoor coil temperatures can be checked by reading the incoming and discharge air temperatures. You should expect about a 20° difference (10° wet bulb). If you have that, there is no need to connect the gauges.

On the outside, a temperature check of the suction and liquid lines is a good idea. The suction line should be cool enough to cool the compressor (less than 70° on an A/C system) and the liquid-line temperature should be warm to the touch (depending on how warm it is outside), about 5° to 10° warmer than the outdoor ambient. A hot liquid line indicates a problem and a very cool (or warm) suction line indicates a problem, so it may be time to connect the gauges.

Typically, a hot liquid line indicates not enough air flowing through the outdoor coil and a very cool suction line indicates poor indoor airflow, while an ambient temperature suction line indicates a low refrigerant charge.