In my column that ran in the February issue I suggested it would be better for HVAC service techs to use thermometers rather than manifold gauges when doing preventive maintenance calls.
As I explained, connecting the manifold creates all sorts of contamination problems. It too often results in refrigerant leaks that require a callback. And on the new high-pressure systems, a lot of refrigerant often is lost when connecting and disconnecting the hoses.
Shortly after the magazine came out, I received an email from a reader pointing out the obvious — that obtaining superheat and subcooling readings usually requires the use of both gauges and a thermometer. Yes, this is true. But perhaps I should have gone on to add the following.
There are, of course, many service problems that will require the use of pressure gauges. But understand that I was talking about preventive maintenance calls.
There are some simple facts that should be considered when checking the refrigerant charge and the overall operating condition of an air-conditioning system. For example:
- Subcooling can be read on a thermometer by checking the temperature at a middle bend of a condenser coil against the temperature at the liquid line.
- Typically, the liquid line will be just slightly warm to touch – about 5° F to 10° F warmer than the outdoor ambient temperature.
- While it often is impossible to read the low-side saturation temperature with a thermometer since evaporator coils are hard to access, the suction line should be slightly cool to the touch after the system has been running a while – about 5° to 10° cooler than the indoor ambient temperature as read at the air return.
Understand that using such a “rule of thumb” is a better idea than connecting the manifold because the point of such a call is to do more good than harm.
So, if the liquid line is clearly being subcooled and the returning refrigerant is cooling the compressor with no floodback (not too cold), the system is best left alone. Of course, I realize that a homeowner watching the procedure may not be too impressed. But thermometer readings and an explanation should suffice.
One of the nice things about most of the new systems is that they use thermostatic expansion valves, not the capillary tubes or metering pistons that were employed on many of the old R22 systems (the charge is far less critical).
The biggest challenge to service techs performing preventive-maintenance checks is overcoming the desire to “top-off” the charge. This usually results in an over-charge that degrades system performance. Remember: if the charge is low, there must be a leak in the system that should be located and repaired!
Of course, a liquid line that is too warm indicates a low charge or a dirty or deteriorating condenser coil and this might (but might not) require taking some pressure readings.
The same is true of the suction line. If it is too warm, there may be a low charge or something wrong with the metering. If it’s too cold, the tech should check the indoor airflow, the metering adjustment, the charge, or the air or refrigerant distribution around and through the evaporator coil.
What I’d like to see in our industry is preventive maintenance techniques that make systems live longer, not shorter.