A quick feel of the liquid line can tell you a lot about how the system is operating, not everything, but a lot nonetheless. You may not be able to diagnose a system by grabbing refrigerant lines and playing "Human Thermometer Man" but a little touchy-feely can point you in the right direction. Before we go too far let me say this; I've always hated it when technicians say a good suction line temperature is "Beer-Can-Cold". Using slang like beer-can-cold just reinforces the image of trades people as beer drinking, football watching, pickup-truck driving...OK, maybe they're right on this one. Let's get back to liquid lines...
There are a few "loose" rules when using liquid line temperature as a quick check-
- Indoor and outdoor temperatures need to be typical for summer operation; this won't work if its forty-five degrees outside and sixty-four degrees inside.
- The liquid line leaving the condenser will never be lower than the outdoor temperature unless there is a restriction in the coil or the metering device is mounted in the outdoor unit (mixed-phase line, usually found on some brands of duct free splits)
- The temperature must be felt close to the outdoor unit, not fifty feet away.
So, what should the liquid line feel like on a properly charged system? It should be slightly warm to the touch; around 100 degrees. If its cool to the touch, the system is probably over charged. If the liquid line is warm or hot to the touch, it's probably undercharged.
This "quick-check" is one of the ways to get a idea of how the system is operating. Can it be used to diagnose the system? No, but combined with other quick checks it may guide you to the potential problem.
Always remember; super-heat and sub-cooling values are the only way to determine how the refrigerant side of the system is working.
By the way; how should the suction line typically feel on a properly running system? Uh, beer-can-cold isn't far off the mark...