In a perfect world your A/C unit should not lose any refrigerant. Unfortunately, leaks do occur and sometimes they are virtually impossible to find. The problem could be in the evaporator coil, line set, or condenser.
Typically leaks are found around brazing joints, fittings, or service valves. Refrigerants are in an oil base so any oily areas are usually indicative of a leak. Those are the easy ones to fix. The difficult ones are usually in the evaporator coil or condenser. A common problem with older systems is when dogs urinate against a condenser. The urine is highly corrosive and will erode the condenser fins which will lead to a refrigerant leak.
The methods used to find leaks are:
- Visual — Look for oily residue
- Audible — Listen for a hissing sound
- Soap Bubbles — Shows if a leak is present
- Electronic — Instruments that emit an audible sound when sensing escaping refrigerant
- Dye — Coloring agent injected and after 24 hours using a UV light to see signs of dye on surface around leak.
If these methods fail to find the leak then it may be time to replace the equipment. (Especially if the equipment is over 10 years old.)