Refrigerant Leak check

Looking for Refrigerant leaks
Finding leaks can be quite difficult, and often very time consuming. If doing this for a customer, let them know that it can be expensive and you may still not find the leak. For residential systems, I typically limit it to 1 hour at most. Patience is key for small leaks. Keep the system powered off during the search.

Using a refrigerant leak detector
This is by far the quickest and most reliable for smaller leaks and the system still has some refrigerant left. A leak detector such as the inficon d-tek lineup. Most models require a warmup time which should be done in fresh air as they self calibrate. When the detector is ready, SLOWLY move along the across any place there might be a leak
 * Focus on places that have oil stains.
 * Joints in the refrigerant piping.
 * Valves
 * Accessories connected to the piping.
 * Places where the piping passes through walls or is able to rub against hard surfaces.
 * Evaporator / Inside coil. This is the most likely place
 * Best to leave the fan off for a few days to allow any leaking refrigerant to settle in low lying areas. It is heavier then air and will make finding leaks easier.
 * Mechanical joints (for the metering device)
 * The small distribution tubes wherever they touch something else
 * Return bends on the coil
 * Face of the coil. This is the most common with manufacturing defects but hardest to actuality find.
 * Condensor / Outside coil
 * Same as the evaporator coil above, but is less likely.

Using leak detector soap
This is specialized spray on soap which easily foams bubbles. Eg. Nu Calgon's Cal-Blue. Finding small leaks are possible but difficult. Add compressed nitrogen if the system no longer contains pressure. Large and medium sized leaks will be nearly instantly detectable. Very large leaks can now blow the soap away making the hole hard to isolate when you can't directly see it. Very small leaks still require some patience for the bubbles to foam.
 * Listen for leaks and focus there
 * Focus on oil stains
 * Field welded joints
 * Mechanical joints
 * Accessories connected to the piping
 * Metering devices
 * Places where the piping might have rubbed on hard surfaces

Using liquid Dish Soap
Sometimes you just don't have a choice. Without access to the leak detectors above this is a last resort but can work with enough patience. It will not find very small leaks. Medium sized leaks will be difficult to find. Non acidic based soaps are preferred. All soap must be throughly rinsed / wiped off after use. Any soap left on the piping can damage the copper. If the system no longer contains pressure, add compressed nitrogen. Do NOT use compressed air as this will introduce harmful contaminates. The procedure is similar to using leak soap as described above. Do not dilute the soap and apply it with your fingers or a dispenser. Use a towel of any kind to hold the liquid in place. You will have to wait on each location for a few minutes, longer is better.

Eyeballs and Ears
The built in sensors of the human body can detect larger leaks. If you don't have the tools, but want to reduce the search area for a hired service tech, this can be very helpful. Isolating a leak to even a general area can save quite a bit of time and money.
 * Look for oil stains
 * Listen for hissing
 * Glide your fingers over suspect areas to feel for the leak. Start at palm distance away from any leaks, to prevent larger leaks forcing gas through your skin. Never try to hold a leak, fingers and repair kits will not work. Be careful of the hot components.