What causes Aircon Gas Leak?

Your aircon shouldn’t leak gas at all; if it was properly set up, gas will not leak even after 10-20 years!
Now, lets take a good look at high risk areas where gas leaks often occur.

1) At the locking nut area.

Picture of the nuts removed. The ‘o-ring’ seal will 
leak after some years. Therefore, the outer nut is 
vital to be properly tightened.

Located at the condenser, you’ll see all these nuts. Some condenser has a cover so you may not see them. These nuts must be properly tightened. If they are not properly tightened, gas can leak from there.

Since the copper pipe is soft, once tightened, it forms a very good seal. But over tightening can cause it to crack.

Description of the faulty flare:
The nut which i’m holding was overly tightened, causing the flare to crack. The pipe which is ~1mm thick is now so compressed; the flare is in fact razer 
sharp!

Great care must be taken when forming the flare; if the flare is not formed well, with ripples, it may cause gas leak too.

Thus, if it leaks from here, it is caused by poor workmanship and not sabotage. DO NOT conduct chemical cleaning by dismantling the entire fan coil. You will risk damaging the flare joints and subsequently leading to gas leak. You may like to read up on keeping your copper pipes.

2) Inside the condenser

If any 2 pieces of copper pipes were accidentally touching one another, the vibration caused by the compressor can soon rupture a hole in the pipes, causing gas leak. Since the pressure inside the condenser is a lot higher when compressor is running (~250psi for r22 refrigerant and 350psi for r410 refrigerant), a leak in the condenser will deplete the system very quickly, rarely lasting more than a week or two. PS: the pressure of your car tire is only ~30psi

The compressor sits on top of 3 pieces of rubber shock absorbers. Since the compressor is hot, and if the shock absorber is made of inferior quality, it may melt or soften. This will cause the compressor to either tilt to one side of simply lower by an inch. It is often this condition that causes two copper pipes to touch one another.

When technician over-charged the gas system, it causes the compressor to overheat, thus increasing the risk of damaging the shock absorbers. On top of that, an over charged system causes the compressor to vibrate vigorously, thus touching pipes that were marginally apart.

3) It leaks at the pipes, severity determined by rate of leak:.

a) It leaks every few years.

If it runs out of gas every few years, it is a really tiny gas leak. Don’t bother conducting pressure test nor any other sort of test, it is just not worth spending that kind of money. Assuming your aircon will last you 10 years, so at most you’ll top it up 10 times. Troubleshooting and replacing the pipes will cause you more than that. Don’t worry, modern refrigerant gas is not flammable and is not harmful to the human body unless taken in huge amount (i mean really HUGE amount). In the olden days, ammonia gas was used as refrigerant and this gas is highly toxic (fatal), but those are history.
.
b) It leaks in a couple of months time.

This rate of leak is definitely disturbing and most likely frustrating. But technically speaking, this is still a very small leak and any attempt to isolate the problem (not pin point) will be very costly especially if it is a multi split aircon. If you are lucky, it leaks at places that are not concealed.  There will be traces of oil at the place of leak. If leak can be picked up visually, all it takes is to spot weld the leak and recharge the gas, total repair cost will be $180/hr (inspection) + brazing ($168), gas recharge ($90 to $250), total ~$400-$700.

But, If visual inspection cannot detect any traces of oil, the leak might be occurring in the concealed copper pipes. 
Pressure testing ($200 per set of pipe) will need at least half a day before we see any significant drop in gas pressure.

Note: your condenser liquefies the refrigerant gas by compressing gas by ~600 times. Therefore, if we conduct 
pressure test by using compressed air instead of liquefied gas, the rate of leak will “improved” significantly by 
approximately 600 times. Therefore, what that took 90 days (2160 hours) to leak can be revealed in ~2160/600 = ~3.5 
hours.

Therefore, we come in, set it up and back in the evening to monitor if pressure drops. But such test can only isolate which 
pipe and blower set is leaking, it does not pin point. If you want to further isolate whether it is the pipe or blower, the 
pressure test must be repeated at $200, and another day without aircon.

If it leaks at the connecting pipes, the pipes must be replaced. Depending on whether you want to run the new pipes 
concealed or externally (with casing), pipe replacement is not cheap, and depending on the level of difficulty, pipe 
replacement, will cost a minimum of $300 – $500.

But, the test itself (assuming a system 4) would have cost you $800. Making total repair cost ~$1300.

Therefore, gas leak can be a very painful experience for any house owner. Always avoid dismantling your air conditioner for chemical cleaning, gas leak is not something you’ll like to experience.

c) It leaks in a few days.

This is better indeed, leak will most likely be occurring in the condenser and can easily be picked up visually. Provided that it leaks at copper pipe sections and accessible, all it takes is to spot weld the leak and recharge the gas, total repair cost 
will be $90-$300 (inspection) + blazing ($168), gas recharge ($90 to $250), total ~$350-$700