During a recent checkup, my mom's Lennox 10 SEER HS29-261-2P 2 ton with Armstrong CAU24-1 indoor coil has developed a leak after 13 years of perfect service.

I confirmed very low refigerant with 60 psi high side and 0 psi low side.

I've charged to 200 PSI nitrogen pulled back the insulation and checked every joint with bubbling solution including the schrader valves. I've been unable to check the most of indoor coil and get inside the outdoor unit to check the compressor. I left the gauges on and after 3 days now see a 10-15 PSI loss.

This is looking like a very difficult leak to find.

My next thought is to evacuate, fill with small amount of r-22 and check with a leak detector, but given I've lost no nitrogen so far I'm not so sure this will pan out as I am not that skilled at finding leaks. Alternatively, I've also been told I could safely charge to 300 PSI since the coils are factory tested to 350.

Any thoughts or suggestions are welcome.

Just to clarify, I have lost 10-15 PSI, so obviously I have lost some nitrogen. I mistyped, sorry for the confusion.

Has she had to get the unit charged yearly? Bi yearly? Or is this the first time it has been low. Odds are it is a small leak in the high pressure side of the outdoor unit. Look where the coil and the metal on the ac from loop that is a prime place for a small leak to start. Also if it is 14 yrs old or so. I see no problem with putting in the stop leak to the system . I have used it a few times and it seems to work well.

I would agree with the above ideas.

Thanks for all the responses.

As an update I charged the system to 300 psi, got a Tif 5550A leak detector and began sniffing around. After many hours with no signs of oil residue it looks like the leak is in the outdoor unit, on the end of the coil with the 180 elbows around a bracket that holds the coil assembly to the sheet metal. I've not been able to pinpoint the exact spot due to a windy day, the bracket and maybe the leak being below and behind the bracket or a fin. But the leak detector goes off regularly in that area.

After some research it would not be cost-effective to replace the coil assembly with new due to the high cost, so my options are to find and fix or replace the entire outdoor unit.

Since Mom doesn't want to spend big bucks, I will try to find the leak. Any ideas on how to find a really, really (did I mention really) small leak is greatly appreciate. Thanks again, this one's for mom.

After you take the top off and the door panels you should be able to see all the loops if you take your leek sniffer and go around them get close to the area. Then take a pair of needle nose pliers and twist the metal frame gently at each loop. By doing so you should cause it to loosen up and leak faster so you can see witch one it is exactly. Cut the metal away from the copper. And burn of any alumanum before you start then just braze over the whole. Do not use soft sodier.

after you take the top off and the door panels you should be able to see all the loops if you take your leek sniffer and go around them get close to the area. then take a pair of needle nose pliers and twist the metal frame gently at each loop. by doing so you should cause it to loosen up and leak faster so you can see witch one it is exactly. cut the metal away from the copper. and burn of any alumanum before you start then just braze over the whole. do not use soft sodier.

Thanks for the aggressive ideas.

Not aggressive just the fastest and easyest way to do it. Also there is a small chance there is more then 1 loop leaking. The last 2 I did each had 3 loops apiece leaking

Good news! I found the leak!

It is on the high side where the compressor 3/8" piping connects to the coil. It looks like a shoddy factory braze because the leak is on the bottom of the joint, leaking around the solder where the solder was not properly sucked into the joint.

Even better news it is definitely repairable. The joint is up high, very accessible and not near anything else that could melt.

I would like to take a shot at brazing/rebrazing this joint. From some of the soft soldering I've done and all the reading I've done, the tools I can get, I think this is doable, especially since I plan to practice on a few joints before tackling this.

So if anyone can guide me, I'd appreciate it.