Home AC - very slow leak

TL;DR: How do you fix a slow leak in your AC when you can’t find where it’s leaking? Or... how do I find where it is leaking?

We bought our house in September 2016. When we bought it, the AC blew cold; on the inspection, I made sure that it had been turned off the day before and was there as it cooled the house from about 77 to 67. We used it maybe 4-5 times that year. This unit was installed sometime in 2012.

In June 2017, by about the 2nd or 3rd time we used it, it was no longer blowing cold - it had leaked some refrigerant. So we had it recharged. The tech used dye but couldn’t find a leak when he came back the next week. 

In 2018... same story. Worked the first time, not the second. Recharged and dye - but I never got the tech back out to check for the leak. 

2019... Same story, but this time he did come back in a week and still, couldn’t find it. Oh, and this is 3 different companies by the way. First one we didn’t like, 3rd one had bought the 2nd. So I doubt they’re trying to scam me.

Now we are getting close to AC season. I anticipate I’ll have the same problem. I want to stop this paying for a recharge every year. We haven’t been able to find a leak. 

How do I solve this? A guy at a bar told me there is something you can inject with the recharge that will close pinhole leaks - is this a thing? He owns a commercial Hvac operation, but it also sounds too good to be true, and it’s rhe ubiquitous guy in a bar, so....

I borrowed a halogen sniffer tester thing from a friend a number of years ago to track down a leak that I wound up finding in the evaporator coil.  Apparently it's got a super sensitive nose on it and it starts beeping in the presence of refrigerant.  There's a lot of line going out to the compressor/condenser, though, and most of that is insulated, so that may pose a problem.

My experience has been they leak in the evaporator.  They must make them out of crap metal these days.  

Leak checking can be tedious and time consuming.  Step one is to recover the remaining refrigerant, then pressurize with nitrogen to at least 150 psi along a trace of refrigerant.   A good quality electronic leak detector will find even the smallest leaks but it takes time; you have to check every fitting which can mean removing insulation on the suction line and assumes you can access the whole piping run.  Black light UV leak detectors work too but I’m personally not a fan of putting additives in a refrigeration system.  There are no “miracle stop leak” answers either. 

Once a leak is located and repaired we leave a nitrogen test on the system for as long as possible (overnight is great if the timing works) and then evacuate to 500 microns or lower with an electronic vacuum gauge.  The vacuum gauge won’t lie, it’s the best indicator of a tight system.   The tech should replace the liquid line filter drier once the leak is found (before final evacuation), not an expensive part and critical to removing any moisture or contamination from the system. 

If your system uses R410a, it operates at much higher pressures than older R22 systems and requires all fittings to be brazed.  If the system was replaced and re-used older piping with soldered connections that could be the source of your problem.   If this ends up being the case, consider replacing the piping with a new line set to eliminate as many fittings as possible.   Good luck, hopefully you can find a company/ tech that will put the time in to find the leak and fix it correctly. 

When I purchased my house, went thru the same thing. A recharge would last a year or so... but would last a shorter duration every charge. Eventually tracked down the leak to the a coil, which of course isn't sold separately. Got a new unit last year.

spitfirebill said:

My experience has been they leak in the evaporator.  They must make them out of crap metal these days.  

Similar experience here.  Seems I heard the term "dissimilar metals" used in the construction of the evaporator coils.  They reliably corrode to swiss cheese in about 7 years and the installer gets to bill $1k or $2k for a nice coil replacement.  I've had it happen to both Carrier and Trane coils. 

Same issue... Purchased house in 2018, AC worked well. First run in 2019 diminished cooling diagnosed with a leak that they couldn't readily detect but suspected coil. Options included recharge ($300), in-depth detection and fill ($700), coil replacement ($$), or unit replacement ($$$). I took the 'cheap' way out but know I'm on borrowed time. Not sure what I'll have this year but planning to bite the bullet I guess...

Uggg... I wonder what my wife would say to us running window units this summer. Already have 1, 2 more would be sufficient for the entire house. 

In reply to 1988RedT2 :

Aluminum coils with copper lines coming in. Carrier is a double pain in the ass because half the time it's cheaper to buy an entire new air handler than it is to buy the coil as a stand alone part.

There are stop leak things you can put in but I'm not actually sure they work. Chances are good it's leaking in the evap coil though.

mtn (Forum Supporter) said:

Uggg... I wonder what my wife would say to us running window units this summer. Already have 1, 2 more would be sufficient for the entire house. 

That's my plan if our 30-year old unit dies before we're ready to replace it. 

Pete Gossett (Forum Supporter) said:

mtn (Forum Supporter) said:

Uggg... I wonder what my wife would say to us running window units this summer. Already have 1, 2 more would be sufficient for the entire house. 

That's my plan if our 30-year old unit dies before we're ready to replace it. 

Same thing we did when ours finally wouldn't hold a charge. Got us through the summer. 

I had to have the evap coil in the house replaced 5 years ago due to a slow leak. $900 later it still runs pretty well for a 15 year old unit. Next major failure will probably mean replacing it. 

The mini-split in my shop has a super slow leak as well and needs to have the charge topped off every 2-3 years. I converted it from R22 to propane several years ago, so topping it off is as simple as hooking up a can of Coleman gas and letting it suck the can dry. I strongly don't recommend doing this to your house unit.