hunting leaks in a R-410A air conditioner

[Friend of Pyewacket]

My system seems to run OK during the cooling season but loses 4-6 lbs of R-410A over the winter.

What is the recommended procedure for hunting slow leak(s) in an air conditioning system with HFC mixture R-410A?

I'm getting inconsistent information on this topic.

1. The installer tried "listening" for the leak by pressurizing the system with several hundred pounds of nitrogen gas.

2. The Carrier service manual for Puron (R-410A) systems suggests hunting with a HFC sesntitive electronic leak detector in a system pressurized to 150-375 psi with R-410A or with a mix of nitrogen and R-410A.

I'm working with a different service provider and would like to find the leak in a cost efficient and environmentally responsible manner.

[exsearsguy]

Do you see any signs of oil? That much gas loss should  leave a sign.

[Jedi Appliance Guy]

What part of the country are you in?  Do you get snow in the winter?  Couldn't hurt to stick a little teflon tape on those service ports.  A nitro isolation would find it.  What kind of coverage (if any ) do you have on it?  Extended warranty maybe?

[Friend of Pyewacket]

about visible signs of leaks:

The service tech says there's no oil on the expansion valve or the ~2 ft of copper pipe leading into it. The rest of the piping is in foam insulation and behind drywall.

about geography:

We're in suburban Chicago where winter=snow and the other 3 seasons = road construction.

about service fittings:

The service tech made a point of using a BIG wrench to secure them.

about warranty:

Alas, we just have Carrier's parts warranty, the installer is long gone.

[Jedi Appliance Guy]

  Although I have no first hand knowledge of what it must be like in the cold (I reside in the tropics) the Force is telling me in might be possible  for the extreme cold to shrink the rubber o-rings and such in those service valves enough for gas to escape.

  When your  A/C Guy charged your unit, did he measure the liquid line temperature and use the sub-cooling method, or did he put his hand on your suction line and use the beer can method?

 Now that is working well you can get a feel for what it is supposed to be doing.  Feel the air coming off the condenser unit.  Feel the skinny pipe.  Feel how cold the large copper pipe near the suction service valve is.  The air coming out of your air handler should be 18 to 20 degrees colder that the air going in.  Now you can monitor things during the summer cooling season.