I had a 2 or 3 rats in my attic this winter.

I put my son on Rat Patrol and we got em all. But not before they chewed through the washer drain line and two plumbing lines under the upstairs floor ... flooding two rooms downstairs.

Anyway, started running the AC a couple of weeks ago and it wouldn't cool to below 78.
Had AC guy out and it was 3 lbs low. Should have 14.

Any chance a rat could chew through the line? I figure if it did, all the freon would escape, but not sure.

It would be 100% out of freon if rat-chewed, and would not recharge.

But do you have a water drain line from the condensor to outside? Might want to check it for being chewed. Would hate for that water to drip inside.

That's what I figured, Canyon.

He used a leak detector on the outside unit and in the attic and found nothing.

He said if if keeps leaking they'd disconnect both ends and pressure test the line from outside to inside.

There is a drain line. It is fine. Good tip, though.

Leaks in an AC system are usually where the line set is brazed onto the coil or AC unit, or the bends in the evap coil or the condensing coil.

The leaks tend to be very difficult to find and even more difficult to repair.

It's much cheaper/easier to add refrigerant when needed.

Agreed with EMY on this one... Depending on the rate of loss, it's a lot easier/cheaper to just charge it back up than to do anything else. Leaks can be hard to find... and when you do find them, they're most oftentimes in places that can't be fixed very easily or cheaply.

We had a leak in our upstairs unit that I nursed along for 3 summers before I finally got a new complete system. The leak was in the evaporator coil and the unit was so old it basically couldn't be repaired. So I just added freon once or twice a year and let it be.