Water and refrigerant, and the same principle under both: a leak gets found, not fed.
This is the single most useful thing a homeowner can know about air conditioning. A refrigerant circuit is sealed. The gas is not consumed, not burned, not worn out, and there is no normal seasonal loss. If your system is low, that refrigerant is outside the system, which means there is a hole in it. “It needs a pound every summer” is not a maintenance requirement; it is a description of an unrepaired leak with a payment plan attached.
It is also not a gray area. Knowingly venting refrigerant to the atmosphere is illegal under Section 608 of the Clean Air Act. A contractor who adds a pound each year, knowing full well the previous pound went into the sky, is charging you annually to break the law slowly. We will not do it, at any price, and the request goes on the report as declined at $0.00 so there is a record of the conversation.
Recover what is left properly into a recovery machine and cylinder — not vented. Pull a vacuum, then pressurize the system with dry nitrogen, typically around 350 psig, and leave it standing. A system that will not hold nitrogen has a leak large enough to find quickly; a system that holds it for hours has a small one, and small ones are the patient work.
Then walk it with an electronic detector: every joint, both coils, the line set, the service ports, the accumulator and the filter drier. Where the detector alarms, soap it and watch. The usual culprits are worth naming because most people are never told them — a Schrader core in a service port, weeping past a worn seal, which is a part costing a few dollars; a flare or brazed joint that was never quite right; and evaporator coils, which corrode from the inside out in Florida and are the expensive answer.
Once it is repaired: evacuate to 500 microns, isolate, and watch for decay. If the reading climbs and settles, there is still moisture boiling off. If it climbs and keeps climbing, there is still a leak and the job is not finished. Then the charge is weighed in to the nameplate and verified on superheat and subcooling — because weighing in gets you close and the readings tell you whether close was right.
The classic call is a warm patch on the floor, a water bill that has jumped, and a meter that will not stop turning. Slab leak is the obvious conclusion, and it is a serious one: jackhammers, or a reroute through the attic, and days rather than hours.
But a hot-water recirculation system produces the same symptoms and costs almost nothing to fix. Recirculation exists so hot water arrives quickly at a distant fixture, and many systems do it with a thermostatic crossover valve under the furthest sink instead of a dedicated return line. When that valve fails open, hot water runs continuously from the hot line into the cold line. Result: a warm floor where those lines run under the slab, cold taps that run warm, a gas or electric bill that climbs, and — if there is a check valve or PRV at the meter — a meter that does not settle.
One of those is a small valve and an afternoon. The other is concrete. So the loop gets ruled out first, every time, before anybody talks about cutting a floor.
Leak location gear is impressive and it is the part people picture. But most of the wrong answers in this trade come from skipping the isolation steps and going straight to listening, because then the equipment is being asked to find something in the wrong system. The cheapest possible outcome is discovering at step two that it is the recirculation loop, and that outcome is only available to somebody who did step two.
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