Service valves.

Service valves are where the line set connects to the outdoor unit. There are two, one on the liquid line and one on the suction line, and they let the refrigerant charge be isolated in the outdoor unit for installation or service.

They are operated with a hex key under a cap, and each usually carries a Schrader port for connecting gauges.

Frost at the valve

This is the observation worth knowing.

Frost that begins at the suction service valve and works its way back along the insulated line is a recognised sign pointing at the refrigerant charge, as opposed to frost that starts on the indoor coil and spreads, which points at airflow.

Neither is a certainty from the driveway. Both tell you where to look first, and the ice guide covers the distinction.

Why they leak

Three reasons, all common on Valley equipment.

The valve stem seal ages. Every time somebody opens or closes the valve, that packing works.

The cap is missing. On most service valves the cap is a sealing cap, not a dust cover, and it is the primary seal on the stem. Caps left off after a service visit are a genuine and avoidable slow leak.

The flare or brazed connection at the line set, which sees thermal cycling every day for decades plus whatever the Phoenix sun does to anything exposed.

What to look at yourself

You can see them without tools. Look for:

• Caps present on both valves and both service ports
• Oil staining around the valve body or the connections, since refrigerant leaks carry oil with them and leave a dark greasy residue that collects dust
• Insulation intact on the suction line right up to the valve

Oil at a valve is one of the more useful things a homeowner can spot, because it localises a leak somebody would otherwise spend time hunting.

What not to do

Do not turn them. A service valve closed on a running system is how compressors get destroyed, and opening one on a system with charge in it vents refrigerant, which is both expensive and illegal to do deliberately.