What it is

The two compressor designs you'll meet most are the reciprocating (recip) and the scroll. A recip uses pistons and valves, like a tiny engine running in reverse. A scroll uses two spiral plates, one orbiting inside the other, to squeeze refrigerant. Scrolls took over most residential and light-commercial AC because they're quieter, more efficient, and more tolerant of a little liquid. But each has quirks you need to know — a scroll can be ruined by running backward, and a recip's valves are a classic failure point. Knowing which one you're working on changes how you diagnose it.

How it works

A reciprocating compressor pulls refrigerant vapor into a cylinder on the down-stroke through a suction valve (reed), then compresses it on the up-stroke and pushes it out a discharge valve. It's a positive-displacement pump with mechanical valves that open and close every stroke. Those reed valves are precise and thin, and they're the part that tends to fail — a broken or leaking valve kills pumping capacity. Recips are robust and were the standard for decades, but they're louder and don't like liquid refrigerant slugging into the cylinder.

A scroll compressor nests a fixed spiral and an orbiting spiral. As the orbiting scroll moves, it traps pockets of vapor at the outer edge and walks them inward, shrinking the pocket and compressing the gas until it's pushed out the center port. There are no suction/discharge reed valves in the conventional sense — compression is continuous and smooth, which is why scrolls are quieter and have less vibration. The smooth, continuous squeeze also tolerates a slug of liquid better than a recip because the scrolls can separate slightly to pass it. The catch: a scroll only compresses in one direction of rotation. Spin it backward and it doesn't pump — it just makes noise and builds no pressure — which matters on three-phase scrolls where phasing sets rotation.

In the field

Identifying which you have: scrolls are typically rounder, smoother-running, and quieter; recips often have a more pronounced thump and vibration. The model/nameplate confirms it.

Scroll-specific checks:

  • On a three-phase scroll, rotation direction is set by phase sequence. If it was recently wired or a disconnect was changed and the scroll is loud, not pumping, and pulling odd amps, suspect reversed rotation — swap any two of the three line leads to correct it. A scroll run backward makes noise, builds little to no pressure differential, and can overheat.
  • Scrolls often have a built-in internal discharge check valve, so they can be quieter on shutdown but may need different start considerations; some equalize slowly, which is where a hard-start kit earns its keep.

Recip-specific checks:

  • A recip that runs but won't build much pressure differential, with suction and discharge pressures wanting to equalize while it's running, often has failed reed valves. Amp draw may be lowish because it isn't doing much work.
  • Recips are less forgiving of liquid floodback and slugging — repeated liquid return breaks valves and washes out bearings.

Common to both: confirm windings, ground, cap, and voltage before condemning. A compressor that won't pump could be electrical, not mechanical.

Normal values & targets

  • A healthy compressor of either type pulls suction down and pushes discharge up, creating a clear pressure differential within seconds of starting.
  • Running amps at or below nameplate RLA; a compressor doing little work (bad valves or wrong scroll rotation) may pull oddly low amps while making noise.
  • Three-phase scroll: correct rotation is mandatory; reversed rotation = noise, no/low differential, rising compressor temperature.
  • Scrolls generally run quieter and smoother; a normally quiet scroll suddenly loud is a red flag (rotation, mechanical damage, or liquid).
  • Liquid tolerance: scrolls handle minor floodback better than recips, but neither is designed to pump liquid continuously — both can be destroyed by sustained slugging.

Common faults & what they mean

  • Three-phase scroll loud, not cooling, low/no pressure differential after rewiring → reversed rotation; swap two line phases.
  • Recip runs but suction and discharge drift toward each other under load → broken/leaking reed valves; the compressor can't pump.
  • Either type making metallic knocking on start → liquid slugging into the compressor; find the floodback cause (overcharge, low load, failed metering device).
  • Scroll that's normally silent now growling → internal mechanical damage, liquid, or reversed rotation; investigate before it self-destructs.
  • Compressor won't pump and is electrically fine → mechanical failure (valves on a recip, scroll set/bearing on a scroll).

Tech tips & gotchas

The scroll rotation gotcha bites techs after any three-phase work — a new disconnect, a utility change, a rewire. If a three-phase scroll comes up loud and isn't building pressure, don't condemn it; check rotation first and swap two legs. Single-phase scrolls don't have this issue (their start circuit fixes rotation), so this is strictly a three-phase concern.

A recip with bad valves fools people because it runs smooth and quiet and the windings test fine — but it won't build a differential. If a recip is electrically healthy, spinning, and the gauges won't spread, think valves.

Protect both types from liquid. A scroll's better liquid tolerance is a margin, not a license — chronic floodback still kills them. Address the root cause (charge, airflow, metering device) rather than relying on the compressor to survive it.

Scrolls can have a low-side or high-side internal pressure relief and an internal discharge check; sometimes a "won't restart" or a hiss equalizing on shutoff is normal scroll behavior, not a fault. Know the design before you chase a ghost.

Safety / code notes

For any compressor R&R, recover refrigerant per EPA Section 608 — never vent. De-energize at the disconnect within sight of the unit (NEC Article 440) and discharge capacitors before touching terminals. On three-phase equipment, verify rotation safely with the panel buttoned up appropriately; a backward scroll will overheat quickly, so don't let it run reversed for long while diagnosing.