Scroll compressor

For homeowners

The scroll compressor is the pump at the heart of your air conditioner — the part that moves refrigerant around the loop and does the actual work of pulling heat out of your house. It sits inside a sealed steel can in the outdoor unit. You can’t open it, you can’t service it, and if it dies you replace the whole compressor (often the whole outdoor unit, since it’s usually cheaper). It uses two interleaved metal spirals — one fixed, one orbiting — that squeeze refrigerant gas into a smaller and smaller space as the orbit progresses. By the time the gas reaches the center, it’s at high pressure and high temperature, and it exits through the top of the can.

The reason this matters to you as a homeowner: most everything else in your AC exists to support the compressor. Keep the air filter changed, keep the outdoor unit clear of debris, and don’t let the system run while low on refrigerant. Those three things determine whether your compressor lasts ten years or twenty. The compressor is the single most expensive part on the system — typically $1500 to $3500 to replace — so the maintenance that protects it is the maintenance worth doing.

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For technicians

This is a scroll. It has been the dominant residential compressor topology for thirty years now and there’s a reason — fewer moving parts, smoother torque, quieter operation, and much better tolerance of the abuse residential systems take in the field. The reciprocating piston compressor it replaced is still around in older units and some commercial applications, but for a new straight-cool split system installed in the last twenty years, the odds are overwhelming that what’s inside the dome is a scroll.

How it compresses refrigerant. Two interleaved spiral plates, one fixed to the top of the housing, the other free to orbit (not rotate — orbit, in a small circle, while staying angularly fixed). When suction gas enters the outer edges of the spirals, the orbital motion of the moving scroll creates crescent-shaped gas pockets between the two scrolls. As the orbit continues, those pockets migrate inward toward the center and simultaneously get smaller. Pressure rises as volume decreases — that’s compression. By the time the pocket reaches the center of the spiral, the gas is at full discharge pressure, and it exits through a port in the center of the fixed scroll into the discharge plenum at the top of the housing.

What makes this elegant: the compression is continuous. A piston compressor takes in gas, compresses, discharges, repeats — pulsing the refrigerant in slugs. A scroll has multiple pockets at different stages of compression at all times, so suction is being drawn in at the outer edge while gas is being discharged from the center simultaneously. The output is a smooth flow, not a series of slugs. That’s why scrolls don’t need suction or discharge mufflers the way reciprocating compressors do, and why they’re quieter overall.

The motor. Single-phase induction motor — three windings: start (S), run (R), common (C). Run capacitor wired between S and R, line voltage applied across R-C, start winding gets its current phase-shifted by the capacitor, motor spins. Same principle as the dual cap. The rotor sits inside the stator with refrigerant gas flowing around it.

The part most people don’t realize: the suction gas is routed directly over the motor windings on its way to the scroll set. The motor is cooled by suction-temperature refrigerant, which is typically around 50°F at the compressor inlet on a properly charged cooling system. This is brilliant from a thermal standpoint — you get free motor cooling, and the slight heating of the suction gas doesn’t matter because it gets compressed and dumped to the condenser anyway. But it also means low refrigerant charge starves the motor of its coolant. A system running low on charge has a compressor running hot, drawing more current, working harder, with the motor temperature climbing into territory that the manufacturer didn’t design for. The internal overload eventually trips, the unit shuts down on thermal protection, the customer calls. The fix isn’t the compressor — it’s finding and fixing the leak.

The internal overload is a small disc-style thermal switch buried inside the compressor housing, usually clipped to the top of the motor windings. It opens at around 250°F internal temperature. When it opens, the common line is interrupted and the compressor stops drawing power. When the windings cool back down, the disc closes again and the compressor will try to restart — usually after fifteen to thirty minutes. A compressor “cycling on internal overload” is a unit that runs for a while, shuts off without the thermostat calling for it to, sits there for half an hour, restarts, runs a bit, repeats. Customer description is usually “it works sometimes.” The internal overload is non-replaceable — it’s inside the welded housing.

The terminal block is the three-pin port where line voltage enters and connects to the three motor windings. The pins are glass-sealed through the steel housing. If a pin’s glass seal cracks, refrigerant leaks out around the pin into open air — sometimes audibly, sometimes silently. Burned terminals are also a thing — the pin gets pitted from current arcing on a loose wire connection, eventually carbon tracks across the glass seal, the seal shorts, and you have a compressor that trips the breaker the moment it’s energized. Don’t tug on compressor terminals. Don’t leave a wire loose against the terminal. Replace any wires showing heat damage.

Oil lives in the bottom of the housing. The compressor is a sealed system — refrigerant and oil circulate together through the entire refrigeration loop, with oil clinging to the inside walls of the suction line and being drawn back to the compressor with the returning gas. The crankshaft has an oil pickup tube that dips into the sump and feeds oil to the bearings and the orbiting scroll. A scroll compressor’s oil also acts as a sealant between the two spiral plates — there’s no metal-to-metal contact between the fixed and orbiting scrolls; they’re separated by a thin film of oil that handles the sealing. If oil return is bad — improperly sloped suction line, oil traps, low refrigerant velocity — the compressor sump starves and the scrolls scuff. That’s how scroll compressors die from a design or installation flaw rather than a manufacturing one.

How a scroll fails, in order of how often you see it:

Locked rotor. The compressor won’t turn at all — usually because the scrolls have seized due to lack of lubrication, contamination from a previous compressor burnout that wasn’t cleaned up, or liquid refrigerant slugging that snapped the orbiting scroll’s drive. Symptom: unit hums for a few seconds, internal overload trips, repeat. Resistance check across S-R, S-C, and R-C shows windings are intact. The mechanical side is dead.

Open winding. One of the three windings (usually start) has burned open from sustained overcurrent. Symptom: hum on attempted start, sometimes a single click and silence. Resistance check shows infinite resistance on one of the three measurements. Compressor is electrically dead.

Shorted winding. Insulation between turns of a winding has broken down internally and current is bypassing turns. Symptom: drawn current is higher than nameplate, sometimes much higher, breaker trips. Resistance reads lower than spec.

Winding-to-ground (grounded compressor). The motor windings have shorted to the housing through degraded insulation. Symptom: drawn current is high, the breaker trips, and a continuity check from any winding terminal to the housing shell shows continuity (instead of the infinite resistance you should see). This is the worst failure because it usually means refrigerant has contaminated, the system is acidic, and the entire refrigerant charge needs to be recovered and the lines flushed before installing the replacement compressor. Failing to do that means the new compressor is dead within months.

Valve fatigue. The compressor isn’t pumping properly — gauges show low head pressure and high suction, performance is poor, but the motor is still running. The internal check valve that prevents reverse rotation on shutdown has failed, or the scroll itself is worn enough that gas is bypassing back across the seal. Less dramatic failure mode. The customer notices the system “just isn’t keeping up.” Diagnosis: high suction, low head, low compression ratio.

Compressor replacement is the biggest single service event a residential system goes through. The whole charge is recovered, the suction and liquid lines are isolated, the old compressor is unbolted and de-brazed, the new one is brazed in under nitrogen flow to prevent oxidation, the filter drier is replaced (always — non-negotiable), the system is vacuum-pulled to deep vacuum to remove moisture, the new charge is weighed in. Eight hours of work in the field, $1500 to $3500 in parts and labor depending on tonnage. Half the time the whole condenser unit is replaced instead because it costs less than the compressor swap and gives a new warranty.

The compressor is also the part the rest of the system is designed around. Everything else in the unit — the contactor, the run cap, the wiring, the condenser fan — exists to serve the compressor. Take care of the compressor — don’t let it run low on charge, change the filter so the indoor coil doesn’t ice and starve it, keep the outdoor coil clean so it isn’t fighting high head pressure, replace a marginal run cap before it kills the motor — and a compressor lasts twenty years. Don’t, and you’re looking at the most expensive service call in residential HVAC.