In short
An AC compressor is the pump in the outdoor unit that circulates refrigerant and raises its pressure so heat can be moved out of the house. It is the most expensive single part of the system. When one fails out of warranty on a system 10 to 15 years old, replacing the outdoor unit and coil usually makes more sense than replacing the compressor alone.
Of all the parts in a cooling system, this is the one that decides whether the unit gets repaired or replaced. What is an AC compressor? It is a sealed, motor-driven pump inside the outdoor cabinet, the black steel shell you can see through the top grille under the fan. Everything else in the refrigerant circuit exists to feed it or to use the pressure it creates.
What an AC compressor does in the refrigerant circuit
The compressor draws in cool, low-pressure refrigerant vapour returning from the indoor coil and squeezes it into a hot, high-pressure vapour. That vapour goes to the condenser coil, gives up its heat to the outdoor air and turns to liquid. In a heat pump the same compressor runs year-round and the flow is reversed in winter, so the heat is delivered indoors. Residential compressors come in three types, and the type explains most of the price tier on a quote.
- Single-stage: on at full capacity or off. Simplest and least costly to replace.
- Two-stage: a low stage for mild days and full capacity for hot ones, giving longer, quieter cycles.
- Inverter (variable-speed): ramps up and down continuously; see the inverter compressor entry.
Nearly all modern residential units use a scroll compressor: two spiral plates, one orbiting inside the other. Scrolls tolerate more abuse than the older piston type, but they do not tolerate liquid refrigerant, overheating or low voltage for long.
How an AC compressor fails and what usually caused it
- The breaker trips the moment the outdoor unit tries to start.
- A loud clunk or shudder on start-up, with the house lights dimming.
- The unit hums for a few seconds, clicks off on its internal overload and tries again minutes later.
- The outdoor fan and compressor are both running, but the air indoors is not cold and the larger copper line is not cool to the touch.
- A new grinding, rattling or screeching noise from inside the shell.
Compressors seldom die of old age. The root cause is usually upstream: a failed capacitor that left it stalling, a matted condenser coil that ran head pressure up for years, a low charge that left it running hot, or liquid refrigerant slugging back into it after an airflow problem froze the indoor coil. A technician who replaces a compressor without finding the cause is setting up the next one to fail.
The same care applies to diagnosis. Several of these symptoms come from a capacitor or contactor fault, a $150–$350 repair, which is why the electrical parts are tested before anyone condemns the compressor.
Repair or replace: the compressor decision on 10- to 15-year-old systems
Start with the warranty. Many manufacturers carry a longer parts warranty on the compressor when the unit was registered, which leaves you paying labour and refrigerant only. Check the paperwork or give us the model and serial number.
Out of warranty, the job means recovering the refrigerant, cutting out the old compressor and brazing in the new one, replacing the filter-drier, evacuating and recharging. On a 10- to 15-year-old R-410A system that bill approaches the cost of a new outdoor unit, and you still own an old coil and an old fan motor. On an R-22 system the answer is replacement, because the refrigerant is phased out and what remains is costly reclaimed stock. Our rule: when the repair is more than half the cost of a new system and the unit is over 12 years old, replace the outdoor unit and indoor coil together.
We do not publish a flat compressor price because it depends on the tonnage, the refrigerant and the warranty status; you get the figure in writing before work starts. See how much an AC compressor costs and whether a 10-year-old AC is worth repairing.
What to do if the compressor will not start
Switch the system off at the thermostat and do not keep resetting the breaker; each forced start risks the windings. Note what you hear: a hum, a click, or nothing at all. Then book AC repair or contact ZK Mechanical. We test the capacitor, contactor and wiring first and show you the readings.
Questions people ask about AC compressor
How long does an AC compressor last?
A compressor on a system that is correctly charged, kept clean and not short-cycling commonly lasts the life of the unit. Early failures trace to something else: a dirty outdoor coil, a slow refrigerant leak, a weak capacitor or an oversized unit that starts and stops all day. Annual maintenance protects the compressor more than any other part.
Can a compressor be repaired, or only replaced?
The compressor in a home AC is hermetically sealed, a welded shell with the motor and pump inside, so it cannot be opened and rebuilt. Faults outside the shell can be repaired: a capacitor, a contactor, burnt wiring at the terminals, a hard-start kit for a tight compressor. Once the windings are shorted or the pump is seized, it is replaced.
Why does my AC compressor trip the breaker?
A breaker that trips at start-up points to a compressor drawing far more current than it should: shorted or grounded windings, a seized pump, a failed capacitor or a loose, burnt connection. A breaker that trips after long running on a hot day points more to a dirty condenser coil or a failing fan motor. Leave it off and have it tested.