Common Compressors Reference
Why this matters
The compressor is the most-expensive HVAC component. Customer compressor failure = + repair OR full equipment replace. The tech who understands compressor types + diagnoses failure correctly recommends accurate repair OR replace. This is the field card.
Compressor types
Reciprocating piston: pistons + crankshaft; oldest design. Used in some refrigeration + older AC + heat pumps.
Scroll: two interlocking scrolls compress gas; smooth + reliable; quiet. Most modern residential AC + heat pump.
Rotary: rotating vane; small + quiet; mini-splits + window units.
Screw: twin helical rotors; commercial + industrial > 5 tons.
Centrifugal: impeller spins refrigerant; large commercial (chillers).
Residential compressor characteristics
Scroll compressors (most common residential):
- Reliable + smooth + quiet
- Single-stage OR 2-stage (modulating capacity)
- Common brands: Copeland, Bristol, Mitsubishi
2-stage scroll: matches capacity to load; better dehumidification + efficiency.
Variable-speed (inverter-driven): continuously modulates compressor RPM; matches exact load. Premium efficiency + ultra-quiet.
Common failure modes
Burned motor (electrical):
- Burned winding
- Open OR shorted
- Resistance test (winding-to-winding, winding-to-ground)
- Smell of burnt insulation
Mechanical failure:
- Locked rotor (seized scroll/piston)
- Broken connecting rod (rare)
- Worn bearings
Refrigerant contamination:
- Moisture + acid (from leak + dehydration not done before charging)
- Particulate (from debris)
- Air (from improper evacuation)
Loss of refrigerant:
- Compressor overworks; overheats; insulation breaks down
Liquid slugging:
- Liquid refrigerant returns to compressor
- Hydraulic compression destroys compressor
- Cause: improper subcooling, expansion valve fail, evaporator load too low
Acid contamination:
- From overheating event; refrigerant oil breaks down to acid
- Damages internal parts; flushes required before new compressor
Compressor diagnosis
Step 1 - Visual + electrical:
- Burned smell?
- Visible damage?
- Insulation discoloration?
Step 2 - Electrical test (with power off + capacitor discharged):
- Common, run, start terminals identified
- Winding resistance: Common-Run, Common-Start, Run-Start
- Compare to OEM nameplate
- Ground fault: each terminal to compressor body (should be ∞ Ω)
Step 3 - Mechanical:
- Spin freely by hand (if accessible)
- Listen for grinding OR slamming
Step 4 - Operational test:
- Energize; measure amp draw
- Compare to nameplate FLA + LRA
- Watch for trip / hum / stall
Hard start kit
For older compressors with weak starting torque:
- Start capacitor + relay added externally
- Provides extra torque burst on startup
- Solves "compressor starts then trips" issues
- Used as diagnostic; rules out start cap failure
Common compressor amps
3-ton AC scroll:
- RLA (Rated Load Amps): 12 - 16A
- LRA (Locked Rotor Amps): 70 - 90A (5 - 8x RLA)
- Running below RLA = healthy
Heat pump compressors: similar; slightly higher at heating mode.
Replacement decision tree
Compressor failure on system < 5 years old:
- Likely under warranty
- OEM compressor replacement
Compressor failure on system 5 - 10 years old:
- Possibly out of warranty
- Consider partial vs full replacement (compressor only vs new outdoor unit)
Compressor failure on system > 10 years old:
- Often less cost-effective to replace compressor vs full system replacement
- Newer system more efficient + new warranty
- Consider customer's long-term plan
General rule of thumb: if compressor replace cost > 50% of new equipment, replace equipment.
Reciprocating compressor scenarios
Refrigeration: walk-in cooler / freezer; replaced as serviceable unit.
Window AC + small heat pumps: reciprocating OR rotary; often replaced as full unit.
Variable-speed inverter compressors
Inverter-driven: AC line → DC → AC at variable frequency.
Continuously variable: from 30% to 110% of rated capacity.
Premium efficiency: SEER2 18 - 26 typical.
Communicating thermostat: required (custom protocol).
Failure modes: inverter board failure (separate cost from compressor); compressor itself.
Hermetic vs serviceable
Hermetic (sealed): most residential; not field-serviceable; replace as unit.
Semi-hermetic (bolted access plate): commercial + some larger residential; serviceable parts.
Fully serviceable (open drive): commercial; belt-driven; rare residential.
Compressor refrigerant + oil compatibility
R-22 compressors: mineral oil OR alkylbenzene oil.
R-410A compressors: POE (polyolester) oil; hygroscopic (absorbs moisture).
R-32 + R-454B: POE oil; similar to R-410A.
Cross-contamination risk: mixing oils OR refrigerants damages compressor.
Always verify: nameplate before adding any refrigerant.
Compressor protection devices
Internal thermal protector: opens at high temp; internal to compressor.
External thermal protector: bolt-on; replaces internal in some designs.
High-pressure cutoff (HPC): manifold-mount; trips at high head pressure.
Low-pressure cutoff (LPC): trips at low suction; protects against loss of charge.
Crankcase heater: keeps refrigerant from migrating to cold compressor (winter); short cycle prevention.
Common compressor mistakes
Replacing compressor without addressing root cause (acid, moisture, contamination = new compressor fails again). Skipping refrigerant flush after acid contamination. Skipping vacuum + leak test post-install. Wrong refrigerant OR oil (R-410A compressor + R-22 refrigerant = damage). Not nitrogen purging while brazing (oxide contamination). Skipping filter-drier replacement on compressor swap.
Brazing-in new compressor
Procedure:
- Recover existing refrigerant
- Remove old compressor (cut copper)
- Install new compressor (matching specs)
- Replace filter-drier (always)
- Braze copper connections with nitrogen purge
- Pressure test with nitrogen
- Evacuate to 500 microns
- Charge per nameplate weight
- Start + verify operation
- Recheck superheat/subcool
Warranty considerations
Most residential compressors: 5 - 10 year warranty (parts only).
Installed by certified contractor: required for warranty.
Registration: required within 60 - 90 days for full warranty.
Failure due to misuse / improper install: warranty void.
Truck stock for compressor work
Heavy-duty wrench set (compressor mount bolts), nitrogen tank + regulator, brazing supplies, leak detector, refrigerant scale, vacuum pump + micron gauge, compressor diagnostic kit (multimeter, capacitance meter, hard start kit), filter-driers (matching system refrigerant).
References
- ASHRAE Refrigeration Handbook
- AHRI 540 (compressor standards)
- Manufacturer technical data (Copeland, Bristol, LG, Mitsubishi)
- Manuall internal: Refrigerant Types Reference, Common Gauges + Meters