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Knocking / slugging / broken valves, blown head gasket

What is going on

Possible causes and checks for: knocking / slugging / broken valves, blown head gasket.

Likely causes

  • Liquid refrigerant or oil slugging — flooded start (refrigerant condensed in crankcase on off cycle)
  • Leaking expansion valve on off cycle
  • Overcharge, TXV superheat set too low / hunting
  • Failed accumulator
  • Liquid migration through oil separator

Check in this order

  1. Crankcase heater working
  2. Frost on suction line at start-up
  3. Superheat
  4. Charge
  5. Accumulator level
  6. Oil separator insulation/check valve/solenoid | 478 (452), 484 (458), 505 (479), 520 (494)

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Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.