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High superheat, starved evaporator, low suction, partial frost on coil

What is going on

Possible causes and checks for: high superheat, starved evaporator, low suction, partial frost on coil.

Likely causes

  • Superheat set too high
  • Undersized valve
  • Bulb losing contact / poorly insulated bulb
  • Gas-charged head colder than bulb (charge condensed in head)
  • Low head pressure in cold weather
  • Restriction/plugged inlet strainer or drier
  • Evaporator pressure drop >4 psi without external equalizer
  • Loss of bulb charge

Check in this order

  1. Superheat and subcooling
  2. Bulb mounting and insulation
  3. Warm the power head (valve should open)
  4. Head pressure, ΔP/temperature across drier
  5. Equalizer line presence/connection
  6. Valve capacity vs coil | 505 (479), 508–510 (482–484), 513 (487), 515 (489), 571 (545)

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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.