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Valve won't open on warm start / long pull-down

What is going on

Possible causes and checks for: valve won't open on warm start / long pull-down.

Likely causes

  • MOP (gas-charged or pressure-limited) valve holding closed until suction drops below MOP — normal
  • Or lost bulb charge

Check in this order

  1. Suction pressure vs MOP rating, warm bulb in hand and watch for response

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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.