Heating Fault Decision Tree (Core Diagnostic Flow)

Heating Fault Decision Tree

Purpose

To quickly identify whether a heating fault is caused by controls, wiring, or the boiler itself.

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Step 1: Identify Symptom

• No heating at all
• No hot water
• Intermittent heating
• Heating stuck ON
• Boiler not firing

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Step 2: Check Demand Exists

• Raise room thermostat above ambient temperature
• Check programmer is calling for heating/hot water
• Observe:
  • Valve movement
  • Relay clicks
  • Boiler attempt to fire

IF NO DEMAND: → Control issue (thermostat/programmer/wiring)

IF DEMAND PRESENT: → Move to Step 3

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Step 3: Check Zone Operation

• 2-port valve opens? (S-Plan)
• 3-port valve moves correctly? (Y-Plan)
• End switch activates?

IF NO MOVEMENT: → Valve or actuator fault

IF MOVEMENT BUT NO BOILER FIRE: → Electrical interlock fault

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Step 4: Check Boiler Signal

• Verify switched live present at boiler terminal
• Check boiler enable input
• Confirm no internal lockout

IF NO SIGNAL: → Wiring/control issue upstream

IF SIGNAL PRESENT: → Boiler fault likely

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Step 5: Final Isolation

• If controls and wiring are correct:

→ Treat as boiler-side issue

• If signal missing or incorrect:

→ Control system fault confirmed