Glow-worm Fault Code F.20 — Diagnose and Fix
Fault code F.20 on a Glow-worm boiler indicates no ignition. Gas valve opens but spark or flame detection fails. Here is the diagnostic order UK Gas Safe engineers follow.
Step-by-step method
1. Confirm the F.20 lockout
Power off at the fused spur for 60 seconds, restore and reset. If F.20 returns within 5 minutes, proceed. Intermittent F.20 points to a sensor or connection, not the PCB.
2. Check pressure, condensate, gas
System pressure 1.0–1.5 bar, condensate unfrozen, gas proven at another appliance — accounts for ~60% of lockouts.
3. Test what F.20 points at
Gas valve opens but spark or flame detection fails.
4. Reset and prove
Reinstate covers (interlocks matter), reset and observe full ignition. Flame current 1–8 µA DC.
5. Gas Safe callout
Gas-side work on a Glow-worm requires Gas Safe. Provide the fault code, GC number and serial so parts are pre-ordered.
Frequently asked questions
Reset Glow-worm F.20 myself?+
Hold reset 5 seconds. If it returns, fault is still present.
Repair cost?+
£120–£280 typical; PCBs £180–£320; sensors £60–£100.
Warranty cover?+
Yes if within warranty and annual services logged.