Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
F.20 is overheat protection. The boiler has gone above the safety limit (or the limiter circuit has been fooled by a bad earth, NTC, or ignition tracking). Do not keep resetting it without finding why it overheated.
Vaillant meaning: Safety switch-off: temperature limiter
Possible causes
- Poor earth between harness and boiler (including ignition earth)
- Flow or return NTC loose, drifted, or swapped so the PCB under-reads temperature
- Tracking / black discharge on the ignition lead, plug, or electrode
- True overheat from no circulation (pump, air, sludge, closed valves) — often seen with F.23 / F.24 / F.76
Possible solutions
- Do not keep firing into lockout. Check system pressure and that TRVs / zone valves are not isolating all flow.
- Prove earth continuity to the heat exchanger, burner door, and electrode earth tag.
- Inspect the ignition lead and electrode for tracking. Replace if carbon-tracked.
- Measure flow and return NTCs (about 12 kΩ at 20 °C) and confirm they are on the correct pipes.
- If circulation is poor, diagnose pump, air, and heat-exchanger blockage before changing electronics.
Possible parts needed
- Flow/return NTC 193592 or 0020068120
- Ignition electrode (e.g. 090709 or 0020133816 by range)
- Ignition lead / plug
- Pump / debris in the hydroblock
- PCB (last)
