Error S.20

Safety precautions:

- If you smell gas, turn the gas supply off at the meter, do not operate electrical switches, ventilate the area, leave the property and call the gas emergency number immediately. Do not attempt internal repairs.

- If you suspect a CO (carbon monoxide) problem (unusual smells, occupants feeling unwell when boiler runs), turn the boiler off, ventilate and get a qualified engineer immediately.

- Do not remove the boiler cover or attempt internal electrical/gas work yourself.

Initial homeowner checks you can do:

1) Confirm the boiler display: note S.20 and check for any accompanying F (fault) codes. If any F codes are present, record them and pass them to the engineer.

2) Check your programmer/timer and room thermostat: ensure the schedule is set to call for heat/hot water and the thermostat set temperature is above room temperature. Check that any heating override is not accidentally active.

3) Check wireless controls: if you use wireless thermostats or a smart controller, check batteries, signal, and that the device is paired correctly.

4) Check boiler pressure: the central heating pressure gauge should normally be around 1.0–1.5 bar (check your manual). If pressure is below ~1.0 bar, the boiler may refuse to fire; repressurise following the manual instructions or call an engineer if you are unsure.

5) Listen for pump or fan operation when the boiler is called: you should hear the pump or fan start when a demand is made. If there is silence, that indicates a possible circulation or electrical issue.

Simple steps to try before calling an engineer:

1) Reset the boiler: consult your boiler manual for the correct reset procedure and try a single reset. If the boiler starts and runs normally, monitor. If the fault returns immediately or frequently, do not continue resetting.

2) Check wiring at accessible external controls: ensure thermostat and programmer wiring is secure at their terminals (do not open the boiler). For a wired thermostat, a loose or shorted wire can create false demands.

3) Check batteries and signal for wireless devices and thermostats. Replace batteries if low.

4) If pressure is low, top up using the filling loop as described in your manual. After topping up, bleed any radiators that are cold at the top to remove trapped air and then recheck pressure.

When to call a professional and what to tell them:

- Call a Gas Safe registered engineer or Vaillant support if S.20 persists and the boiler does not fire, if you see any F fault codes, if pressure repeatedly drops, if you suspect ignition/flame issues, or if you are uncomfortable with any checks above.

- Provide the engineer with the exact display messages (S.20 and any F codes), what you observed (no noise, pump silent, low pressure, etc.), and any recent work or changes to controls.

- If your boiler is under warranty or you want Vaillant support directly, have your model and serial number ready and contact Vaillant customer service (number shown in your manual or via Vaillant support).

Important final notes:

- S.20 itself is usually a normal status message (a call for heat). If the boiler does not respond to that demand, the root cause is likely a separate fault (controls wiring, pump, pressure, ignition, sensors or PCB) that requires a qualified engineer. Do not attempt internal repairs or diagnosis involving gas, burner or mains wiring yourself.