Error S.20

Safety precautions:

- If you smell gas, leave the building immediately and call the gas emergency number. Do not operate electrical switches or the boiler.

- Isolate electrical power to the boiler before removing any covers. If you are not competent, do not open the boiler or touch internal parts.

- Do not attempt to replace gas or high-voltage components yourself. Only a Gas Safe registered engineer should carry out internal repairs.

Initial homeowner checks (safe, simple steps):

1. Note the exact code and any other codes shown on the display. This helps the engineer.

2. Reset the boiler: switch it off at the mains or use the reset function, wait 30 seconds, then switch back on. Attempt a hot tap while watching the display. If S.20 clears and hot water runs normally, monitor for recurrence. If the fault returns repeatedly, stop resetting and call an engineer.

3. Check boiler pressure on the gauge. If pressure is low (typically under 1.0 bar when cold) top up the system using the filling loop following the boiler handbook. If topping up clears the fault, re-check for leaks — do not repeatedly top up if the boiler is losing pressure.

4. Confirm room thermostat/timers and hot water programmer are calling for hot water. Make sure any cylinder thermostat (if fitted) or external controls aren’t inhibiting DHW.

5. Check external faults: ensure the gas supply and gas isolator are open and the meter hasn’t been turned off, and that the condensate discharge pipe isn’t frozen or blocked (common in cold weather).

6. Check for obvious visual issues: visible water leaks under/around the boiler, tripped external isolation valves (pump/valve service valves) in the piping to the boiler, or blocked inlet strainers on external feed valves if you can access them.

Specific diagnostic steps (what an engineer will do or that you can prepare for them):

1. Reproduce and record: with a hot tap open, record whether boiler attempts to fire, any display messages, and whether other fault codes appear (F-codes often give more detail — e.g., F.22 = low pressure, F.10/F.11 = sensor faults).

2. If the issue looks like a temperature sensor fault (DHW/flow/cylinder NTC related), an engineer will check sensor resistances and wiring continuity to the PCB, and replace the defective NTC or harness if required.

3. If the fault suggests no water flow or excessive temperature rise, the engineer will check the DHW circulation pump, check for blockages/air locks, inspect the DHW heat exchanger and sediment/filters, and may perform a system flush of the exchanger if blocked.

4. If the diverter or 3-port valve is fitted and the fault only happens on DHW changeover, the valve or its actuator may be checked and replaced if seized or electrically faulty.

5. The engineer will also inspect the PCB and related wiring for intermittent faults, moisture, or damage that could cause incorrect signals.

6. If low system water pressure was a factor, the engineer will test for leaks, check the automatic filling valve, expansion vessel condition, and the boiler pressure sensor.

When to call a professional:

- If the basic checks above don’t clear the fault, or if the fault returns after a reset, call a Gas Safe registered engineer. Do not attempt to open the combustion chamber, remove sensors, or work on gas/electrical parts yourself.

- Provide the engineer with the exact model (Vaillant EcoTEC 65 kW), serial number if available, the S.20 code, any accompanying F-codes, and a summary of checks you have already carried out.

Other important notes:

- Avoid repeatedly resetting the boiler — it is intended to protect the appliance and the property. Frequent resets can mask a developing fault and increase risk.

- If you have no hot water but heating still works, use an alternative hot water source until a qualified engineer can attend.

- Keep a record of fault codes, dates/times, and any patterns (cold mornings, after power cuts, only when hot taps used simultaneously) to help diagnosis.

In short: S.20 indicates the boiler cannot correctly satisfy a DHW demand. Do safe, simple checks (reset, pressure, programmers, visible leaks, condensate) but call a Gas Safe engineer for any internal diagnosis or repair of sensors, pumps, valves, wiring or gas-related parts.