Error S.08

Safety precautions:

- If you smell gas, evacuate the property and call the gas emergency number immediately — do not operate the boiler or electrical switches.

- Isolate the boiler electrical supply at the fused spur if you need to power-cycle the unit, and never open the boiler casing or attempt internal repairs unless you are a qualified gas engineer.

- If you are unsure about any step, stop and call a qualified engineer.

Initial checks a homeowner can do:

1. Look at the display closely and note the exact S.08 message and remaining minutes. Take a photo for reference.

2. Check your central heating controls: programmer/timer (time and on/off schedule), room thermostat, TRVs (thermostatic radiator valves) and any smart thermostat or wireless relay. Ensure the system is not scheduled to be off.

3. Check the boiler’s heating mode: some Vaillant units have Comfort, Off, or timed modes. Make sure central heating is not set to Off or a temporary holiday mode.

4. Confirm hot water availability. If hot water is present but heating is off, the issue is likely control/schedule related.

5. Check mains power and boiler power button — make sure the boiler is powered on and the digital interface is active.

6. Check system water pressure on the boiler display. If pressure is very low (eg below 0.5–0.7 bar) the boiler may show other safety shutdown codes (F22) and you should top up the system pressure or call an engineer.

Specific diagnostic and simple fix steps:

1. Cancel scheduled cut-off: On the boiler or external programmer, temporarily set the CH program to On or boost central heating manually. Turn the room thermostat up to force a call for heat. If S.08 disappears and the boiler fires, the cause was a deliberate schedule or thermostat setting.

2. Reset the boiler display: If the code looks stuck, perform a controlled reset. On many EcoTEC Plus models press and hold the power button for about 5 seconds (or use the reset button if fitted) to restart the boiler. Wait for the startup sequence and see if S.08 clears. Only reset once or twice; repeated resets are not recommended if the message returns.

3. Check external controls and wiring (basic): If you have a separate programmer, Hive/Smart thermostat or wireless receiver, ensure they are powered and calling for heat. If possible, temporarily bypass smart controls by setting the programmer to call for heat to test whether the boiler responds.

4. Inspect TRVs and radiator valves: If all TRVs are turned fully down or individual zones are closed, the boiler may not be required to run. Open a radiator valve and try calling for heat again.

5. Confirm boiler is not in a cooling or anti-cycle delay: Modern boilers sometimes impose short delays between firing cycles to protect components. If S.08 shows a very short countdown, wait and see if normal operation resumes.

6. If the boiler will not produce heat after the displayed time, or if other fault codes appear after S.08, note those codes and proceed to call a professional.

7. If system pressure is low, and you are confident and have a filling loop, you can top up the system to the recommended pressure (usually about 1.0–1.5 bar). Follow the boiler handbook for your model. If you are not comfortable doing this, call a Gas Safe engineer.

When to call a professional:

- If S.08 won’t clear after checking schedules, thermostats, and performing a single controlled reset.

- If S.08 is accompanied by F-codes, flashing warning triangles, loss of hot water, flame or ignition warnings, or persistent low pressure (F22) — these indicate faults requiring a qualified engineer.

- If you detect gas smells, leaks, or any unsafe condition.

What to provide to the engineer or Vaillant support:

- The exact display message and any other fault codes, photos of the display, when the problem started, recent changes to controls or installations (new thermostat, programmer), and whether hot water is affected.

Final note: S.08 is usually informational (a timed heating cut-off) and often resolved by changing the programmer/thermostat settings or a simple reset. If the message persists, or if other fault indicators appear, contact a Gas Safe registered heating engineer or Vaillant support for diagnosis and repair.