Error F.21

Safety precautions:

- If you smell gas, do not operate switches, appliances or phones inside the property. Leave immediately and call the gas emergency number from a safe location. Do not attempt any internal repairs yourself.

- Turn off the boiler electrical supply at the isolator if you need to open cupboards or do visual checks. Do not open boiler casing or touch internal components unless you are a Gas Safe registered engineer.

Initial homeowner checks you can safely do:

1. Check if other gas appliances in the property (hob, gas fire) are working. If none of them work, there may be a supply issue from the gas meter or supplier—contact your gas supplier.

2. Check the external gas isolation valve (if you know its location) to ensure it is open. Do not tamper with gas supply pipework beyond turning an accessible isolator on/off.

3. Inspect the external flue and air intake areas for obvious blockages (bird nests, debris, snow). Clear any safe, external obstructions. Do not force anything into the flue.

4. In cold weather check the condensate discharge pipe for freezing. If frozen, carefully thaw with warm (not boiling) water or a warm cloth. A thaw may allow the boiler to restart.

5. Check the boiler’s pressure gauge to confirm the system pressure is roughly in the normal range (typically around 1–1.5 bar when cold for many domestic boilers). Low pressure usually gives a different fault but it is useful to note.

Reset and simple diagnostic steps:

1. Try a single reset: with the boiler powered on, press and hold the reset button as the manufacturer describes (commonly up to 10 seconds). Wait for the boiler to attempt ignition and observe whether it lights. Do not repeatedly reset more than once or twice if the fault persists—repeated resets can mask a dangerous ongoing issue.

2. Note any other indicators or codes on the display and the exact behaviour (does it spark and then lock out, does it not spark at all, are there unusual noises?). Take a photo or write down the code and sequence; this helps the engineer.

3. If the boiler attempts ignition (you hear the fan and spark) but the flame is not detected, that suggests an ignition electrode, flame probe, or gas valve issue and requires professional diagnosis.

4. If the boiler does not attempt ignition at all, it may be an ignition transformer/spark generator, wiring fault, or PCB problem—again a Gas Safe engineer must inspect.

When to call a professional and what to tell them:

- Call a Gas Safe registered engineer if the fault returns after a single reset, if you cannot clear external blockages safely, if you suspect faulty components (electrode, gas valve, flame probe, PCB), or at any time you are unsure. Do not attempt to replace ignition electrodes, gas valves, or internal wiring yourself.

- Tell the engineer the model (Vaillant Ecomax Pro 828E), the fault code (F.21), what you observed, what checks and resets you tried, whether other gas appliances work, and whether any external flue or condensate issues were found.

Other important notes:

- Do not keep resetting a locked-out boiler repeatedly; persistent lockouts indicate a fault that must be diagnosed.

- Any work on gas-carrying components, the gas valve, electrodes, or internal PCB must be carried out by a Gas Safe registered engineer. They have the correct testing equipment and safety procedures to diagnose and repair ignition and flame detection faults safely.

- If the engineer needs parts or the boiler is old and recurring faults occur, discuss repair options and safety implications with them.

Remember: F.21 means the boiler could not detect a safe flame. It can sometimes be cleared by simple external checks and one reset, but repeated faults or any sign of gas smell or safety concern require immediate professional attention.