Error F.27

SAFETY FIRST

- If you smell gas, leave the property immediately, ventilate doors/windows on the way out if safe to do so, do not operate electrical switches or use phones inside the building, and call the national gas emergency number. Call a Gas Safe engineer before using the boiler again.

- Do not open the boiler casing or attempt internal electrical or gas work unless you are a qualified Gas Safe registered engineer. Working on gas appliances or live PCBs is dangerous.

- Do not repeatedly reset the boiler. One attempt to reset for testing is reasonable; repeated resets can hide intermittent faults and create unsafe conditions.

- If you see active water leaks or significant condensation inside the boiler area, isolate the water supply to the heating system and switch the boiler off until a professional attends.

INITIAL HOMEOWNER CHECKS (safe, no internal access required)

1. Check the display and note the full error code and any other codes. Photograph the display and record how and when the fault appears and whether a reset temporarily clears it.

2. Power and gas supply: confirm the boiler is receiving mains power (clock, lights) and that the gas supply to the house is on. Confirm other gas appliances (if any) light correctly. If the gas supply is off or intermittent, call your gas supplier or a Gas Safe engineer.

3. Boiler pressure: check the pressure gauge. Most domestic systems should sit around 1–1.5 bar cold. Very low pressure could indicate a leak; very low water levels can allow moisture ingress or other faults. Do not top up if you are unsure — consult your service manual or an engineer.

4. Condensate pipe: in freezing weather a blocked/frozen condensate pipe can cause lockouts. If the external condensate discharge is frozen, thaw it with warm (not boiling) water and clear any visible blockage.

5. Visible water or damp: look around the boiler for evidence of leaks or water pooling near the PCB or wiring. If you find water ingress, isolate mains power to the boiler and call an engineer.

6. Single reset: follow the boiler manual to perform one reset. If the fault clears but returns, record how long it runs before faulting and contact a Gas Safe engineer.

DIAGNOSTIC AND PROFESSIONAL REPAIR STEPS (for the engineer)

- The engineer will undertake safe isolation and open the boiler to inspect the PCB and connections for signs of moisture, corrosion, burn marks or loose connectors. Moisture/damp on the PCB is a common cause and may require drying, cleaning, or replacement of the board.

- Flame rectification testing: the engineer will test the ionisation/flame detection circuit (flame rectification) to confirm whether the flame is being detected correctly by the PCB. They will measure the microamp signal and check electrode continuity.

- Ignition components: check spark electrode alignment, condition and ignition leads for continuity and correct spark gap; however, the electrode assembly rarely needs replacement if it conducts and is intact.

- Gas valve and gas supply: the engineer will check the gas valve operation and that gas pressure and delivery are within specification. A leaking or sticking gas valve can cause incorrect flame conditions and may require valve repair or replacement.

- PCB diagnosis: if other components test OK, the PCB (flame monitor/ignition control) is often the culprit on affected models. The engineer will verify the PCB behaviour, confirm fault codes and may replace the PCB if defective. If moisture caused the PCB failure, the source of the moisture must also be addressed.

- After repair: the engineer will run combustion and safety checks, confirm correct flame detection and stable operation, check for leaks, and reset any fault history. They should also advise on the root cause and any preventative actions (e.g., addressing leaks, improving ventilation, insulating external condensate pipes).

WHEN TO CALL A PROFESSIONAL

- Call a Gas Safe registered engineer if the F.27 code appears. This code involves ignition and gas control systems and is not safe for unqualified DIY repair. If the boiler was recently serviced and still under warranty, contact the installer or Vaillant support as well.

DOCUMENTATION FOR THE ENGINEER

- Provide the engineer with: how many times you reset the boiler, any preceding events (power cut, freezing weather, water leak), photos of the error code display, and any visible signs you observed (damp, corrosion, frozen condensate).

FINAL NOTES

- F.27 is a protective lockout. It prevents operation until the cause is fixed. While the fix can be as simple as drying out connections or as involved as PCB or gas valve replacement, only a competent Gas Safe engineer should diagnose and carry out repairs. Avoid guessing at component replacement to prevent unnecessary costs and safety risks.