What does F28 mean on a Vaillant boiler?
F28 is Vaillant’s code for a failure during start-up: ignition unsuccessful. Each time the boiler needs heat it runs the fan, opens the gas valve and sparks the ignition electrode. A separate monitoring electrode then checks for a flame. If no flame is proved after the set number of attempts, the boiler closes the gas valve and locks out with F28.
Vaillant’s ecoTEC plus operating instructions tell the user to press the fault clearance button for one second so the boiler makes another attempt, and to consult a competent person if the fault cannot be cleared after three attempts.
What causes F28?
Vaillant’s installation manual lists a wide range of possible causes. Grouped simply:
- Gas supply: a prepayment meter out of credit, the gas isolating valve closed, or low gas pressure reaching the boiler.
- Condensate route: a blocked siphon (condensate trap) or a frozen or blocked condensate pipe.
- Flue and air intake: a blocked or damaged flue or air intake pipe.
- Ignition parts: the ignition electrode, ignition cable, ignition plug or the transformer that makes the spark.
- Flame monitoring: a dirty or faulty monitoring electrode that cannot detect the flame.
- Gas valve, electronics and settings: a faulty gas valve, an earthing or circuit board fault, or an incorrect combustion setting.
Only the first two are things a homeowner can check.
What can you safely check yourself?
- Check the gas. Does your hob light? If you have a prepayment meter, is it in credit?
- Mention any recent gas work in your enquiry, for example a new meter or hob, as the supply may have been left turned off.
- In freezing weather, check the condensate pipe where it leaves the wall and thaw it with warm, never boiling, water. See frozen condensate pipe.
- Reset once by pressing the reset button for one second.
- Note how often it happens: every time, or only sometimes.
When should you get an engineer?
If the gas supply is fine, the condensate pipe is clear and F28 returns after a reset, the fault is inside the boiler. The engineer will check the gas pressure at the boiler, the condition and gap of the electrodes, the ignition lead, the condensate trap, the flue and the combustion. Worn electrodes and leads are common, often inexpensive parts. You will be given a price before any work starts.