Fault guide
Vaillant F28, ignition failure and what actually causes it
F28 on an ecoTEC means the boiler tried to light and failed. Vaillant fault codes are specific, which is helpful: this one narrows the problem to gas, ignition or flame sensing.
What the boiler did
On a call for heat the ecoTEC purges the combustion chamber, opens the gas valve and sparks the electrodes. If the ionisation probe does not confirm a flame within the allowed window, the boiler shuts the gas off and displays F28.
It usually happens on the coldest morning of the year, which is a clue in itself.
The realistic causes
We work through these in order, because they are ranked by how often they turn out to be the answer.
- Frozen or blocked condensate pipe, especially after a hard frost.
- Gas supply: meter credit, an emergency control valve turned off, or low working pressure at the boiler.
- Ignition electrodes worn, sooted or incorrectly gapped.
- Ionisation and earthing faults, where the flame lights but the boiler cannot detect it.
- Gas valve not opening correctly, or an air and gas ratio out of adjustment.
Safe checks before you call
Two minutes of checks can save a call-out, particularly in winter.
- Check the gas works elsewhere, for example on the hob.
- Check meter credit if you are on a prepayment meter.
- Follow the white plastic condensate pipe outside and thaw it with warm, never boiling, water if it is iced up.
- Press reset once. If F28 returns, leave it and call an engineer.
What we test on site
Working gas pressure and gas rate at the meter, condensate route and trap, electrode condition and gap, ionisation current, and a full combustion analysis. That sequence proves the cause instead of guessing at it.
We carry electrodes and common ecoTEC spares, so a good proportion of F28 calls are resolved on the first visit.
Questions we get asked
- What does F28 mean on a Vaillant boiler?
- F28 is an ignition failure at start-up. The boiler ran its ignition sequence, did not establish a flame, and locked out.
- Is F28 dangerous?
- No. The lockout is the boiler protecting you by refusing to keep releasing gas without a flame. What matters is not resetting it over and over without finding the cause.
- What is the most common cause of F28?
- In our experience: a blocked or frozen condensate pipe, gas supply problems, worn ignition electrodes, or a gas valve that is not opening correctly.