Boiler won't start and has no flame: ignition, ionisation and what it costs

Short answer: if the boiler does not ignite or there is no flame after the ticking, the cause is usually small and inexpensive. In order of likelihood: a worn ignition electrode or ionisation probe (€ 20 to € 50), a dirty burner, or no gas because of a closed valve or a gas pressure problem. If the ionisation probe does not detect a flame, the gas valve closes and the boiler locks out after three to five attempts. Only then does the gas valve itself come into the picture. Reset once, no more.

What happens during an ignition attempt

When heat is requested, the fan first starts the pre-purge. Then the gas valve opens, the ignition electrode produces a spark and the ionisation probe must report a flame signal within a few seconds. If that signal does not come, the gas valve closes immediately and the boiler tries a few more times. After three to five failed attempts the appliance locks out. That is not a defect but a safety feature: letting gas flow without a flame is the most dangerous thing a boiler can do.

You can often hear this pattern clearly: the fan speeds up, the gas valve clicks, a ticking spark sound, and then silence. That ticking is the ignition; if you hear it but no flame appears, the problem lies in flame formation or flame detection.

The codes per brand

Brand	Code	Message
Intergas	4	No flame signal after four ignition attempts
Remeha	02	No flame formation or no ionisation after five start attempts
Vaillant	F.01	Ignition fault, no flame detection
Vaillant	F.27	Incorrect ionisation measurement, flame signal while the gas valves are closed
AWB	01	Ignition fault
Bosch	227 V	No flame signal after ignition

What you can check yourself

Start with the gas. Is the gas valve under the boiler open? Has the gas meter recently been shut off, or has there been work in the street? Is the gas hob still working normally? A boiler that gets no gas at all reports the same as a boiler with a worn electrode.

Then look at the water pressure; if the pressure is too low, the boiler sometimes will not start at all. And reset once. If the fault keeps coming back, stop resetting: with every attempt a little gas flows into the burner without igniting.

The parts that are usually to blame

The ignition electrode and the ionisation probe sit just above the burner and get a flame over them at every start. After eight to twelve years the metal is burnt in or burnt away and the spark becomes too weak or the flame signal too small. With many brands, ignition and ionisation are combined in one part, for example the Remeha ignition and ionisation electrode (fabrikantnummer S101566, €47.95). Soms is alleen de afdichtring versleten, zoals de Nefit ring for the ionisation probe (7099556, €21.95 voor vijf stuks).

The ignition cable itself can also lose the spark if the insulation is damaged. All related parts can be found under ignition and ionisation en cables and wiring harnesses.

Only once the electrode, probe and cable are in order does the gas valve come into the picture. That is a more expensive part and always requires a combustion analysis after fitting; our guide covers this under replacing the gas valve.

When should you call an engineer?

With this type of fault, always, unless the problem was simply a closed gas valve. The parts sit in the sealed combustion section, and work there must be done by a certified gas engineer. In addition, the electrode must be at the correct distance from the burner, often to within tenths of a millimetre; too far away gives no spark, too close burns it away faster.

If you smell gas, turn off the boiler and the gas valve, open a window and call for help immediately. More about safe heating can be found at Milieu Centraal.

Frequently asked questions

Why does my boiler not ignite?

If you hear no ticking spark sound, the ignition electrode is getting no or too little voltage: a worn electrode, a damaged ignition cable or an incorrect distance from the burner. If you do hear ticking but no flame appears, first check that the gas valve is open and that other gas appliances work. If the fault returns after one reset, have a certified gas engineer take a look.

What does ionisation do in a boiler?

The ionisation probe detects whether a flame is burning: a gas flame conducts a very small current, and that current is the flame signal. If the signal drops out, the gas valve closes immediately. A worn or dirty ionisation probe therefore gives the same message as a boiler without a flame. More in our guide on the ionisation electrode.

My boiler does start after resetting a few times. Is that problem solved?

No. That is precisely the classic sign of an electrode at the end of its life: when cold it just about works, when warm it no longer does. It will only get worse over the coming weeks, usually at the worst possible moment.

Can I clean the electrode instead of replacing it?

During a service an engineer removes soot from it and checks the gap. If the metal is visibly burnt in or has become thinner, cleaning no longer helps and replacement is the only solution. The part is cheap enough not to hesitate.

Why does my boiler report a flame when it is not burning?

That is the opposite fault, for example Vaillant F.27. Usually the ionisation probe is damp or dirty, or there is a short circuit in the wiring. The boiler then refuses to start because it does not trust itself, and rightly so.

How often should an ignition electrode be replaced?

There is no fixed interval. It is checked during the annual service and replaced if necessary; in practice it lasts eight to twelve years. Boilers that make many short starts wear it out faster.