Vaillant Boiler Not Turning On

Read the display before anything else

A Vaillant tells you more than most boilers if you know where to look. Letters beginning with F are faults: the boiler has stopped itself. Letters beginning with S are status codes: the boiler is fine and is telling you what it is doing, and the same codes help pin down a Vaillant that gives no hot water. On many ecoTEC plus models, pressing the i button shows the current status code if it is not already on the screen.

Which of these you see decides everything that follows. A dark screen is a power problem. An F code is a lockout. A lit screen with an S code and no flame symbol is a boiler waiting for a demand.

Nothing on the display at all

Check the fused spur beside the boiler is switched on, then look at the consumer unit for a tripped breaker or RCD. The spur fuse is normally 3 amps. If it has blown, a replacement that blows again straight away means a fault in the boiler, so leave it off and get it looked at.

Some ecoTEC boilers also have their own on/off button on the front panel, which is easy to knock. Make sure it is on.

If the power is definitely reaching the boiler and the screen is still blank, the fault is behind the casing: the board fuse, the display lead or the main board. That is engineer work.

An F code on screen

These are the Vaillant codes that most often stop the boiler turning on, with what you can safely do about each.

• F.22: low water pressure. The boiler has shut down because the heating circuit is short of water. Top up to between 1 and 1.5 bar cold at the filling loop, then reset. See our Vaillant boiler pressure guide.
• F.28: failed to light on start-up. Check a gas hob works, check prepayment credit, and check the outside condensate pipe is not frozen, then reset once.
• F.29: the flame went out while running and would not relight. The same checks apply, and a repeat points to the gas supply, the electrode or the condensate route.
• F.75: the pump started but the pressure sensor saw no pressure rise, from low pressure, air, a failing pump or a faulty sensor. A reset may clear it briefly, but it usually needs an engineer.

If you can smell gas, do not reset the boiler. Open windows, turn off the gas at the meter if you can reach it safely, avoid switches, then leave and call the National Gas Emergency Service on 0800 111 999.

Screen on, no fault, but the heating is not coming on

This is the most common reason a working Vaillant seems dead. Run a hot tap. On a combi, if the burner lights and the water gets hot, the gas, ignition and fan are all fine. The heating is not coming on because nothing is asking for it, or the boiler has been set not to listen.

• S.31 on screen means summer mode: the heating side of the boiler is switched off. On ecoTEC plus models with a heating dial, turning it right down does this. Turn it back up.
• S.0 means no heat is being requested. Check the programmer is in an on period and turn the room thermostat well above room temperature.
• With a wireless or smart control such as a Vaillant vSMART or sensoCOMFORT, check it has power and is still connected to the boiler.
• If hot water works but radiators stay cold even with a demand, a stuck diverter valve is the usual suspect. See our guide to a boiler not heating the radiators.

Resetting a Vaillant safely

Only reset after you have dealt with the likely cause, such as topping up pressure or thawing a frozen pipe. On ecoTEC models press the reset button on the front panel; older models need about a second, current ecoTEC plus models more than three seconds. The code should clear and the boiler will try to light within a minute or so. Our Vaillant reset guide covers the button on each control layout.

One reset is a test. Two, after fixing something, is reasonable. If the same F code comes back, the boiler is telling you the cause is still there, and more resets only delay the repair.

When a Vaillant that will not start cannot wait

A boiler that will not turn on is rarely dangerous in itself, but losing heating and hot water can be. Get someone out the same day if anyone in the house is very young, elderly or unwell, if the forecast is below freezing, or if the boiler is locking out with the same F code after you have fixed the obvious cause.

Switch it off at the fused spur and leave it off if water is dripping from under the casing (provided the spur is dry), if the pressure climbs towards 3 bar and the relief pipe outside is discharging, or if you hear loud bangs as it tries to light. None of those should be reset.

What gets repaired and what it usually costs

An engineer reads the fault memory, then checks the gas inlet pressure while the boiler tries to fire, the ignition electrode, the condensate trap, the fan and flue, and the water pressure sensor. On older ecoTEC plus boilers, diverter valve and pressure sensor failures are both familiar repairs.

Typical market prices including fitting run roughly £80 to £160 for an ignition electrode, £250 to £500 for a diverter valve and £300 to £650 for a main board. A controls fault or a cleared condensate trap is usually much less.

For Vaillant boiler repair the call-out is £60 at any hour, and the repair price is agreed with you on site before any work starts. Gas work is carried out by Gas Safe registered engineers.