Boiler fault codes
Vaillant F28 Fault Code: No Flame on Start-Up
Published
Vaillant display
F28
What it means
The boiler tried to light on start-up, never detected a flame, and locked itself out. Vaillant describes it as the flame signal not being detected during the ignition phase.
Safe to check yourself
- Light the gas hob or another gas appliance to confirm gas is reaching the property
- On a prepayment meter, check the credit and follow the meter’s own steps to restore supply
- Check the gas isolation valve under the boiler has not been turned off
- In freezing weather, look for ice in the white plastic condensate pipe outside and thaw it with warm, never boiling, water
- Reset the boiler no more than three times in a row, as your model's manual describes
Call an engineer if
- F28 returns after three resets even though the gas and condensate pipe are fine
- Other gas appliances are also failing and the meter has credit (call your gas supplier or 0800 111 999)
- You see soot, scorch marks or water around the boiler casing
- You smell gas: open windows, shut off the gas at the meter if safe, use no switches, then leave and call 0800 111 999 from outside
F28 on a Vaillant boiler means ignition failed at start-up: the boiler tried to light, never detected a flame and locked out. The usual culprits are a gas supply problem, a frozen or blocked condensate pipe, or a worn ignition electrode. Check the first two yourself, reset up to three times, then call an engineer.
What does F28 mean on a Vaillant boiler?
Every time your boiler needs heat it opens the gas valve, sparks the ignition electrode and waits for a flame sensor to confirm the burner has lit. If no flame is seen, it tries again. Vaillant’s latest ecoTEC plus operating instructions say that after five unsuccessful ignition attempts the boiler goes into fault mode and shows F.28; the 2015 edition gave three attempts. The lockout is a safety feature: it stops the boiler pushing unburnt gas into the combustion chamber.
F28 is the start-up failure. If the boiler lit fine, ran for a while, then lost the flame and could not relight, you will see F29 instead. The difference helps an engineer, because a boiler that never lights points more strongly at the gas supply and the ignition parts.
The Vaillant installer manual lists a long set of causes for F.28: a gas meter or gas pressure fault, air in the gas pipe, gas pressure too low at the boiler, a blocked condensate route, a faulty gas valve, a break in the wiring, a failed ignition part (transformer, lead, plug or electrode), poor earthing, or a failed circuit board. Only the first few are things you can check from outside the casing.
What to check before you press reset
Work through these in order. Each one takes a minute. The gas, the meter credit and a frozen condensate pipe are the three things Vaillant itself tells owners to check before calling anyone.
- Is gas reaching the property? Light the hob. If nothing lights anywhere, the problem is the supply or the meter, not the boiler.
- Prepayment meter: a meter that has run out of credit, or has used its emergency credit, cuts the gas. Top up and follow the meter’s instructions to restore supply.
- Gas valve under the boiler: if anyone has worked near the boiler recently, check the isolation valve on the gas pipe has not been turned off. Vaillant’s own first step for F.28 is to check the gas stopcock is open.
- Condensate pipe in cold weather: condensing boilers drain water through a plastic pipe, often to an outside drain. If it freezes, water backs up and the boiler will not light. You may hear gurgling. Pour warm water along the frozen section or hold a hot water bottle against it.
- Reset: on the latest ecoTEC plus, press and hold the reset button for more than three seconds; older models need a one-second press. Vaillant sets a maximum of three repetitions. If it still locks out, stop there.
If you smell gas at any point, do not reset the boiler or touch light switches. Open windows, turn the gas off at the meter if you can reach it safely, then leave the property and call the National Gas Emergency Service on 0800 111 999.
Why does F28 keep coming back?
If the gas is on, the condensate pipe is clear and F28 still returns, a part is usually failing. The commonest is the ignition electrode, which wears and builds up deposits until it no longer sparks cleanly or senses the flame. As a typical market price, a genuine Vaillant ignition electrode is listed at roughly £15 to £40 as a part depending on the model, so this is usually one of the cheaper ignition repairs.
If F28 comes back every cold snap, the fault is the condensate route rather than the boiler. A long or thin external run freezes easily. An engineer can reroute the pipe internally, increase its size or insulate it so the same lockout does not recur each winter.
Other repeat causes are a sticking gas valve, a gas pressure problem at the meter, air in the gas line after work on the supply, or a fault on the circuit board. These all need someone qualified to work on gas.
What an engineer does about F28
The engineer reads the boiler’s fault history, then measures the gas pressure at the boiler inlet with a manometer, both standing and with the burner trying to fire. Low pressure there sends the job back to the meter or the pipework. With the casing off, they check the electrode gap and condition, the ignition lead, the condensate trap and the flue, and finish with a combustion check using a flue gas analyser.
Gas work is carried out by Gas Safe registered engineers. You pay the £60 call-out, and the repair price is agreed with you on site before any work starts. If your boiler shows a different code, our Vaillant boiler repair page covers the other common faults.
Questions people ask
Can I reset F28 myself?
Yes, once you have checked the gas supply and the condensate pipe. Hold the reset button for more than three seconds on the latest models, or one second on older ones. Vaillant allows up to three attempts; if F28 comes back after that, leave it switched off and call an engineer.
Is F28 dangerous?
The code itself means the boiler has made itself safe by refusing to keep trying. It becomes urgent if you smell gas, in which case ventilate, turn off at the meter if safe, get out and call 0800 111 999, or if you have no heating in freezing weather with vulnerable people at home.
Why does my Vaillant show F28 when it is freezing outside?
A frozen condensate pipe is the most likely reason. Water cannot drain out of the boiler, so it will not fire. Thaw the outside section with warm water, reset, and ask an engineer about rerouting or insulating the pipe if it happens every winter.