Ignition Failure - Your boiler has failed to ignite after multiple attempts
Ignition Lockout
Frozen Condensate
Sometimes
Free - 250
The F28 error code on a Vaillant boiler indicates an ignition failure. Your boiler has tried to ignite the gas burner multiple times (usually 3-5 attempts) but has been unsuccessful. As a safety measure, the boiler has "locked out" to prevent any potential hazards.
This is one of the most common Vaillant fault codes, especially during cold weather when frozen condensate pipes are a frequent culprit. The good news is that in many cases, you can fix this yourself without calling an engineer.
There are several potential causes of F28, ranging from simple DIY fixes to issues requiring a Gas Safe engineer:
Modern condensing boilers produce acidic water as a by-product of combustion. This drains away through a plastic condensate pipe, usually to an external drain. In cold weather, this pipe can freeze, causing a blockage that prevents the boiler from igniting.
If there's no gas reaching the boiler, it obviously can't ignite. This could be due to an empty prepayment meter, a closed gas valve, or issues with the gas supply.
The ignition system uses electrodes to create a spark that lights the gas. Over time, these can become dirty, corroded, or cracked, preventing successful ignition.
This repair involves internal boiler components and must be carried out by a Gas Safe registered engineer. Do not attempt to access internal parts yourself.
Typical cost: 80 - 150 including parts and labour
The gas valve controls the flow of gas to the burner. If it's stuck, faulty, or not receiving the correct signal from the PCB, it may not open properly, preventing ignition.
Gas valve work is strictly regulated and must only be performed by a Gas Safe registered engineer.
Typical cost: 150 - 250 including parts and labour
The PCB is the "brain" of your boiler. If it's faulty, it may not be sending the correct signals to ignite the boiler. This is usually diagnosed after ruling out other causes.
PCB diagnosis and replacement requires specialist knowledge and should only be done by a qualified engineer.
Typical cost: 300 - 500 including parts and labour
Once you've addressed the underlying cause (such as thawing a frozen condensate pipe), you'll need to reset the boiler:
If the F28 error returns after resetting, do not keep resetting the boiler repeatedly. This can damage components such as the ignition electrodes and PCB. Call a Gas Safe engineer instead.
If your Vaillant boiler keeps showing F28 after trying the DIY fixes above, it's time to call in the professionals.
If you're seeing a different code, these are related ignition and gas faults:
The F28 lockout itself is a safety feature - it means the boiler has detected a problem and shut down to prevent potential hazards. However, some causes (like gas leaks) can be dangerous. If you smell gas, leave immediately and call 0800 111 999.
If it's a frozen condensate pipe, you can fix it in 10-15 minutes. If an engineer needs to attend, the repair typically takes 1-2 hours depending on what parts need replacing.
If F28 occurs every winter, your condensate pipe is likely freezing regularly. Consider having it insulated with foam lagging, or ask an engineer about rerouting it internally or installing a condensate pipe heater.
No - using boiling water can crack the plastic pipe due to thermal shock. Use warm water from the tap instead, and apply it gradually along the frozen section.
PMD Plumbers are Vaillant specialists covering Harrow, London and surrounding areas. Available 24/7 for boiler emergencies.