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The short answer
The Vaillant F28 fault code means ignition failure: the boiler attempted to light and could not, so it has locked out for safety. Common causes are gas supply problems, a frozen condensate pipe, worn ignition electrodes or a faulty gas valve. One reset is safe to try.
What the law requires
What does F28 mean on a Vaillant boiler?
F28 appears on ecoTEC, ecoFIT and most other Vaillant models when the boiler goes through its ignition sequence and fails to establish a flame. Unlike the pressure code F22, which you can usually resolve yourself, F28 is a lockout with safety built in: the boiler will not keep attempting ignition indefinitely, because repeatedly firing gas at a burner that will not light is exactly what safety systems exist to prevent. That is also why the golden rule with F28 is one reset, not ten. If a single reset does not clear it, the boiler is telling you something upstream is wrong, and the fix belongs to a Gas Safe registered engineer.
What causes a Vaillant F28 fault?
The causes sort neatly into things outside the boiler and things inside it. Outside the boiler: no gas reaching the appliance is the most common of all. A prepayment meter out of credit, a gas emergency control valve knocked partially closed, a supply interruption in the street, or, in winter, a frozen regulator at the meter can all starve the boiler and produce F28. The other great winter culprit is the frozen condensate pipe: when the small white waste pipe outside freezes, the boiler cannot drain and refuses to light, and on Vaillants that often surfaces as F28 or F29. Inside the boiler: ignition electrodes wear with age and stop producing a reliable spark, the flame-sensing electrode can fail to detect a flame that is actually there, the gas valve can stick or drift out of adjustment, and occasionally the fault sits in the PCB or its wiring. Everything in this second group is engineer territory, both legally and practically.
What can I safely check before calling anyone?
Check your other gas appliances. If the hob will not light either, the problem is your gas supply, not the boiler. Prepayment customers: check the meter credit first; it is the most common F28 “fault” in London.
Check the gas emergency control valve near your meter is fully open, with the lever in line with the pipe.
In freezing weather, look at the white plastic condensate pipe where it runs outside. If it is frozen, our frozen condensate pipe guide shows how to thaw it safely with warm (never boiling) water.
Press reset once and let the boiler run its full ignition sequence.
If F28 returns, stop. Do not reset repeatedly, and do not open the boiler casing. Book a Gas Safe engineer.
F28 back after one reset? Our Vaillant specialists carry ignition electrodes and common ecoTEC parts in the van, across all London boroughs. Call 020 3000 0000.
Is F28 dangerous?
The lockout itself is the safety mechanism working: no flame means the gas valve closes and the boiler waits. The danger arrives when people fight the safety mechanism, resetting over and over in the hope the boiler gives in. It will not, and the habit is both pointless and unwise. Leave the boiler locked out, run the checks above, and let an engineer find the cause. If at any point you smell gas, stop everything, avoid switches and naked flames, and call the National Gas Emergency line free on 0800 111 999 before anything else.
Want a real number instead of a range? Describe the fault, get a fixed quote after diagnosis, and pay only what was quoted. No call-out fee across London.
What does an F28 repair cost?
If the cause is your meter credit or a frozen pipe, it costs nothing but the check. Engineer repairs in London typically run: ignition electrode replacement £120 to £240, flame sensor replacement £110 to £200, gas valve work £220 to £400, and PCB replacement £300 to £500. Diagnosis comes first and the quote is fixed before any work starts. On an older boiler where the gas valve and PCB are both suspect, ask for the repair-versus-replacement numbers; our repair or replace guide explains the maths engineers actually use.
Vaillant F28, quick answers
Can I fix F28 myself?
Only the external causes: meter credit, the gas valve position and a frozen condensate pipe. Everything involving the boiler’s internals is Gas Safe work by law.
How many times can I press reset?
Treat it as once. A second attempt after fixing an obvious external cause (topping up the meter, thawing the pipe) is fine; repeated blind resets are not.
Is F28 the same as F29?
They are siblings: F28 is failure to ignite at startup, F29 is losing the flame during operation. The causes overlap heavily and both need the same diagnostic approach.
Does F28 mean my boiler is finished?
Rarely. Most F28s end as an electrode or sensor swap, one of the cheaper repairs on the menu.