The F28 code on a Vaillant boiler — including the ecoTEC range — means the boiler has gone through its ignition sequence, failed to detect a stable flame, and locked out. It won’t attempt to fire again until the fault is cleared and the boiler is reset.
This is your Vaillant’s built-in protection doing its job. The lockout prevents repeated unsafe ignition attempts and protects both the boiler and your household while the cause is identified.
Here at Guscott Heating, our Gas Safe registered engineers have been diagnosing and resolving Vaillant F28 faults across Chatham, Gillingham, Strood, Rochester, Rainham, Maidstone, Sittingbourne and the surrounding areas since 2017. This guide explains exactly what’s happening inside your boiler and what to do about it.
What Causes of the Vaillant Boiler F28 Fault Code to Show?
The F28 spans a broad range of potential causes — from something as simple as a prepayment meter running dry, to a failing PCB. That’s precisely why it needs systematic diagnosis rather than guesswork:
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No gas supply or interrupted supply — nothing gas-powered will work in the property
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Prepayment meter out of credit — easily overlooked and easily resolved
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Low gas pressure — insufficient gas reaching the burner to sustain a flame
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Faulty or sticking gas valve — gas not flowing to the burner despite supply being present
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Blocked flue — restricted airway preventing safe combustion
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Faulty spark generator — no spark produced during the ignition cycle
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Damaged or worn ignition electrodes or leads — weak or absent spark at the burner
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Debris blocking the burner jet — even fine contamination can prevent ignition
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Failed PCB — the boiler’s main control board has stopped functioning correctly
Fixing the Vaillant F28 Fault Code & Where to Start
Confirm Your Gas Supply Is Live
The first thing to establish — before touching the boiler — is whether gas is actually reaching your property. Turn on your gas hob or check any other gas appliance in the home. If they’re working normally, your supply is fine and the issue is specific to the boiler. If nothing gas-powered is functioning, your supply has been interrupted — contact your gas provider.
Prepayment meter users should check credit balance at this stage. It’s one of the most common causes of an F28 callout and one of the few that costs nothing to resolve.
Attempt a Boiler Reset
With gas supply confirmed, press and hold the reset button on the Vaillant control panel for three seconds. Allow the boiler to complete its startup sequence. A successful restart that holds for several hours with no recurrence is a good sign the fault was a temporary interruption.
If the F28 returns within minutes or immediately after reset, the boiler is repeatedly failing ignition for a reason that needs finding. Continued resetting without a diagnosis isn’t just ineffective — it can cause additional wear on ignition components.
Inspect the External Flue Terminal
Head outside and look at where the flue exits the property. Check the terminal for any obvious obstruction — accumulated debris, bird activity, plant growth, or ice during cold periods. A flue guard should always be fitted to the terminal; the absence of one is a common finding on older Vaillant installations and frequently leads to debris entering the flue airway.
If the terminal is clear but the fault persists, blockage may be further inside the flue — which requires an engineer to access and clear.
Check System Pressure
Low water pressure can prevent the Vaillant ecoTEC from completing its ignition sequence safely. The pressure gauge should read between 1 and 1.5 bar on a cold system. Below 1 bar, repressurise using the filling loop beneath the boiler and attempt a reset. If pressure drops repeatedly between top-ups, a system leak needs locating before the F28 will resolve permanently. If your display shows a symbol instead of a fault code, it may simply be a protective pause, as we explain in our guide to the egg timer symbol on a Vaillant boiler.
Deeper Causes of the Vaillant F28 Error Requiring a Gas Safe Engineer
Gas Pressure Issues Specific to Vaillant Boilers
Low gas pressure at the appliance — as opposed to no gas supply — is a subtler problem that requires testing equipment to confirm. There are a few situations where this crops up specifically with Vaillant installations:
Older properties: Air pockets can develop in gas supply pipework over years of use, creating partial flow restrictions that show up as low gas pressure at the boiler. An engineer can identify and clear these.
Recent installations: A newly fitted Vaillant ecoTEC is typically more efficient and may have a higher gas demand than the boiler it replaced. If the existing gas supply pipework is undersized for the new appliance, pressure at the boiler will be insufficient. The solution is upgrading the supply pipework — something your installing engineer should assess during commissioning.
Cold weather and frozen regulators: External gas meters exposed to freezing temperatures can develop a frozen regulator, restricting gas flow to the property. Protecting the meter with appropriate boxing or insulation is a practical preventative measure for any Vaillant owner with an exposed external meter.
Gas Valve Faults on Vaillant Boilers
The gas valve controls how much gas enters the combustion chamber. When it sticks, fails to open at the right point in the ignition sequence, or develops an electrical fault, gas doesn’t reach the burner consistently — and F28 follows. Vaillant gas valve diagnosis requires specific testing equipment and must be carried out by a Gas Safe engineer. Replacement is required if testing confirms a fault.
Ignition Electrode and Lead Failures
Three distinct failure modes affect Vaillant ignition components — and the correct diagnosis matters because each has a different fix:
Vibration damage: A Vaillant boiler that has been running noisily — rattling, vibrating, or humming — before displaying F28 may have suffered vibration-related damage to electrode leads or their connections. The source of vibration (usually the pump or fan) should be identified and addressed alongside any ignition component replacement.
Moisture ingress from internal leaks: Replacing electrodes in a boiler with an active internal leak is a temporary fix at best. Water will reach the new components and cause the same fault again. An engineer must identify and resolve any internal leak before ignition components are replaced — not after.
Natural wear and age: Electrodes degrade over time and simply need replacing. There’s no fix other than a like-for-like replacement by a qualified engineer.
Blocked Burner Jet
The jet inside the Vaillant burner has a very small aperture — fine enough that even microscopic debris can cause a complete blockage. If gas supply, pressure, valve operation, and ignition components have all been checked and found working, the burner jet is the next logical inspection point. Cleaning the burner and jet thoroughly, followed by a reset, typically resolves this.
PCB Failure
A failed PCB on a Vaillant boiler is the diagnosis of last resort — confirmed only after every other component has been tested and ruled out. Replacement costs typically fall between £400 and £600 including parts and labour, making thorough diagnosis beforehand essential. Reputable Vaillant engineers will never recommend PCB replacement without exhausting all other possibilities first.
Vaillant F28 Quick Reference Guide
Frequently Asked Questions About the Vaillant F28 Fault Code
How is the Vaillant F28 different from F29?
Both codes indicate ignition failure, but they point to different stages of the process. F28 means the boiler failed to ignite — no flame was established. F29 means a flame was detected during the startup sequence but was lost during operation, suggesting a different set of causes including flame sensor faults or gas supply instability. If your Vaillant is alternating between F28 and F29, tell your engineer — it provides useful diagnostic information. Glow-Worm boilers use the same F28 code for the same ignition failure, so our guide to the Glow-Worm F28 fault code is a useful cross-reference if you are comparing symptoms.
Can the Vaillant F28 come on in cold weather specifically?
Yes. Frozen condensate pipes and frozen gas meter regulators are both cold weather triggers for F28. A frozen condensate pipe is the more common of the two — thawing it with warm water and resetting the boiler usually resolves it quickly. A frozen meter regulator requires protecting the meter from future exposure.
How much does resolving a Vaillant F28 cost?
Straightforward causes — gas supply, meter credit, system pressure, condensate pipe — cost little to nothing. Electrode or spark generator replacement typically runs £80–£200 including labour. Gas valve replacement usually falls between £200 and £400. PCB replacement is the most significant cost at £400–£600. With a Guscott Heating boiler cover plan, parts and labour are covered so there are no unexpected bills regardless of what the diagnosis reveals.
Should I keep resetting my Vaillant boiler if F28 keeps coming back?
No. Repeated resets without identifying and resolving the underlying fault cause unnecessary wear on ignition components and can mask a worsening problem. If F28 returns after two resets, call an engineer rather than continuing to reset. If faults are becoming a pattern, it is worth reading our Vaillant boiler reviews to see how reliable the brand is overall.
Vaillant boiler showing F28 in Chatham, Gillingham, Strood, Rochester, Rainham, Maidstone or Sittingbourne? Guscott Heating’s Gas Safe engineers are on hand across Kent and Medway to get your Vaillant back up and running. We’ve been solving local heating problems since 2017 — call us today.
