- Boilers
- Gas boiler
F28
Saunier Duval Thema NOx Boiler F28: Ignition Fault Lockout
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On a Saunier Duval Thema NOx gas boiler, fault code F28 (or F29) with a flashing red lamp signals an ignition failure. If there is no gas smell and the gas supply valve is open, switch the ON/OFF button to O, wait 5 seconds, and set it back to I once; if F28 returns, call official service.
Safety first
Stop and check this first
- If you smell gas: never use a naked flame to look for a leak, do not operate electrical switches, plugs, or telephones that could cause a spark, open windows and doors right away to ventilate, close the gas isolation valve, warn neighbours, and call your gas emergency number (0800 111 999 in the UK or your local gas supplier emergency line).
- Never attempt maintenance or repairs yourself, never tamper with safety devices or remove component seals, and keep the boiler switched off until Saunier Duval Official Technical Service has resolved fault F28.
Safe checks you can perform
- Rule out any gas smell and verify external supply valves
- Switch the boiler off to O for 5 seconds and restart once
- Locate the 28-digit serial number before calling official service
Leave these tasks to a qualified professional
- Do not attempt this professional task: Saunier Duval Official Technical Service intervention for persistent F28
- Do not attempt this professional task: Mandatory annual servicing and room-sealed flue inspection
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Saunier Duval Official Technical Service intervention for persistent F28
What they doContact Saunier Duval Official Technical Service with your 28-digit serial number when F28 persists after waiting 5 seconds and restarting once.
They must show youofficial Saunier Duval accreditation or gas registration ID card and the combustion and ignition test readings.
In writingthe repair entry in the boiler maintenance logbook, any genuine Saunier Duval part references fitted, and the safety check result.
Mandatory annual servicing and room-sealed flue inspection
What they doHave an accredited gas engineer or Saunier Duval Official Technical Service inspect the room-sealed air intake and flue exhaust system and complete the annual service.
They must show youthe visual inspection of the room-sealed flue ducting and the flue gas analyser printout.
In writingthe annual service certificate and updated maintenance logbook record confirming safe combustion.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Rule out any gas smell and verify external supply valves
- Who:
- you as the boiler owner, without tools or opening any panel.
- How:
- First check the room for any smell of gas; if there is none, confirm that the external gas supply valve is open, electrical power is on, and the screen pressure reading is not flashing at or below 0.5 bar.
- Result that matters:
- No gas odour is present and external gas, water, and electrical supplies are confirmed active.
- If you smell gas at any point, treat it as an immediate emergency, do not touch electrical switches or phones, open doors and windows, shut off the gas valve, and call the gas emergency service.
- If there is no gas smell and the external gas valve was closed, open the valve and proceed to the single 5-second switch reset allowed by the manual.
- Evidence to keep:
- Written note of the gas valve position, the flashing red indicator status, and the bar pressure shown on screen.
Evidence: HSE gas safety for homeowners · Saunier Duval Thema NOx operating instructions
Switch the boiler off to O for 5 seconds and restart once
- Who:
- you at the front control panel, using only the ON/OFF switch.
- How:
- Move the ON/OFF rocker switch to the O symbol to take the boiler out of service, wait 5 full seconds, and move the switch back to the I symbol a single time.
- Result that matters:
- The control panel runs its initialization cycle for several seconds and attempts to light the burner with a steady yellow indicator.
- If the indicator turns steady yellow on heat or hot water demand and F28 clears, the ignition interruption was temporary and normal operation is restored.
- If the indicator flashes red again and F28 reappears on the screen, turn the switch back to O and book Saunier Duval Official Technical Service.
- Evidence to keep:
- Photo of the F28 screen display and the exact time you carried out the single 5-second restart.
Evidence: Saunier Duval Thema NOx operating instructions
Locate the 28-digit serial number before calling official service
- Who:
- you with the boiler switched off at position O.
- How:
- Look for the 28-digit manufacturing serial number inside the drop-down front flap (bottom right corner), at the top of the outer instruction magnet, or on your warranty application card.
- Result that matters:
- You have the unique 28-digit Thema NOx serial number ready to give to the official service dispatcher.
- If you have both the 28-digit serial number and your maintenance logbook ready, the official technician can verify the exact boiler variant and bring genuine Saunier Duval spare parts.
- If the screen actually displays St33/F33/F37 or F20/F25 instead of F28/F29, note that the manual assigns those codes to air flow or overheating rather than ignition failure.
- Evidence to keep:
- Photo of the 28-digit serial number label and the boiler maintenance logbook.
Evidence: Saunier Duval Thema NOx operating instructions
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Saunier Duval
- Product type
- Gas boiler
Known code variants
- caldera Glow-worm F28
- erreur F28 chaudière
- Error F28 Saunier Duval
- Error F28 Vaillant
- errore F28 caldaia
- F28
- Fallo F28 Thema NOx
- Glow worm boiler F28 fault
- Glow worm F28
- Glow-worm F.28
- Glow-worm F28
- Saunier Duval boiler F28
- Saunier Duval F28
- Saunier Duval Thema NOx F28
- Vaillant F.28
- Vaillant F28
- Vaillant F28 fault
- Vaillant Fehler F28
What it means
According to the official Saunier Duval Thema NOx F25 operating manual (published in Spanish), fault code F28 or F29 appears on the display while the control panel indicator flashes red when the boiler fails to ignite. The manual permits the homeowner only to verify electricity, water, and gas supply and to perform one reset by moving the ON/OFF switch to O, waiting 5 seconds, and returning it to I. On its own, F28 never proves a specific internal component has failed, and opening the sealed casing or adjusting gas parts is restricted to Saunier Duval Official Technical Service.
Warnings and stop conditions
- Highsource verified
If you smell gas: never use a naked flame to look for a leak, do not operate electrical switches, plugs, or telephones that could cause a spark, open windows and doors right away to ventilate, close the gas isolation valve, warn neighbours, and call your gas emergency number (0800 111 999 in the UK or your local gas supplier emergency line).
Evidence: HSE gas safety for homeowners · Saunier Duval Thema NOx operating instructions
- Highsource verified
Never attempt maintenance or repairs yourself, never tamper with safety devices or remove component seals, and keep the boiler switched off until Saunier Duval Official Technical Service has resolved fault F28.
Evidence: HSE gas safety for homeowners · Saunier Duval Thema NOx operating instructions
Symptoms and causes
Probable causes · in order
- Closed gas isolation valve or missing startup supply conditionsCommon
Section 7.3.1 of the Thema NOx F25 manual requires electrical power, an open gas isolation valve, and an open main water valve before the boiler can complete its startup sequence and light the burner.
Evidence: Saunier Duval Thema NOx operating instructions
- Transient burner ignition failure cleared by a 5-second power cyclePossible
Section 8 of the operating instructions groups fault codes F28 and F29 under ignition failure with a flashing red fault light and specifies switching the unit off to O for 5 seconds before switching back to I.
Evidence: Saunier Duval Thema NOx operating instructions
- Persistent ignition or gas regulation fault requiring official servicePossible
When F28 remains on the display after waiting 5 seconds and restarting once, the manual instructs you to leave the appliance off and contact Saunier Duval Official Technical Service for gas and combustion checks.
Evidence: HSE gas safety for homeowners · Saunier Duval Thema NOx operating instructions
Virtual technician
Does F28 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Health and Safety Executive · official_doc
- 2
Saunier Duval Thema NOx operating instructions
Saunier Duval · 2012 · manual
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