- Boilers
- Gas boiler
F68
Vaillant F.68 on a gas boiler: unstable flame signal
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Vaillant F.68 means the flame monitor is getting an unstable flame signal. Check the gas valve is open and there is no smell of gas or fumes; do not open the boiler and call an authorised professional.
Safety first
Stop and check this first
- If you smell smoke or burnt gases indoors: Vaillant says to open doors and windows fully, switch the appliance off and call an authorised professional. A blocked or leaking flue can fill the home with carbon monoxide, which has no smell.
- If you smell gas: do not operate switches, sockets, doorbells or phones, no flames or smoking, open doors and windows, close the meter valve and the appliance gas valve if you can, warn neighbours without using the bell, leave the building and call the gas company from outside. That is what Vaillant asks.
Safe checks you can perform
- Smell of gas or fumes
- Gas isolation valves
- Flue terminal seen from outside
- Combustion check by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Gas flow pressure
- Do not attempt this professional task: Ionisation value
- Do not attempt this professional task: Combustion analysis
- Do not attempt this professional task: Condensate pipe and flue route
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.
Gas flow pressure
What they doThe technician measures gas flow pressure with a manometer while the burner runs and compares it with Vaillant's value for that gas.
They must show youthe measured pressure against the table.
In writingthe value and what was corrected.
Ionisation value
What they doThe technician reads the actual ionisation value (D.044) and checks the electrode and its cable.
They must show youthe ionisation value before and after.
In writingthe electrode's condition and whether it was replaced.
Combustion analysis
What they doThe technician measures CO2 at the flue gas test point and compares it with Vaillant's table.
They must show youthe analyser printout.
In writingthe CO2 measured and whether it was adjusted.
Condensate pipe and flue route
What they doThe technician checks the condensate pipe is not blocked and there is no flue gas recirculation.
They must show youthe siphon and pipe checked.
In writingwhat was cleaned or corrected.
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.
Smell of gas or fumes
- Who:
- you.
- How:
- notice whether there is a smell of gas or burnt gases near the boiler.
- Result that matters:
- whether there is a smell.
- If it smells of gas:
- follow Vaillant's gas warning.
- If it smells of fumes:
- open doors and windows, switch the appliance off and call a professional.
- Evidence to keep:
- a note of the time and what you noticed.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Gas isolation valves
- Who:
- you.
- How:
- check the home's gas valve and the appliance gas valve are fully open.
- Result that matters:
- whether they were open.
- If one was half open or closed:
- open it fully and tell the technician.
- If they were open:
- the cause is in combustion or flame sensing.
- Evidence to keep:
- a photo of the valves.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Flue terminal seen from outside
- Who:
- you.
- How:
- look at the flue terminal from the ground or a window, without touching it or climbing.
- Result that matters:
- whether it is blocked or close to something that sends fumes back.
- If you see a blockage:
- tell the technician.
- If it is clear:
- it points to gas, ionisation or condensate.
- Evidence to keep:
- photos of the terminal.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Combustion check by the technician
- Who:
- authorised gas technician.
- How:
- measures gas flow pressure, CO2 and the ionisation value and checks condensate, nozzle and flue.
- Result that matters:
- which of Vaillant's causes applies.
- If gas pressure is low:
- the installation is checked or the distributor is told.
- If gas is fine:
- ionisation, condensate and recirculation are checked.
- Evidence to keep:
- a report with all readings.
Measuring gas, opening the combustion chamber or touching electrodes is work only for an authorised gas technician.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
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
- Vaillant
- Product type
- Gas boiler
Known code variants
- A68
- BlueHelix Pro F68
- ecoTEC F.68
- Error F68 Saunier Duval
- Error F68 Vaillant
- F.68
- Ferroli A68
- Ferroli F68
- Saunier Duval F.68
- ThemaFast Condens F.68
- Vaillant F.68
What it means
In the ecoTEC plus table, F.68 is fault: unstable flame signal. The flame monitor reports an unstable signal and Vaillant says to check the excess air factor, gas flow pressure, the condensate pipe (blockage), the gas nozzle, the ionisation current (cable, electrodes) and flue gas recirculation. The flame is monitored by ionisation; if combustion or the measurement fails, the signal fluctuates. The code does not say which of those systems is at fault.
Warnings and stop conditions
- Higheditorial warning
If you smell smoke or burnt gases indoors: Vaillant says to open doors and windows fully, switch the appliance off and call an authorised professional. A blocked or leaking flue can fill the home with carbon monoxide, which has no smell.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
- Criticalsource verified
If you smell gas: do not operate switches, sockets, doorbells or phones, no flames or smoking, open doors and windows, close the meter valve and the appliance gas valve if you can, warn neighbours without using the bell, leave the building and call the gas company from outside. That is what Vaillant asks.
Evidence: Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Symptoms and causes
Probable causes · in order
- Gas flow pressure too lowPossible
If the technician measures low gas pressure with the burner running, the flame is unstable; it may come from the network or the installation.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Ionisation: cable or electrodesPossible
If combustion is correct but the ionisation current is low or varies, the fault is the electrode or its cable.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Condensate or flue gas recirculationPossible
A blocked condensate pipe or flue gases drawn back into the air intake disturb the flame; it shows at the siphon and the flue.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Excess air or nozzlePossible
CO2 outside the table or a wrong nozzle changes the mixture; the analyser shows it.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Virtual technician
Does F68 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
Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Vaillant · 2017 · manual
- 2
Vaillant ecoTEC plus VM, VMW y VMI ..6/5-5, instrucciones de funcionamiento 0020243851_02
Vaillant · 2017 · manual
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