- Boilers
- Gas boiler
F23
Saunier Duval F.23 on a gas boiler: temperature spread too high
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Saunier Duval F.23 is a safety shutdown for excessive temperature variation. It points to the pump, air in the appliance or swapped sensors. Check the pressure (1 to 1.5 bar) and bleed the radiators; if it returns, call the official service centre.
Safety first
Stop and check this first
- Top up with water suitable for heating and close the filling valve when you finish. Saunier Duval warns that water with a lot of lime, corrosive substances or chemicals damages the appliance.
- If you smell gas: Saunier Duval says to keep away from the area, open doors and windows, light no flames and do not smoke, do not touch switches, sockets, doorbells or phones, close the meter and appliance gas valves, warn neighbours without the bell, leave the building and call the gas company from a phone outside.
Safe checks you can perform
- Pressure on the display
- Topping up with the filling valve
- Bleeding the radiators
- One reset with the fault key
- Appliance bleeding, pump and sensors checked by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Appliance bleeding
- Do not attempt this professional task: Pump: blockage and output
- Do not attempt this professional task: Flow and return sensor fitting
- Do not attempt this professional task: Fault memory
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.
Appliance bleeding
What they doThe technician bleeds the appliance short circuit with test programme P.07 and the system with P.06, with the pump bleed valve open.
They must show youthe filling pressure after bleeding.
In writingthe programmes used and the final pressure.
Pump: blockage and output
What they doChecks whether the pump turns, reviews its setting on diagnostic code d.14 and reads its actual speed on d.15 with the boiler heating.
They must show youthe d.15 reading with the boiler running.
In writingthe setting found, the one left and, if the pump is replaced, the reference and why.
Flow and return sensor fitting
What they doChecks that each sensor is on its own pipe and compares the flow (d.40) and return (d.41) readings with the burner on.
They must show youboth readings with the burner lit.
In writingwhether the sensors were fitted correctly and what was corrected.
Fault memory
What they doThe technician reads the last 10 codes stored in the fault memory.
They must show youthe list of codes.
In writingthe codes found before they are cleared.
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.
Pressure on the display
- Who:
- you.
- How:
- with no heating or hot water demand, read the pressure on the display. Saunier Duval recommends 1 to 1.5 bar.
- Result that matters:
- whether it is below 1 bar or flashing below 0.5 bar.
- If it is low:
- top up as in the next check.
- If it is correct:
- low pressure does not explain F.23; go on to bleeding.
- Evidence to keep:
- a photo of the display showing the pressure.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Topping up with the filling valve
- Who:
- you.
- How:
- open the radiator valves, slowly open the filling valve under the boiler as the professional showed you, add water to 1 to 1.5 bar and close the valve.
- Result that matters:
- whether F.23 stops and the pressure holds.
- If it holds:
- lack of water fits; note the date.
- If it drops again:
- there is a leak; call the official service centre.
- Evidence to keep:
- photos of the pressure before and after.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Bleeding the radiators
- Who:
- you.
- How:
- bleed the radiators, as Saunier Duval says when there is air in the system. Then read the pressure again and, if it has dropped, top up to 1 to 1.5 bar.
- Result that matters:
- whether air came out and whether the water noises stop.
- If air came out:
- air fits; note which radiators you bled.
- If no air came out:
- the cause of F.23 is more likely inside the boiler.
- Evidence to keep:
- a note of the radiators bled and a photo of the final pressure.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
One reset with the fault key
- Who:
- you.
- How:
- press the fault clearing (reset) key and wait five seconds, as Saunier Duval describes. Do it once only.
- Result that matters:
- whether F.23 comes back.
- If it clears and stays away:
- note the date and tell the technician at the next service.
- If it persists or returns:
- do not keep resetting; call the official service centre.
- Evidence to keep:
- a photo of the display with the code and the time.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Appliance bleeding, pump and sensors checked by the technician
- Who:
- official service centre technician or qualified installer.
- How:
- bleeds the appliance with the test programmes, checks whether the pump turns and its setting, and checks that the flow and return sensors are each on their own pipe.
- Result that matters:
- which of the four Saunier Duval causes is present.
- If the pump was blocked or underpowered:
- it is freed, adjusted or replaced.
- If the sensors were swapped:
- they are refitted and no parts need replacing.
- Evidence to keep:
- a report with what was checked and corrected.
Saunier Duval reserves inspection, maintenance, repair and gas adjustments for a qualified installer: do not open the casing.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
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
- 23
- code erreur F23 chaudière Vaillant
- codice F23 Vaillant
- código F23 Vaillant
- error F23 caldera Vaillant
- Error F23 Saunier Duval
- errore F23 caldaia Vaillant
- F 23
- Fehlercode F23 Vaillant
- panne F23 Vaillant
- Saunier Duval F.23
- ThemaFast Condens F.23
- Vaillant boiler F23
- Vaillant F.23
- Vaillant F23
- Vaillant Fehler F23
What it means
In the ThemaFast Condens table, F.23 is safety shutdown: excessive temperature variation. Saunier Duval gives four causes: pump blocked, pump with reduced output, air in the appliance and flow and return NTC sensors swapped. The first three affect how water circulates through the appliance; the last is a fitting error that distorts the difference between flow and return. The boiler shuts down to protect itself. The code does not say which of the four causes is acting.
Warnings and stop conditions
- Mediumsource verified
Top up with water suitable for heating and close the filling valve when you finish. Saunier Duval warns that water with a lot of lime, corrosive substances or chemicals damages the appliance.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
- Criticalsource verified
If you smell gas: Saunier Duval says to keep away from the area, open doors and windows, light no flames and do not smoke, do not touch switches, sockets, doorbells or phones, close the meter and appliance gas valves, warn neighbours without the bell, leave the building and call the gas company from a phone outside.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Symptoms and causes
Probable causes · in order
- Air in the appliance or the systemPossible
It fits if you hear water noises in the system, which Saunier Duval links to air. Bleeding the radiators confirms or rules it out.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00 · Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
- Pump blockedPossible
Saunier Duval lists it first. It fits if the pressure is correct, no air comes out when bleeding and F.23 returns at every start. Only the technician checks it.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
- Pump with reduced outputPossible
The pump turns but moves less water than needed. The technician tells it apart with the pump setting (d.14) and its actual speed (d.15).
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
- Flow and return sensors swappedPossible
A fitting error: it fits if F.23 started after installation or after sensors were replaced. The boiler reads the temperature difference the wrong way round.
Evidence: Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
Virtual technician
Does F23 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
Saunier Duval ThemaFast Condens, instrucciones de instalación y mantenimiento 0020238203_00
Saunier Duval · 2016 · manual
- 2
Saunier Duval ThemaFast Condens, instrucciones de funcionamiento 0020238204_00
Saunier Duval · 2016 · manual
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