- Boilers
- Gas boiler
F23
Vaillant boiler F23: flow and return temperature difference is too large
Evidence linked to this decision
View source: HSE gas safety for homeownersRisk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
F23 means flow and return temperature difference is too large in the cited model family. Do not buy a part from the display. First note whether the fault appears from cold or only after the burner starts, and whether heat reaches the radiators before shutdown. From-cold shutdown points more toward sensing; a rapid rise with cold emitters supports poor circulation.
Safety first
Stop and check this first
- Gas smell, a CO alarm, smoke, symptoms, escaping hot water or abnormal combustion means no reset: leave if safe and call the local gas emergency service from outside. Never open the boiler casing.
Safe checks you can perform
- Capture the failure pattern before clearing it
- Run the safe discriminating check
- Use recurrence as the decision point
Leave these tasks to a qualified professional
- Do not attempt this professional task: Measured service test for F23
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.
Measured service test for F23
What they doCheck pump operation, air, hydraulic restriction and both temperature channels against the exact model.
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.
Capture the failure pattern before clearing it
Photograph F23, both exact model identifiers and the operating state. Note whether the fault appears from cold or only after the burner starts, and whether heat reaches the radiators before shutdown.
Gas smell, a CO alarm, smoke, symptoms, escaping hot water or abnormal combustion means no reset: leave if safe and call the local gas emergency service from outside. Never open the boiler casing.
Evidence: Vaillant UK boiler fault codes list
Run the safe discriminating check
Stay outside sealed or powered compartments. From-cold shutdown points more toward sensing; a rapid rise with cold emitters supports poor circulation. If the observation is unclear, stop and treat the internal branch as unresolved.
Gas smell, a CO alarm, smoke, symptoms, escaping hot water or abnormal combustion means no reset: leave if safe and call the local gas emergency service from outside. Never open the boiler casing.
Evidence: Vaillant UK boiler fault codes list
Use recurrence as the decision point
Only if the exact user manual permits it and no stop condition exists, make one controlled attempt. If F23 returns, stop; repeated resets destroy evidence and can worsen the fault.
Gas smell, a CO alarm, smoke, symptoms, escaping hot water or abnormal combustion means no reset: leave if safe and call the local gas emergency service from outside. Never open the boiler casing.
Evidence: Vaillant UK boiler fault codes list
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Virtual technician
Does F23 still appear after these checks?
This guided flow uses the exact code and your answers to choose a test, an expected result and the next branch.
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
- 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
Vaillant's official regional documentation associates F23 with flow and return temperature difference is too large. That is the controller's observed condition, not a universal failed component. Confirm the exact model before using this decision path; the same code can mean something else on another product family.
Warnings and stop conditions
- Highsource verified
Gas smell, a CO alarm, smoke, symptoms, escaping hot water or abnormal combustion means no reset: leave if safe and call the local gas emergency service from outside. Never open the boiler casing.
Evidence: Vaillant UK boiler fault codes list
Symptoms and causes
Probable causes · in order
- First branch to separate: restricted heating-water circulationPossible
The documented condition is flow and return temperature difference is too large. Discriminating observation: Note whether the fault appears from cold or only after the burner starts, and whether heat reaches the radiators before shutdown. Expected reading and decision: From-cold shutdown points more toward sensing; a rapid rise with cold emitters supports poor circulation.
Evidence: Vaillant UK boiler fault codes list
- If that branch is ruled out: sensor, pump or protected hydraulic faultPossible
sensor, pump or protected hydraulic fault remains possible only after the safe observation is recorded. The next evidence is measured, not another reset: Check pump operation, air, hydraulic restriction and both temperature channels against the exact model.
Evidence: Vaillant UK boiler fault codes list
Sources and technical references
- 1
Health and Safety Executive · official_doc
- 2
Vaillant UK boiler fault codes list
Vaillant UK · official_doc
- 3
Vaillant ecoTEC plus current installation and maintenance instructions
Vaillant UK · manual
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