- Boilers
- Gas boiler
811
Junkers 811 gas boiler: hot water disinfection failed
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On the Bosch Condens 4300iW, 811 reports failed thermal disinfection of hot water. Record the warning, close any taps left running and arrange a technician to check the cylinder cycle.
Safety first
Stop and check this first
- If you smell gas while dealing with 811, leave the building and call the emergency services and gas supplier outdoors; avoid electrical switches, unplugging equipment, indoor phones and flames. Isolate the main gas supply and open doors or windows only without exposing yourself to danger, and keep others out.
- An 811 warning is not a reason to raise the temperature and test hot water with your hand. Thermal disinfection can cause serious scalds; have qualified personnel carry out and verify the cycle and tell household members about the hot-water hazard.
Safe checks you can perform
- Identify the unsuccessful cylinder cycle
- Look for a hot tap left running
- Have the cylinder circuit investigated
Leave these tasks to a qualified professional
- Do not attempt this professional task: DHW sensor location and contact
- Do not attempt this professional task: Cylinder circulation and heating priority
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.
DHW sensor location and contact
What they doA qualified gas technician checks the cylinder sensor location and its physical contact as specified in the Bosch 811 fault row.
They must show youthe sensor mounting point and contact checked.
In writingwhether repositioning was needed and the condition found.
Cylinder circulation and heating priority
What they doA qualified gas technician checks for air in the loading circuit, DHW priority, exchanger scale and circulation pipe sizing and heat losses.
They must show youwhich check identified a problem in the cylinder installation.
In writingthe circulation or configuration correction and the supervised cycle outcome.
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.
Identify the unsuccessful cylinder cycle
- Who:
- you, using the accessible display.
- How:
- record 811, the model in your paperwork and whether the warning appeared during programmed thermal disinfection; leave service settings alone.
- Result that matters:
- confirmation that this is a Condens 4300iW disinfection warning.
- If confirmed:
- give the maintainer the cycle details with the fault number.
- If not confirmed:
- have the exact model and message identified before altering controls.
- Evidence to keep:
- an image of 811 and the time of the programmed cycle.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266) · Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Look for a hot tap left running
- Who:
- you, without accessing cylinder components.
- How:
- check normal household outlets and close any hot tap that was left open; do not run hot water to test its temperature by touch.
- Result that matters:
- whether continuous DHW draw-off coincided with disinfection.
- If confirmed:
- report the open outlet; closing it does not establish that disinfection has been completed.
- If not confirmed:
- the technician must investigate sensing, circulation and DHW configuration.
- Evidence to keep:
- the outlet involved and the known duration of the draw-off.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266) · Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Have the cylinder circuit investigated
- Who:
- a qualified gas technician.
- How:
- check sensor position and cylinder contact, bleed the circuit where required and inspect DHW priority, the exchanger and circulation as specified for 811.
- Result that matters:
- the condition preventing the thermal disinfection cycle from completing.
- If confirmed:
- correct the identified issue and verify the cycle with scald protection in place.
- If not confirmed:
- complete the remaining checks in the fault row before proposing replacement parts.
- Evidence to keep:
- the 811 diagnostic report and the supervised cycle result.
Internal work on this Condens 4300iW belongs to a qualified gas technician because the casing encloses electrical connections and hot components.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
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
- Junkers
- Product type
- Gas boiler
Known code variants
- Bosch 811
- Condens 4300iW 811
- Junkers 811
What it means
The Condens 4300iW fault table assigns 811 to failed DHW thermal disinfection, separately from a general cold-water complaint. Bosch lists continuous draw-off, cylinder sensor contact, air, DHW priority, scale and circulation heat losses for investigation. This message neither proves a failed temperature sensor nor confirms that the stored water has been disinfected.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas while dealing with 811, leave the building and call the emergency services and gas supplier outdoors; avoid electrical switches, unplugging equipment, indoor phones and flames. Isolate the main gas supply and open doors or windows only without exposing yourself to danger, and keep others out.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
- Higheditorial warning
An 811 warning is not a reason to raise the temperature and test hot water with your hand. Thermal disinfection can cause serious scalds; have qualified personnel carry out and verify the cycle and tell household members about the hot-water hazard.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Symptoms and causes
Probable causes · in order
- Hot water drawn continuouslyPossible
For 811, Bosch instructs the technician to stop continuous hot-water discharge. Establish whether an outlet was left running before attributing the unsuccessful cycle to cylinder components.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Poor cylinder sensor contactPossible
Bosch calls for correct DHW sensor positioning and contact between the cylinder sensor and the cylinder. A mounting problem must be separated from a sensor that actually needs replacement.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Air or unsuitable DHW priorityPossible
Bleeding the cylinder circuit and giving DHW heating priority both appear in the 811 remedies. These are professional circulation and configuration checks, not instructions to dismantle the cylinder yourself.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Scale or circulation heat lossesPossible
The 811 row also directs attention to plate heat-exchanger scale and the sizing and heat loss of the DHW circulation pipework. These installation checks go beyond reading the boiler display.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
Virtual technician
Does 811 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
Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
Bosch Termotecnia (Robert Bosch España) · 2021 · manual
- 2
Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Bosch Termotecnia (Robert Bosch España) · 2021 · manual
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