- Boilers
- Gas boiler
810
Junkers Bosch 810 on a gas boiler: hot water stays cold
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Bosch Condens 4300iW code 810 means the hot water stays cold. Check the hot water setting, carry out the reset the user manual allows and wait; if the code persists, have the valves and cylinder connection checked.
Safety first
Stop and check this first
- If you smell gas while code 810 is showing, do not touch switches or plugs, avoid flames and sparks, close the gas valve, open windows, leave the home and call the gas supplier and service company from outside.
- Do not dismantle valves or cylinder connections to clear 810; cold hot water points to a circuit that a technician must inspect.
Safe checks you can perform
- Photograph 810 and the appliance details
- Check the hot water setting
- Reset and wait
- Technical check of the hot water circuit
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check the 3-way valve
- Do not attempt this professional task: Check DHW valves and cylinder connection
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.
Check the 3-way valve
What they doAn authorised technician verifies that the 3-way valve switches to the cylinder on a hot water demand and replaces it if it does not respond.
They must show youthe valve switching during a hot water demand.
In writingthe 3-way valve condition and whether it was replaced for 810.
Check DHW valves and cylinder connection
What they doThe technician checks the domestic hot water valves and confirms the cylinder is connected as Bosch specifies.
They must show youvalve positions and the cylinder connections.
In writingwhich valve or connection was corrected and the cylinder heat-up result.
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.
Photograph 810 and the appliance details
- Who:
- you, from the Condens 4300iW panel.
- How:
- note the 810 shown with the fault symbol and copy the appliance designation and serial number from the panel rating plate, as the user manual asks.
- What matters: the service company receiving 810 together with your Condens 4300iW details.
- If the code persists:
- if 810 is still displayed, move on to checking the hot water setting.
- If it has cleared:
- if 810 has cleared, keep the photo in case the water runs cold again.
- Evidence to keep:
- photo of 810 and the rating plate.
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)
Check the hot water setting
- Who:
- you, using the hot water key.
- How:
- check the set hot water temperature and whether eco mode is active, as described in the hot water section of the Condens 4300iW user manual.
- What matters: ruling out a low setting before blaming a part for 810.
- If the code persists:
- if the setting is correct and 810 continues, go on to the reset the manual allows.
- If it has cleared:
- if the setting was wrong and the water heats without 810, note the change you made.
- Evidence to keep:
- photo of the configured hot water temperature with 810 visible.
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)
Reset and wait
- Who:
- you, following the fault clearing section.
- How:
- switch the boiler off and on again, or press both arrow keys together until the fault symbols disappear; the technical table for 810 then says to wait 24 hours.
- What matters: seeing whether the Condens 4300iW heats the cylinder again after the reset.
- If the code persists:
- if 810 returns or the water is still cold after that period, call the service company with the code.
- If it has cleared:
- if hot water returns without 810, mention the episode at the annual service.
- Evidence to keep:
- time of the reset and hot water condition a day after 810.
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)
Technical check of the hot water circuit
- Who:
- authorised heating and gas technician.
- How:
- checks the DHW valves, 3-way valve operation and the cylinder connection, following the table for 810 in the installation manual.
- What matters: identifying what stops the Condens 4300iW from heating the cylinder.
- If the code persists:
- the technician should replace or correct the affected item and verify a full cylinder heat-up.
- If it has cleared:
- ask for the check to be recorded even if 810 does not recur during the visit.
- Evidence to keep:
- job sheet with the valve or connection checked for 810.
Removing the 3-way valve or cylinder connections on the Condens 4300iW is technician work.
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 810
- Bosch Condens 810
- Condens 4300iW 810
- Junkers 810
What it means
In the Condens 4300iW fault table, 810 means the hot water stays cold. Bosch proposes resetting the appliance and waiting 24 hours, checking the DHW valves, checking the 3-way valve and replacing it if necessary, and confirming the cylinder is correctly connected. Unlike 811, it does not concern thermal disinfection: it covers stored water that does not heat up at all.
Warnings and stop conditions
- Criticalsource verified
If you smell gas while code 810 is showing, do not touch switches or plugs, avoid flames and sparks, close the gas valve, open windows, leave the home and call the gas supplier and service company from outside.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
- Highsource verified
Do not dismantle valves or cylinder connections to clear 810; cold hot water points to a circuit that a technician must inspect.
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)
Symptoms and causes
Probable causes · in order
- 3-way valve not switchingPossible
Bosch lists checking the 3-way valve and replacing it if necessary, because it diverts boiler heat to the hot water cylinder.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Domestic hot water valvesPossible
The table for 810 calls for checking the DHW valves in the installation, whose position or condition can stop the water from heating.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Cylinder not connected correctlyPossible
The manufacturer includes confirming that the hot water cylinder is correctly connected to the boiler as part of the remedy for 810.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
Virtual technician
Does 810 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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