- Boilers
- Gas boiler
2964
Junkers Bosch 2964 on a gas boiler: insufficient water flow
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Junkers Bosch 2964 means too little water is flowing through the heat exchanger. Press OK to see the pressure: cold, it should be between 1 and 2 bar. Check that the radiator valves are open and call the technician if the code comes back.
Safety first
Stop and check this first
- Do not refill a hot boiler: Bosch warns that cold water in a hot boiler can cause thermal stress cracks. Fill only when the system is cold, with water at 40 °C at most, and never above 3 bar at maximum temperature.
- If you smell gas: Bosch says do not smoke or use lighters or matches, do not operate switches or pull plugs, do not use the phone or doorbell, close the gas supply at the main valve or meter, open doors and windows, warn the neighbours, leave the building and keep others out. From outside, call the fire brigade, the police and the gas company.
Safe checks you can perform
- Gas or flue gas smell near the boiler
- Operating pressure on the display
- Cold refill with the blue lever
- Radiator valves and bleeding
- Flow, pump and sensor by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Purge with the venting function
- Do not attempt this professional task: Pump check
- Do not attempt this professional task: Heat exchanger sensor
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.
Purge with the venting function
What they doThe technician turns on service function 4-A1 and purges air from the appliance and the system.
They must show youthe venting function running.
In writingthat it was purged and the final cold pressure.
Pump check
What they doChecks that the pump runs and moves enough water, and reads the pump modulation in menu 1-C2.
They must show youthe pump running and its modulation.
In writingthe pump's condition and whether it was replaced.
Heat exchanger sensor
What they doMeasures the resistance of the heat exchanger sensor (primary thermistor) and checks its plug and the harness continuity.
They must show youthe resistance measured against the manufacturer's table.
In writingthe value and what was replaced.
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.
Gas or flue gas smell near the boiler
- Who:
- you.
- How:
- walk up without pressing switches or turning lights on and notice whether there is a smell of gas or combustion fumes.
- Result that matters:
- whether there is a smell or not.
- If there is a smell:
- do not touch the boiler and follow the gas warning on this page.
- If there is no smell:
- go on to read the pressure.
- Evidence to keep:
- the time and what you noticed.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Operating pressure on the display
- Who:
- you.
- How:
- with the system cold, press the OK key; the user manual says the display then shows the operating pressure.
- Result that matters:
- whether it is below 1 bar, between 1 and 2 bar or above.
- If it is below 1 bar:
- lack of water becomes more likely; go to the refill.
- If it is between 1 and 2 bar:
- water is not missing; air, valves or the pump become more likely.
- Evidence to keep:
- a photo of the display with the pressure and with 2964.
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)
Cold refill with the blue lever
- Who:
- you, or the technician; Bosch recommends leaving filling to a qualified technician.
- How:
- with the system cold, push the blue filling lever down and release it when the pressure reaches the target, between 1 and 2 bar.
- Result that matters:
- whether the pressure holds over the next days.
- If it drops again:
- Bosch says to check the expansion vessel and the system for leaks; call the technician.
- If you overshoot:
- stop; when hot it must never exceed 3 bar.
- Evidence to keep:
- the pressure before and after, and the date.
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)
Radiator valves and bleeding
- Who:
- you.
- How:
- check that the radiator valves and thermostatic valves are open; if a radiator does not heat evenly, bleed it, as the user manual says.
- Result that matters:
- whether any valves were closed or air came out.
- If air came out:
- check the pressure again, because bleeding lowers it.
- If everything was open and there was no air:
- the pump or sensor are for the technician.
- Evidence to keep:
- which valves you opened and which radiators you bled.
Evidence: Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700) · Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Flow, pump and sensor by the technician
- Who:
- authorised technician.
- How:
- checks the flow through the appliance, purges with venting function 4-A1, checks the pump and measures the heat exchanger sensor resistance.
- Result that matters:
- whether the flow is really low or the sensor reads wrongly.
- If the flow is low:
- air, valves or pump.
- If the flow is correct:
- sensor, plug or wiring harness.
- Evidence to keep:
- a report with the tests done and the final cold pressure.
Bosch says never remove the boiler casing and leave work to an authorised company. Inside there are gas parts and mains voltage.
Evidence: Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700) · 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 2964
- Bosch Condens 2964
- Condens 2300i W 2964
- Condens 4300iW 2964
- Error 2964 Junkers
- Junkers 2964
What it means
On the Bosch Condens 4300iW and 2300i W, 2964 is a blocking fault (category B): insufficient flow in the heat block, that is, the heat exchanger. The boiler stops and restarts by itself once the cause clears. Bosch says to check the flow and that the system is full, air locks, the position of the heating circuit valve, the pump, and the heat exchanger sensor with its resistance, plug and wiring harness. The same code can come from the water side or from a false sensor reading.
Warnings and stop conditions
- Highsource verified
Do not refill a hot boiler: Bosch warns that cold water in a hot boiler can cause thermal stress cracks. Fill only when the system is cold, with water at 40 °C at most, and never above 3 bar at maximum temperature.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
- Criticalsource verified
If you smell gas: Bosch says do not smoke or use lighters or matches, do not operate switches or pull plugs, do not use the phone or doorbell, close the gas supply at the main valve or meter, open doors and windows, warn the neighbours, leave the building and keep others out. From outside, call the fire brigade, the police and the gas company.
Evidence: Bosch Condens 4300iW, instrucciones de uso de la caldera de condensación a gas (6720891218)
Symptoms and causes
Probable causes · in order
- Too little water in the systemCommon
Bosch says to check that the system is full and the water pressure. With little water, less flows through the heat exchanger and the boiler protects itself.
Evidence: Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700) · Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Heating circuit valves closedPossible
For the 2300i W, Bosch says to check the position of the heating circuit valve and open it if needed. This happens after building work, bleeding or a radiator change.
Evidence: Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700)
- Air in the circuitPossible
Bosch names air locks in the system that need purging. Air slows circulation even when the pressure shown by the boiler is correct.
Evidence: Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
- Pump or heat exchanger sensorLess common
Bosch also says to check the pump and the heat exchanger sensor (primary thermistor), its resistance, plug and harness. A false reading gives the same code.
Evidence: Bosch Condens 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700) · Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
Virtual technician
Does 2964 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 2300i W GC2300iW 24/30 C, instrucciones de instalación y de mantenimiento (6721836700)
Bosch Termotecnia (Robert Bosch España) · 2025 · manual
- 2
Bosch Condens 4300iW GC4300iW 24 P, instrucciones de mantenimiento y de instalación (6721835266)
Bosch Termotecnia (Robert Bosch España) · 2021 · manual
- 3
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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