ErrorBase
Independent technical referenceOfficial site
  • Boilers
  • Tankless gas water heater

CA

Junkers CA gas water heater: water flow above the maximum

Verified with sources3 sourcesReviewed Oct 4, 2026
Technician coming? See the 2 steps they must show you
Two wall-mounted tankless gas water heaters with their pipework
BehratCC BY-SA 3.0Image source

Risk level

Medium

Proceed carefully and stop if the check is unclear.

Urgency

Medium

Quick answer

On Junkers Hydronext water heaters, CA means water flow is above the maximum specified for the appliance. Close the taps, note how many were open and ask a technician to check the water filter and the flow limiter.

Safety first

Stop and check this first

  • If at any point you smell gas, put out any flame, do not operate switches or sockets and do not use a phone or doorbell indoors. Close the main gas valve, open doors and windows, warn neighbours on the way out and call the fire service and gas supplier from outside.
  • Do not remove the filter or the limiter to get more flow: the manual forbids running the heater without its water filter and leaves its maintenance to technicians. Do not take off the front panel either.

Safe checks you can perform

  • Note which taps were open
  • Check the fault history
  • Reset once as the manual describes
  • Filter and flow limiter inspection

Leave these tasks to a qualified professional

  • Do not attempt this professional task: Flow limiter check
  • Do not attempt this professional task: Flow measurement after the work

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.

  1. Flow limiter check

    What they doA trained technician inspects the water filter and the flow limiter at the heater inlet and how they are fitted.

    They must show youthe removed limiter and filter and their condition.

    In writingwhether they were cleaned, refitted or replaced.

  2. Flow measurement after the work

    What they doA trained technician runs hot water and confirms flow stays within the model maximum without CA returning.

    They must show youthe flow achieved with taps open.

    In writingthe measured flow and the fault-free test.

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.

  1. Note which taps were open

    No special tools

    Who:
    you, around the home.
    How:
    when CA appears, close the hot taps and write down how many were open at once, which ones, and whether any plumbing work or change was done recently.
    Result that matters:
    linking the code to a specific water demand.
    If confirmed:
    share it with the technician along with the model.
    If not confirmed:
    tell them the code appears even with a single tap open.
    Evidence to keep:
    list of taps and time of the code.

    Evidence: Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438) · Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)

  2. Check the fault history

    No special tools

    Who:
    you, using the 5700S or 5600S user menu.
    How:
    open the FH menu and note the stored codes (1F to 10F) to see how often CA has repeated; on the 2400S just record each occurrence.
    Result that matters:
    knowing whether CA is isolated or frequent.
    If confirmed:
    give the list to the technician.
    If not confirmed:
    say it is the first time.
    Evidence to keep:
    photo of the FH history.

    Evidence: Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)

  3. Reset once as the manual describes

    No special tools

    Who:
    you, at the panel.
    How:
    on the 5700S and 5600S hold the reset symbol for 5 seconds; on the 2400S switch the appliance off and on again. Then open a single hot tap.
    Result that matters:
    whether the heater runs on one tap without showing CA.
    If confirmed:
    use fewer taps at once and note whether it returns.
    If not confirmed:
    stop resetting and call a technician.
    Evidence to keep:
    result of the reset and the single tap test.

    Evidence: Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438) · Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)

  4. Filter and flow limiter inspection

    Tools required

    Who:
    a trained technician dealing with the CA flow fault.
    How:
    closes the water valve, removes the filter cover at the inlet, checks the filter and flow limiter and refits them; the manual forbids running the heater without its filter.
    Result that matters:
    whether the limiter is missing, damaged or misplaced.
    If confirmed:
    fits or replaces it and checks flow with the heater running.
    If not confirmed:
    moves on to the flow sensor and the water installation.
    Evidence to keep:
    report on filter and limiter condition and measured flow.

    The filter and limiter belong to the maintenance the Hydronext manual leaves to technicians; the water must be shut off and the heater never run without its filter.

    Evidence: Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438) · Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)

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
Tankless gas water heater

Known code variants

  • Bosch CA
  • calentador Junkers CA
  • Hydronext 2400S CA
  • Hydronext 5600S CA
  • Hydronext 5700S CA
  • Junkers CA

What it means

The Hydronext 5700S, 5600S and 2400S fault tables assign CA to water flow above the specified maximum. The only remedy Bosch lists is to check the water filter and flow limiter, which sit at the water inlet of the appliance. Like the other display codes, CA puts the heater into a safety lockout that is reset once the problem is fixed. It does not indicate a gas leak or a combustion fault.

Warnings and stop conditions

  • Criticaleditorial warning

    If at any point you smell gas, put out any flame, do not operate switches or sockets and do not use a phone or doorbell indoors. Close the main gas valve, open doors and windows, warn neighbours on the way out and call the fire service and gas supplier from outside.

    Evidence: Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438) · Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)

  • Mediumeditorial warning

    Do not remove the filter or the limiter to get more flow: the manual forbids running the heater without its water filter and leaves its maintenance to technicians. Do not take off the front panel either.

    Evidence: Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438) · Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)

Symptoms and causes

Probable causes · in order

  • Water filter or flow limiter disturbedCommon

    Bosch asks for the water filter and flow limiter to be checked when CA appears. If the limiter is missing, damaged or badly seated, the heater can take more water than it is designed for.

    Evidence: Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438) · Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)

Virtual technician

Does CA 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.

Start diagnosis

Sources and technical references

  1. 1

    Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438)

    Junkers (Bosch Termotecnia) · 2020 · manual

  2. 2

    Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217)

    Junkers (Bosch Termotecnia) · 2022 · manual

  3. 3

    Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)

    Junkers (Bosch Termotecnia) · 2021 · manual

Sources: Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438) · Junkers Hydronext 5600S WTD12/15/17..., manual de instalación y uso del calentador de gas (6721841217) · Junkers Hydronext 5700S WTD12/15/17..., manual de instalación y uso del calentador de gas (6720884104)reviewed 2026-10-04 · verified

Was this entry useful?