- Boilers
- Tankless gas water heater
CF
Junkers CF gas water heater: flue blocked or too little air
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On Junkers Hydronext water heaters, CF means the flue outlet is blocked or there is not enough air for start-up. Close the hot tap, open it again once and, if CF stays, leave the flue for a gas technician to check.
Safety first
Stop and check this first
- If you smell gas, do not switch lights on or off, use sockets, the doorbell or an indoor phone, and do not strike a flame. Close the main gas valve if it is within reach, open doors and windows, warn neighbours as you leave and call the fire service and gas company from outside.
- With CF, flue gases may not be leaving the building, and carbon monoxide (CO) cannot be seen or smelled. If anyone gets a headache, dizziness or nausea, or a CO alarm sounds, close the gas valve, ventilate, leave the home and call the service technician.
Safe checks you can perform
- Record the code and exact model
- Close and reopen a hot tap once
- Make sure ventilation openings are not covered
- Professional flue and combustion air inspection
Leave these tasks to a qualified professional
- Do not attempt this professional task: Flue setting and gas pressure check
- Do not attempt this professional task: Full flue system inspection
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.
Flue setting and gas pressure check
What they doA qualified gas technician checks parameter F9 against the flue length and, on butane heaters, measures that the cylinders deliver 25 mbar at maximum output.
They must show youthe F9 value and the pressure gauge reading.
In writingthe values found and the values left set.
Full flue system inspection
What they doA qualified gas technician confirms that the flue outlet and air intake are free of dirt or objects and that the joints do not leak.
They must show youthe section or terminal where the blockage was.
In writingwhat was removed and the result of the tightness 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.
Record the code and exact model
- Who:
- you, at the control panel.
- How:
- photograph CF on the display and use the rating plate or invoice to find whether the heater is a Hydronext 5700S, 5600S or 2400S and whether it burns natural gas, propane or butane.
- Result that matters:
- knowing which manual row applies and which gas the heater uses.
- If confirmed:
- pass those details on when you book the visit.
- If not confirmed:
- ask for the model to be identified before any setting is changed.
- Evidence to keep:
- photo of CF, model and gas type.
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)
Close and reopen a hot tap once
- Who:
- you, using only your household taps.
- How:
- close the hot water outlet, wait a few seconds and open it again; on the 2400S the manual says to switch the heater back on after 10 minutes. Do not keep repeating it.
- Result that matters:
- whether the heater lights without showing CF again.
- If confirmed:
- note it as a one-off event and watch for it on the next uses.
- If not confirmed:
- leave the heater off and call a technician instead of retrying ignition.
- Evidence to keep:
- time of the fault and the outcome of the single attempt.
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)
Make sure ventilation openings are not covered
- Who:
- you, without dismantling anything.
- How:
- look at the vents in doors, windows or walls of the room and make sure they are not closed off or hidden behind furniture or boxes; the manual forbids reducing them. Do not touch the flue pipe or the outdoor terminal.
- Result that matters:
- whether household items were blocking an air opening.
- If confirmed:
- move the item away and tell the technician.
- If not confirmed:
- the flue and terminal still need a professional inspection.
- Evidence to keep:
- photo of the vents as you found them.
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)
Professional flue and combustion air inspection
- Who:
- a qualified gas technician handling the CF flue fault.
- How:
- inspects the flue outlet and air inlet along their whole length, their sealing and the flue setting F9, and on butane heaters measures cylinder pressure at maximum output.
- Result that matters:
- the specific cause of the blockage and whether the flue is sealed.
- If confirmed:
- clears or corrects the flue and tests operation with the front panel fitted.
- If not confirmed:
- works through the other CF conditions before replacing parts.
- Evidence to keep:
- report on the flue condition, any butane pressure reading and the final test.
Flue work and butane pressure measurement are gas work: the manual forbids the user from removing the front panel and reserves repair for trained specialists.
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 CF
- calentador Junkers CF
- Hydronext 2400S CF
- Hydronext 5600S CF
- Hydronext 5700S CF
- Junkers CF
What it means
The Hydronext 5700S and 5600S fault tables group three situations under CF: a blocked flue gas outlet, insufficient air flow at start-up and, on butane models, cylinders that do not supply enough pressure. On the Hydronext 2400S, CF reports a blocked flue outlet while the heater is running. The appliance goes into a safety lockout, and the code alone does not tell you which of those conditions is present.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas, do not switch lights on or off, use sockets, the doorbell or an indoor phone, and do not strike a flame. Close the main gas valve if it is within reach, open doors and windows, warn neighbours as you leave and call the fire service and gas company 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)
- Higheditorial warning
With CF, flue gases may not be leaving the building, and carbon monoxide (CO) cannot be seen or smelled. If anyone gets a headache, dizziness or nausea, or a CO alarm sounds, close the gas valve, ventilate, leave the home and call the service technician.
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
- Blocked flue or air intake ductCommon
Bosch tells the technician to remove dirt or any foreign object from the flue outlet or air inlet duct. A nest, leaves or a covered terminal stops combustion gases from leaving the heater normally.
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)
- Not enough air at start-upPossible
On the Hydronext 5700S and 5600S, CF also covers insufficient air flow when the burner starts. The manual suggests closing and reopening a hot tap before calling a technician.
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)
- Butane cylinder pressure too lowLess common
For butane heaters the same row lists cylinders that do not deliver enough pressure. Elsewhere in the table Bosch asks for 25 mbar at maximum output to be verified, which is a professional measurement.
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)
Virtual technician
Does CF 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
Junkers Hydronext 2400S WD8/11..., manual de instalación y uso del calentador de gas (6720882438)
Junkers (Bosch Termotecnia) · 2020 · manual
- 2
Junkers (Bosch Termotecnia) · 2022 · manual
- 3
Junkers (Bosch Termotecnia) · 2021 · manual
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