Boilers and water heaters, Junkers
Display messages on Junkers Cerapur boilers: Lo, In, siphon, purge and others that are not malfunctions
boiler Junkers" it's because your Cerapur boiler is displaying something on the screen that doesn't look like a fault code: a "Lo," an "In," an unfamiliar symbol, or it runs at low power for a few minutes after being switched on. Before calling a technician, it's worth knowing that condensing boilers Junkers often use the display for many things that aren't faults: maintenance alerts, automatic safety functions, or special modes.
boiler Junkers and other display indications that are most frequently seen in the Cerapur Excellence (ZWBE/ZSBE…-2A) and Cerapur Acu (ZWSB 28-3A), what each one usually means and when it is advisable to worry.
Quick summary
- Lo: This usually indicates that the key lock is activated. It's not usually a malfunction; it can normally be deactivated from the panel itself.
- In: This is usually a notification for an inspection or maintenance. It's typically due for a periodic service.
- Siphon program: When switched on or after a repair, the boiler usually operates at low power for a few minutes to fill the condensate trap. This is normally not a problem.
- Purge: an automatic function that is usually activated to remove air after filling the system.
- Monitoring gradients: The boiler most likely stopped for about 2 minutes because the water was heating up too quickly. If this happens frequently, there may be a circulation problem.
- Floor drying and thermal disinfection: special functions that are usually activated intentionally.
Junkersboiler: key lock
In many cases, it's a safety lock, designed to prevent children (or anyone passing by) from changing the settings. While it's active, the boiler usually functions normally, but it won't respond to the keys. It's activated and deactivated from the control panel itself; the exact combination depends on the model and you'll find it in your boiler's user manual. If it appears without anyone having activated it, it could be because someone accidentally pressed the combination while cleaning the boiler.
In: inspection notice
Typically, the boiler tracks operating time and alerts you when maintenance is due. This is usually not a malfunction, and the boiler will continue to function, but it's a good time to schedule a service. For a condensing boiler, this usually includes cleaning the condensate drain, checking the electrodes, and verifying combustion. The technician will typically reset the alert upon completion.
Siphon filling program
Condensing boilers produce water (condensate) that drains through a trap. This trap must be full of water to act as a seal: if it were empty, fumes could escape through the drain into the house. Therefore, when you turn on the boiler for the first time, after a long power outage, or after a repair, it will usually run for a few minutes at minimum power until the trap fills. During this time, you may see the corresponding warning light and notice that it takes a while to heat up. In our experience, this is normal behavior.
What is not normal is if water drips from under the boiler or if there is a smell of smoke near the drain: in that case, check the siphon and its seals, or call an authorized technician.
Purge function
After filling or topping up the system, the boiler may activate an automatic function that runs the pump intermittently to draw air towards the air vent. This usually lasts a few minutes, and it's normal to hear some running water. If noise persists or the pressure drops after this process, bleed the radiators as well.
Gradient monitoring: a 2-minute pause
This is the most important indicator to understand. The boiler monitors how quickly the water temperature rises. If it rises too quickly, the water is probably not circulating properly (or is circulating very little), so the boiler stops for about 2 minutes before trying again.
Once it happens in isolation, it's usually nothing to worry about. If it occurs frequently, it could be an early warning sign of a circulation problem: low pressure, air in the system, a loose pump, clogged radiators, or limescale buildup. It's a similar group of causes to those often behind the AA error or, in more serious cases, the E9. It's best to address it now.
Floor drying function
In underfloor heating systems, after laying new flooring, the installer may activate a program that gradually heats the floor over several days to dry it and prevent cracking. While this program is active, the boiler typically follows its own temperature schedule, not the one you select. If you see this function and you're not currently undertaking any work, consult your installer.
Thermal disinfection (Cerapur Acu)
The Cerapur Acu has a hot water storage tank. To prevent Legionella, it may periodically heat the tank to a high temperature. While this is active, the water may be much hotter than usual: be careful with children and the elderly in the shower.
Other indications
- Power output: The boiler may display the operating power or the maximum set power. This is usually information, not a fault.
- Pressed keys: This usually indicates that a key has been held down for too long. If it appears without any input, a key may be stuck or the panel may be damp.
- Value logging: a mode for querying stored data, which is usually used by the technician.
When is it a breakdown?
If a combination of letters and numbers appears on the screen (for example, EA, E9, F0, A7, or C6), it's most likely a fault code. You can find them all, with a brief explanation for each model, in our error code index Junkers. And if the warning indicates a reset or a key being pressed for too long (Fd or FC), we explain it in a specific guide that we'll publish soon.
Related spare parts
For all other parts for condensing boilers, consult spare parts Junkers, including electrodes from the Cerapur family. Always check the reference number of your old part.
Frequently Asked Questions
Is the Junkers boiler broken?
Usually not. When "Lo" appears, it's most likely a key lock and the boiler is still functioning normally. If it also fails to heat or displays a letter and number code, it should be checked by an authorized technician.
Can I continue using the boiler with the "In" indicator?
In many cases, yes, because it's usually just a reminder. But a condensing boiler without maintenance will eventually develop problems with the siphon, electrodes, and combustion.
Why does my boiler take a long time to heat up right after a power outage?
It's probably running the siphon filling program: it usually operates at low power for a few minutes as a safety precaution. After that, it should function normally.
I often see the pause due to gradients. Is it serious?
It's not usually a breakdown, but it can indicate that the water isn't circulating properly. Check the pressure (it's normally between 1 and 1.5 bar when cold), bleed the radiators, and if the problem persists, have the water pump checked.
Important notice
This article is a guide based on our experience as a spare parts store. It is not a substitute for professional diagnosis. Every appliance and every installation is different, and the same error code can have different causes.
Any work on the gas circuit, combustion, or flue system must be carried out by a qualified installer, in accordance with the Technical Regulation for the Distribution and Use of Gaseous Fuels (RD 919/2006) and the Regulation on Thermal Installations in Buildings (RITE). Manipulating these elements without the necessary qualifications can be dangerous and will void the appliance's warranty.
The decision to purchase a replacement part based on this information is the sole responsibility of the customer. ElectroCholo is not liable for incorrect diagnoses or damage resulting from improper installation. Electrical and electronic parts that have been assembled or installed are non-returnable if the fault turns out to be something else, in accordance with our general terms and conditions.
