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How to Increase Pressure in BOSCH JUNKERS Natural Gas Boiler?

hkpconsultation 7041 11
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  • #1 21227135
    hkpconsultation
    Level 2  
    Posts: 3
    Close-up of the control panel of a Bosch Junkers gas boiler with knobs and a digital display.
    Hello Dear all,

    I have a natural gas boiler called BOSCH JUNKERS as can be seen in the attached picture.

    My natural gas boiler is too old. The pressure decreases day by day especially when I activate the heating system.

    How can I increase the pressure inside the system?

    Please help me because winter is coming :) :).

    Hallo liebe Leute,

    ich habe eine Erdgastherme namens BOSCH JUNKERS wie auf dem beigefügten Bild zu sehen ist.

    Mein Erdgaskessel ist zu alt. Der Druck sinkt von Tag zu Tag, besonders wenn ich die Heizung einschalte.

    Wie kann ich den Druck im System erhöhen?

    Bitte helfen Sie mir, denn der Winter steht vor der Tür :) :).
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  • #3 21227199
    hkpconsultation
    Level 2  
    Posts: 3
    >>21227196 Hello Dear Jony, Thank you so much but I saw that it is not the same combi as mine. Unfortunately, I do not have this blue tap.
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  • #4 21227258
    jony202b
    Level 6  
    Posts: 16
    Rate: 1
    Yeah. I realized that it is not the same model. I just wanted it to serve as a reference for you to look for something similar. Let's see this other video:

    https://www.youtube.com/watch?v=KgQXy_6WpWU

    Here it is black.

    It is also possible that the knob has been lost and that is why you do not see it. You will have to try.

    Or this:

    https://repuestoscalefaccion.com/es/cerastar-...2236-conjunto-llenado-junkers-zwn24-7mfa.html
    https://www.youtube.com/watch?v=rC8ohFrhDbg

    Added after 2 [hours] 49 [minutes]:

    https://junkers-es-es-c.boschtt-documents.com/download/file/file/6720604285.pdf

    page 4:
    "1.5 Esquema de construcción"
    mark "38"
    page 9:
    "fig. 7 Placa de conexión"
    mark "38"

    However in:

    https://www.intec-heizung.de/media/pdf/33/75/...R_ZSR-18-5-24-5-KE-Installationsanleitung.pdf

    page 13:
    "Montage-Anschlußplatte, fertig montiert"
    mark "38"

    I'm sorry. I can't reach more.
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  • #5 21227758
    serwisszulborski.pl
    Heating systems specialist  
    Posts: 1844
    Help: 360
    Rate: 1442
    This model of boiler has no internal water supply. You will need to look externally (on the installation) for a valve or two valves which serve to supplement the required pressure of approx. Often installers also use a connecting hose. In Poland, according to building law, this should be a disconnected connection (but no one obeys this these days). Look under the boiler or close to it, the water admittance person should watch the pressure gauge while filling....You can also connect to a drain valve plugged with a nut, but you need a hose to connect to the water.
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  • #6 21228188
    jony202b
    Level 6  
    Posts: 16
    Rate: 1
    Good. Now, I understand the diagram on page 13
  • #7 21229570
    hkpconsultation
    Level 2  
    Posts: 3
    >>21227758 Hello Dear,
    Thank you so much.
    I have a hose to supply water to the boiler to increase pressure but I do not understand where I should connect my hose in the natural gas boiler that is why I published these pictures. Look at my picture please and if you see the location where I can connect my hose. Please let me know
  • #8 21230338
    serwisszulborski.pl
    Heating systems specialist  
    Posts: 1844
    Help: 360
    Rate: 1442
    Under the boiler, on the right hand side you have a valve closed with a 3/4" chrmo-nickel plug .This is probably used to fill water into the system. After unscrewing the cap, turn it slightly to the left and see if water flows. Of course, it's better to prepare a rectangular bowl and used towels for the laundry early on so as not to flood everything. Of course, switch the Junkers off during the entire operation. Bleed the filling hose and fill the water to 1.5bar on the pressure gauge.
  • #9 21256647
    drackos2
    Level 2  
    Posts: 3
    I have the same Junkers ZSR 18-5 KE model and the same question about how to add water to increase the pressure. However, my setup includes a water reservoir.

    Nameplate of Austria Email AG water heater with technical specifications. Control panel of Junkers ZSR 18-5 KE boiler with a knob and informational label. Section of Junkers ZSR 18-5 KE boiler with visible pipes and installation. Vortex BlueOne water pump connected to a plumbing system under a boiler. View of the underside of a Junkers ZSR 18-5 KE water heater with visible water pipes. Close-up of plumbing installation under Junkers ZSR 18-5 KE boiler. Close-up of pipes and cables in a heating system with Junkers boiler. Junkers ZSR 18-5 KE water heater with plumbing installation and Vortex Blue One service cap.
  • #10 21257327
    serwisszulborski.pl
    Heating systems specialist  
    Posts: 1844
    Help: 360
    Rate: 1442
    In these photos, there is no possibility of a water inlet. The water inlet must be somewhere nearby, e.g. on the floor or in a completely different room, possibly a hose connection.... Generally, this connection is a tragedy, I think that even in Poland it is rare for an installer to make such a flock (loading the tank on 1/2" hoses, no valves, security and order).
  • #11 21258194
    drackos2
    Level 2  
    Posts: 3
    I looked everywhere in my flat for possible valves that could help increase the pressure. Here’s what I found in the radiators.

    Valve on a radiator in an apartment. Close-up of a radiator valve, showing its dirty surface.
  • #12 21258991
    drackos2
    Level 2  
    Posts: 3
    >>21258194 I used the second valve and water inlet to increase the pressure. Thanks all. Now I need to bleed out the radiators.
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Topic summary

✨ The discussion revolves around increasing the pressure in a BOSCH JUNKERS natural gas boiler, particularly as the user experiences a daily decrease in pressure, especially when the heating system is activated. Various users provide guidance on locating the water supply valve necessary for refilling the system. It is noted that the specific model may lack an internal water supply, requiring an external connection for pressure adjustment. Users suggest checking under the boiler for a valve, using a hose to connect to a water source, and ensuring the pressure gauge reads approximately 1.5 bar after filling. Some users share their experiences with similar models and the challenges of finding the correct connection points for water supply.
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FAQ

TL;DR: Owners of older Bosch Junkers ZSR/ZWR boilers with falling pressure: top up to 1.5 bar cold via an external fill point; "fill the water to 1.5 bar" with the boiler off. This FAQ shows where to connect and how to do it safely. [Elektroda, serwisszulborski.pl, post #21230338]

Why it matters: Correctly topping up and bleeding prevents PRV discharges, leaks, and heating lockouts during cold months.

Quick Facts

How do I increase pressure on a Bosch Junkers ZSR/ZWR without a blue tap?

Switch the boiler off. Look under the right side for a 3/4" chrome‑nickel plug hiding the fill valve. Place a tray and towels, unscrew the cap, connect a bled hose, then crack the valve slightly. Watch the gauge and stop around 1.5 bar. Close, disconnect, and check for leaks. [Elektroda, serwisszulborski.pl, post #21230338]

Where is the filling loop on these older Junkers boilers?

These models have no internal water supply. Find the external fill point on the pipework: one or two valves and a connecting hose linking mains water to the heating return. It may sit under the boiler or in a nearby room. Use the gauge while filling. [Elektroda, serwisszulborski.pl, post #21227758]

What pressure should the gauge show when cold and when heating?

Typical sealed systems run about 1.0–1.5 bar when cold. As water heats, pressure commonly rises to roughly 1.5–2.5 bar. If hot pressure pushes near 2.5 bar or more, check expansion capacity and avoid overfilling. [“What should my boiler pressure be?”]

How do I top up pressure step-by-step?

  1. Turn the boiler off. Attach and bleed the hose to the fill valve or a drain point.
  2. Open slightly and watch the gauge. Fill to about 1.5 bar cold, then close.
  3. Remove the hose, bleed radiators, and recheck to 1.2–1.5 bar cold. “Switch the Junkers off during the entire operation.” [Elektroda, serwisszulborski.pl, post #21230338]

I have a water reservoir near my Junkers—where do I add water?

In the example photos, no inlet was visible on the reservoir assembly. The system’s fill point is likely elsewhere: floor level, a different room, or a dedicated hose link. The shown connection lacks proper valves, so locate the mains‑to‑heating link before topping up. [Elektroda, serwisszulborski.pl, post #21257327]

Can I fill via a drain valve if I can’t find the loop?

Yes. You can connect a hose from mains water to a drain valve that is capped with a nut. Bleed the hose, open gently, and monitor the gauge while filling. Close the valves and disconnect afterward to prevent backflow or flooding. [Elektroda, serwisszulborski.pl, post #21227758]

Should the boiler be on or off while filling?

Keep the boiler off during the entire operation. Attach and bleed the hose, open the valve slightly, and fill while watching the gauge. This avoids burner starts and sudden pressure spikes. [Elektroda, serwisszulborski.pl, post #21230338]

How do I bleed radiators after topping up?

Turn heating off and let radiators cool. Use a bleed key to open the valve until air stops and water appears, then close it. Work upstairs to downstairs. Recheck the boiler pressure and adjust to about 1.2–1.5 bar cold. [“How to bleed a radiator”]

Why does my pressure drop when the heating is on?

Likely causes include small leaks at radiators or joints, a passing pressure relief valve, or a failed expansion vessel precharge. Repeated top‑ups indicate a fault and can accelerate corrosion. Have a technician test the PRV and vessel. [“Why does my boiler keep losing pressure?”]

What if my pressure rises above 3 bar or the discharge pipe drips?

Around 3 bar, the safety valve opens and discharges water. Do not keep venting to reduce pressure. Remove any overfill and have the expansion vessel and PRV inspected for faults or debris. [“Boiler pressure too high”]

The filler tap/knob is missing—what can I do?

Sometimes the plastic knob is gone, but the valve spindle remains. Identify the correct fill valve, then carefully turn the spindle with an appropriate tool. Replace the knob or valve for safe future use. [Elektroda, jony202b, post #21227258]

Where can I buy a filling assembly for a Junkers Cerastar?

Some Cerastar models use a dedicated filling assembly kit. Match the part to your exact model and have a competent installer fit it. Check availability by model reference on reputable spares sites. [“Conjunto llenado Junkers ZWN24-7MFA”]

Is a permanent flexible hose allowed between mains and heating circuit?

Regulations vary. In Poland, building law expects a disconnectable connection to prevent backflow. Use a removable braided loop with isolation valves rather than a permanent link. [Elektroda, serwisszulborski.pl, post #21227758]

What tools and parts do I need to top up safely?

Use an adjustable spanner, a braided hose with intact washers, a tray, and towels. Open the fill valve slowly, watch the built‑in gauge, and close at about 1.5 bar. Bleed radiators and recheck pressure after filling. [Elektroda, serwisszulborski.pl, post #21230338]
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