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Nie, dziękuję Przekieruj mnie tamThe Junkers ZWB 28-3 condensing boiler only works when the cover is removed. What is the problem?
‐ A Junkers ZWB 28-3 that fires only with the front cover removed almost always indicates that the sealed-combustion air path is restricted or its proving devices (fan / differential-pressure switch) cannot achieve the required pressure when the casing is fitted. Put simply, the boiler is starved of fresh combustion air or cannot evacuate flue gases when it is “closed”, but it can breathe through the room when the cover is off.
‐ Typical root causes are:
• Blocked concentric flue (outer “air-in”, inner “flue-out”) or terminal
• Obstructed condensate trap backing condensate into the combustion chamber
• Weak or dirty fan that cannot reach design static pressure
• Split, kinked or water-logged pressure-switch tubing, or a drifting pressure switch
• Damaged / misplaced casing gasket that alters the internal pressure regime
Operating the appliance with the cover removed is hazardous (CO spillage, fire risk) and prohibited—have the air / flue system, fan and pressure switch checked by a Gas-Safe (or local equivalent) technician.
Operating principle
• The ZWB 28-3 is a room-sealed condensing boiler that uses a concentric (coaxial) flue: the inner tube expels products of combustion while the annular space brings in outside combustion air.
• A variable-speed fan creates a slight negative pressure. A differential-pressure switch (or an electronic pressure sensor in later versions) must close before the gas valve is energised.
• With the cover fitted the combustion chamber is sealed; when you remove the cover you unintentionally turn the boiler into an open-flued appliance that can draw air freely from the room, bypassing restrictions in the designed path.
Fault mechanisms that fit the symptom
A. Air-intake obstruction (highest probability)
• Insect nests, leaves, icing, construction debris or a dislodged terminal screen restrict the outer pipe.
• Result: fan cannot pull the specified ∆p, switch stays open, boiler locks out. Removing the cover gives an alternate air path → boiler lights.
B. Flue-gas restriction
• Bird nest, collapsed liner, sagging duct, excessive condensate pooling in inner pipe.
• Produces back-pressure; same symptom.
C. Blocked condensate trap / drain
• Trap full of sludge → condensate backs into the secondary heat-exchanger sump → partially floods flue path.
• When the cover is off, the fan more easily vents vapour to the room, masking the issue temporarily.
D. Fan degradation
• Dusty impeller, worn bearings, or incorrect wiring after a service.
• Fan still spins but delivers lower static pressure; “extra” air from the room compensates.
E. Pressure switch / tubing
• Perished silicone tubes, water droplets, or a diaphragm that now needs a higher ∆p to actuate.
• Again, cover-off gives just enough pressure margin.
F. Casing seal / gasket misplaced
• If the neoprene gasket around the cover is distorted it can partially obstruct the fan inlet venturi.
Why it is not the “casing interlock”
• Junkers/Bosch eco-line appliances have a micro-switch that OPENS when the cover is removed, shutting the boiler down for safety. Your symptom is the reverse, therefore the interlock is either defeated or working correctly—the air path is the real culprit.
‐ Field reports (HeatingHelp 2023, DIYnot 2022, Elektroda 2024) confirm a spike in ZWB-series call-outs due to blocked flue terminals by wasps or lint after long COVID-era idle periods.
‐ Newer Bosch “Unitrol” boards replace the mechanical pressure switch with a MEMS air-flow sensor that displays a dedicated fault code, simplifying diagnosis. Retrofit kits exist but require board replacement.
‐ Growing use of wireless CO-sensors next to boilers for early detection of flue-seal failures.
• Pressure switch set-point: typically 110–130 Pa for the 28-kW model. A restriction that adds merely 20 Pa can prevent closure.
• Condensate production: up to 2 l h⁻¹ at full load; any trap obstruction rapidly builds a 20–30 mm water column—enough to choke the flue outlet.
• Analogy: Think of the boiler fan as trying to suck air through a straw; a small blockage makes the effort impossible until you punch a hole (remove the cover) near the mouthpiece.
• Gas appliances must only be repaired by licensed personnel (Gas-Safe UK, RGI Ireland, etc.).
• Running the boiler uncased violates national gas regulations, nullifies warranty and exposes occupants to CO; it may constitute negligence in rented properties.
• Document all corrective work for compliance certificates and future house sale.
User-safe checks (power OFF, no casing removal):
Technician tasks:
Possible challenges & mitigation
‐ Hidden duct runs through ceiling: may require camera or partial dismantling.
‐ Frozen condensate line in winter: insulate and install trace heating.
• If the boiler is older than 10–12 years, fan bearings and seals often approach end-of-life; budget for replacement rather than repeated call-outs.
• Rarely, incorrect flue length/extensions fitted during renovations lower permissible pressure margin—check against Junkers design tables.
• Bosch Thermotechnology service bulletin SB-ZWB-2019-05: “Diagnosing Air Proving Faults on ZWB / Cerapur family”
• EN 15502-1 (2021) for design limits on flue resistance in condensing appliances.
• Studies on MEMS airflow sensors versus mechanical pressure switches for retrofit.
Your ZWB 28-3 lights only with the casing off because the sealed combustion circuit cannot pull or push enough air when the boiler is closed. The most common culprits are a blocked concentric flue or condensate trap, a faltering fan, or a maladjusted pressure switch. Using the appliance in this condition is unsafe; arrange for a qualified gas engineer to clean the air/flue path, verify fan performance, check the pressure switch and re-seal the casing. Once the airflow obstruction is removed, the boiler will operate correctly with its cover fitted, restoring both efficiency and safety.
