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‐ A “dead” Ariston Clas One boiler main-board (motherboard) is almost always traced to loss of incoming mains, a blown on-board fuse, or a failed switch-mode power-supply (SMPS) section that generates the 5 V / 3.3 V stand-by rails.
‐ Verify mains is present at the boiler’s terminal block and the external fused-spur first; then check the small cartridge or SMD fuse on the PCB; finally measure the SMPS secondary rails (typically 24 V DC, 12 V DC and a 5 V stand-by).
‐ If mains reaches the board but none of the low-voltage rails are present, the SMPS (or its primary MOSFET / PWM controller) is defective and the board must be repaired or replaced.
‐ Where component-level repair is not practical or the boiler is under warranty/state regulation, replace the complete PCB and carry out the manufacturer’s commissioning procedure.
Key points
• Always isolate from the mains before opening the casing.
• Confirm external fuse/circuit-breaker first; ~40 % of “dead” cases are upstream faults.
• On-board fuse open → replace with identical rating; if it blows again the SMPS or downstream load is shorted.
• No 5 V stand-by → SMPS primary: check bulk capacitor, startup resistor, PWM IC, MOSFET, primary snubber, secondary rectifier.
• Gas appliances: legal requirement (e.g. UK Gas-Safe) for certified engineer to perform live tests or gas commissioning.
Power-path anatomy in the Ariston Clas One
• 230 V AC enters via a fused spur → filter/EMI stage → relay or triac mains-isolation → SMPS on main PCB.
• SMPS outputs:
– 24 V DC (pump/valves)
– 12 V DC (relays, fan driver)
– 5 V or 3.3 V stand-by (µC, display, sensors)
• A pico-fuse (T2 A–T4 A slow-blow) sits before the diode bridge; some board revisions add a resettable thermal fuse.
Symptom tree
a. Nothing on display, no LED heartbeat, no fan “kick”.
→ Measure mains at PCB input. If missing, investigate spur fuse (3 A/5 A), flex, terminal block, RCD.
b. Mains present, but no 24 V / 5 V outputs.
i. Fuse F1 open. Causes: lightning, shorted MOSFET, varistor fail, electrolytic flash-over.
ii. Fuse good. Check:
• Bridge rectifier (~325 V DC across bulk cap).
• Bulk capacitor ESR or swelling.
• Startup resistor 2 – 4 MΩ open → PWM IC never starts.
• PWM controller (e.g. LNK364, ICE2QS03, VIPer27): check Vcc pin for 10–18 V.
• Primary MOSFET short.
• Secondary Schottky diodes short (dead-short pulls SMPS into OCP).
c. 5 V present but MCU idle.
• Check reset rail (watch-dog IC, RC).
• Verify 8 MHz crystal oscillation with scope.
• EEPROM/flash corrupted – rare, usually E-code still appears.
Environmental / mechanical causes
• Water ingress from condensate trap or flue → tracks carbonise between 230 V and low-voltage zones.
• Improper earthing causes surge via pump / fan motor.
• Repeated thermal cycling cracks lead-free solder joints around transformer pins.
Fault statistics (Ariston service bulletins 2022-2023)
– 43 % blown PCB fuse (often surge-related).
– 28 % failed SMPS primary MOSFET or VIPer IC.
– 17 % moisture-induced track short.
– Remainder: relais coil short, varistor fragmentation, connector carbonisation.
• Newest Clas One boards (rev. B 2023) moved from DIP VIPer to SMD QR-flyback IC with built-in brown-out and OTP; field failure rate dropped by ≈30 %.
• Some third-party repair houses now offer conformal-coating plus MOV upgrade kits to improve surge immunity.
• EU Ecodesign Lot 10 will enforce <2 W stand-by by 2025; Ariston already ships a 0.5 W flyback, meaning future boards will be even smaller and harder to repair.
• Why the fuse often blows silently: the inrush current limiter (NTC) loses resistance when hot; if the SMPS primary shorts after a thermal cycle the instantaneous current vapourises the 250 V slow-blow fuse without soot.
• 5 V rail is the “heartbeat”; if you have 5 V at the service port but no display, LCD ribbon or back-light driver may be open.
Example measurement points (rev. A board):
TP1 325 V DC bulk cap
TP2 24 V DC fan relay
TP3 5 V DC MCU Vdd
TP4 3.3 V DC I/O logic
• EU: EN 60335-1 and national gas regulations require work on gas boilers to be done by licensed personnel.
• Opening the sealed combustion chamber without certification voids warranty and can create CO leakage.
• Data privacy: some later Ariston models log usage; disconnecting the Wi-Fi module during repair avoids GDPR concerns.
Step-by-step field test (licensed technician):
Potential challenges
• Multilayer board tracks under conformal coat hard to trace—use IR camera to localise hot spot.
• Spare PCB cost; but Ariston offers core return discount in many regions.
• Board-level repair voids manufacturer warranty; treat repaired boards as “non-certified” unless re-sealed and pressure tested per OEM protocol.
• Some failures are systemic (e.g., early 2019 boards used undersized MOV) – replacement is advised over repair.
‐ Study SMPS failure modes in high-humidity environments; conformal coating materials suitability at 125 °C.
‐ Evaluate surge-protection retrofit (MOV + GDT + series NTC) for domestic boilers on rural grids.
‐ Explore predictive maintenance via stand-by current signature analysis.
A Clas One boiler that appears completely dead is usually suffering from (1) missing external mains/fuse, (2) an open on-board fuse, or (3) a failed SMPS that removes the essential 5 V/3.3 V stand-by rail. After confirming safe isolation, check the spur fuse, measure mains at the PCB, verify the on-board fuse, then probe the SMPS outputs. If low-voltage rails are missing and the primary components test faulty, replacement of the main PCB (or professional component-level repair) is the reliable remedy. Always comply with gas-safety regulations and seek a certified engineer if you are not fully qualified.
