Intermittent Lockout on a New Fan-Assisted Boiler
A gasfitter commissions a new fan-assisted condensing boiler serving hydronic heating in a two-storey house. On first start the boiler fires and runs for about four minutes, then locks out. Restarting produces the same pattern. The gasfitter measures 1.95 kPa at the boiler inlet with the boiler running and 2.00 kPa standing. The manufacturer requires 1.75–2.00 kPa. The condensate drain runs 6 m horizontally to an internal tundish with no fall. The flue is 8 m of concentric PVC terminating through a wall directly beneath a first-floor window that the occupants open in summer. The boiler is in a cupboard with a louvred door. The fault code indicates a blocked flue or condensate fault.
1. Identify the most likely causes of the lockout and rank them.
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The fault code points to two systems, and both have defects here.
1. Condensate disposal - most likely. The drain runs 6 m horizontally with no fall. Condensate cannot drain by gravity, so it backs up in the boiler's condensate trap. Once the trap fills, the pressure switch or float detects the blockage and the boiler locks out. The four-minute run time matches the time needed to accumulate enough condensate to trip the trap. A condensing boiler produces roughly 1 L of condensate per hour per 30 kW at high fire, so a short run can still generate enough to back up a flat drain.
2. Flue termination - likely contributing. The flue terminates beneath an openable first-floor window. Even if the boiler's own flue-pressure switch is satisfied, the termination is non-compliant and allows recirculation of combustion products, which can cause the fan to see abnormal pressure and trip.
3. Gas pressure - not the cause. 1.95 kPa running is within the 1.75–2.00 kPa band. The small drop from standing is normal.
4. Cupboard ventilation - possible but secondary. A louvred door usually provides adequate air for a room-sealed appliance, but confirm the free area against the manufacturer's requirement.
2. Outline the corrective work and the recommissioning checks.
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Corrective work:
- Re-route the condensate drain with a continuous fall of at least 2.5° toward the discharge point, or fit a condensate pump if gravity fall is impossible. Ensure the trap is primed and the discharge terminates at an approved point (tundish or external).
- Relocate the flue terminal so it meets the manufacturer's clearance from openable windows and the standard's termination requirements. An 8 m run may also need the flue size and equivalent length checked against the manufacturer's tables, since each bend adds resistance.
- Verify cupboard ventilation free area against the instructions.
Recommissioning checks:
- Confirm gas pressure at the inlet under full and minimum fire.
- Run the boiler through a full cycle and confirm no lockout over an extended run.
- Check combustion readings (CO₂, CO, O₂) against the manufacturer's commissioning figures.
- Confirm the condensate trap holds water and drains freely.
- Confirm the flue is sound, correctly sloped back to the boiler, and terminated compliantly.
- Record all readings and the rectification on the certificate.