Open-Flue Boiler Spilling in a Shared Masonry Chimney
A 45 kW atmospheric open-flue boiler in a basement plant room is connected to a masonry chimney shared with a 20 kW open-flue water heater on the floor above. On start-up, smoke from the boiler draught diverter enters the plant room. A CO alarm in the adjacent corridor reads 35 ppm. The chimney has no liner and debris is visible at the base. The plant room has two 300 mm × 300 mm ventilation grilles, one high and one low; both are partly blocked by stored paint. The boiler passes a visual check and the gas rate is correct. The caretaker says the problem started after the chimney was swept and a bird guard was fitted. There is no flue-flow test record.
1. What are your immediate actions and how would you classify the situation?
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Smoke from the draught diverter plus 35 ppm CO in the corridor indicates spillage of products of combustion: immediately dangerous. Turn off the boiler and the water heater, isolate the gas supply, ventilate, keep people away from ignition sources, and call the gas emergency service if CO is present. Attach warning labels and record the CO reading. Do not re-light the boiler to 'see if it settles'.
2. Diagnose the likely causes using the evidence in the scenario.
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The shared masonry chimney probably cannot carry both appliances, especially after the bird guard was fitted. The guard may reduce the flue area, and debris at the base indicates the chimney is blocked or deteriorating. The partly blocked ventilation grilles reduce combustion air and worsen spillage. A shared chimney also allows one appliance to interfere with the other. The absence of a flue-flow record means the chimney's adequacy has never been proven; a visual check and correct gas rate do not prove flue performance.
3. What remedial work and testing would you require before returning the appliances to service?
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Clear the chimney and remove any unsuitable guard; line the chimney or provide a separate flue for each appliance; restore the full ventilation free area; preferably replace with room-sealed appliances. Then perform flue-flow and smoke tests, check draught and combustion, and confirm CO alarms. Do not simply open a window or add a fan. Record the flue route, test results and any limitations, and leave the appliances off until the tests pass.