Guesthouse ensuite water heaters and CO alarm
A 12-room guesthouse in Sai Kung is being renovated. Ten ensuite bathrooms will each receive a new 16 L/min instantaneous gas water heater. The original high-level louvres in the bathrooms are being sealed, and four central exhaust fans, each rated 250 m3/h, are being connected to the ensuites. The water heaters are open-flue models with a 1.2 m vertical flue terminating in a covered light well 3 m from a bedroom window. During a trial, a guest reports headaches and nausea; a portable CO monitor in one bathroom peaks at 85 ppm after 10 minutes with the fan running and the bathroom door closed. The contractor says the CO alarm is faulty because the flame looks blue. The guesthouse owner wants to reopen for a long weekend in three days.
1. Diagnose the likely cause of the CO reading and explain why a blue flame does not rule out dangerous CO production.
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The pattern points to depressurisation and spillage, not a faulty CO monitor. Four exhaust fans at 250 m3/h each can extract about 1,000 m3/h from small bathrooms. With sealed high-level louvres and only a 10 mm door undercut, replacement air is inadequate; the bathroom goes negative relative to the flue/light well, the open-flue heater's draught diverter spills combustion products into the room, and CO rises. The flue terminating in a covered light well 3 m from a bedroom window can also suffer downdraft and recirculation. A blue flame does not prove safe combustion: colour reflects mainly the primary aeration and flame temperature, not necessarily CO. Oxygen depletion, incomplete mixing, a blocked or downdrafted flue, or a spillage at the diverter can produce CO with a blue-looking flame. The correct diagnosis needs a room pressure differential measurement, a spillage test at the draught diverter, flue draught and CO/CO2 readings before and after fans operate, and a check of the permanent ventilation path. The 85 ppm peak after 10 minutes is already a dangerous exposure and the appliance should be isolated.
2. Apply the Hong Kong flueing and ventilation requirements and decide whether the installation can be used.
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Under the Hong Kong gas safety framework and the applicable EMSD code/manufacturer's instructions, open-flue instantaneous water heaters in bathrooms/ensuites are a high-risk arrangement; room-sealed (balanced-flue) appliances are the expected solution where the flue can be terminated safely to external air. Permanent ventilation openings must not be sealed or reduced, and mechanical exhaust must not cause spillage. The installation as described fails on at least three points: sealed permanent ventilation, inadequate replacement air with powerful exhaust, and a flue termination in a covered light well close to a window. A CO alarm is a backup, not a substitute for safe flueing. Therefore the affected heaters should be disconnected and labelled 'do not use' until the flue and ventilation are redesigned. The contractor's statement that the alarm is faulty because the flame is blue is not an acceptable technical response; it ignores the pressure regime. Notify the owner in writing, keep the CO record, and require a registered gas contractor to submit a revised installation with room-sealed appliances, external flue termination, permanent ventilation and, if exhaust fans remain, an interlock or proven make-up air design.
3. Propose interim and permanent actions before the guesthouse reopens.
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Interim: isolate gas at each affected heater, lock off or cap, post warning labels, ventilate the rooms, do not rely on opening windows as a permanent fix, and advise guests not to use the ensuites. A CO alarm should remain in each room as a secondary warning. Permanent: replace with room-sealed balanced-flue heaters, terminate the flue to an external wall or roof with the code clearances from windows, openings, light wells and adjacent buildings, provide permanent ventilation sized to the appliance and room, and either remove the high-capacity exhaust fans from the ensuites or provide proven make-up air and an interlock that prevents heater operation if the fan fails. Recommission with spillage/backdraught tests under worst-case fan operation, CO/CO2 and gas-rate checks. The three-day reopening target cannot be met unless the replacement and tests are complete. Record the isolation, the reason, and the final commissioning; reopening with the existing open-flue units would be a safety and regulatory failure.