The Sealed Mechanical Room
A 1,900 sq ft two-storey house has a 40,000 Btu/h natural-draft water heater and a 75,000 Btu/h induced-draft furnace in a basement mechanical room measuring 2.4 m by 1.8 m by 2.4 m. The room door is weatherstripped and has only a 12 mm undercut. The house has been air-sealed, and a new HRV serves the bathrooms. A CO alarm in the adjacent hallway sounds intermittently when the clothes dryer and both bathroom exhaust fans run. The water heater flue has rust stains at the draft hood, and the homeowner reports a “musty” smell in the room. The contractor proposes adding a louvered door and returning the appliances to service the same day.
1. What is the likely mechanism, and why is adding a louvered door not automatically the right fix?
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The likely mechanism is depressurization and spillage. The mechanical room is small, the door is effectively sealed, and the house has been tightened. When the dryer and two bathroom fans operate, the house draws replacement air from wherever it can, including the chimney and vent connector. A natural-draft water heater is especially vulnerable: its draft hood can spill combustion products into the room, producing CO and moisture. The rust stains are evidence of long-term spillage, not just a cosmetic issue.
A louvered door may help, but it is not automatically compliant. The opening must be sized for the appliances and the house conditions; it must not create a fire, smoke or privacy problem; and it does not correct a blocked or improperly sized chimney, a poor vent connector slope, or a venting system that is unsuitable for the remaining appliance. In a tight house, the better solution may be sealed-combustion or direct-vent appliances rather than relying on a gravity opening. Do not return the appliances to service until the cause is verified.
2. Outline the diagnostic sequence, including measurements and conditions, before returning the appliances to service.
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Use a combustion-appliance safety approach, not a guess-and-fix approach:
- Keep the appliances off if CO is present or spillage is active. Ventilate the space and protect occupants.
- With the door closed, operate all exhaust devices - dryer, bathroom fans and HRV - and measure the pressure difference between the mechanical room and outdoors. Repeat with the door open to see whether the room is starved for air.
- Perform a draft and spillage test at the water heater draft hood and furnace vent connector under worst-case conditions. Measure flue CO and ambient CO.
- Inspect the vent connector slope, supports, joints, chimney liner and termination. Check whether the water heater is orphaned on a chimney sized for a larger appliance.
- Verify combustion-air openings, if any, and confirm they are unobstructed and sized for both appliances.
- Verify the HRV is balanced and not creating excessive negative pressure.
Record the results. If spillage occurs under worst-case conditions, the installation is not ready for service.
3. Recommend a compliant remediation strategy that addresses both appliances and the tightened envelope, and state what must be verified after the work.
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The strategy should address appliance venting, combustion air and house pressure together. Options include converting the water heater to a direct-vent or power-vented appliance, converting the furnace to sealed combustion if it is not already, providing a properly sized and located combustion-air duct, and ensuring exhaust devices do not depressurize the mechanical room below safe limits. If a louvered door or grille is used, it must be sized by calculation and must not compromise required separation or house durability.
After the work, verify worst-case depressurization, draft and spillage at each appliance, flue CO and ambient CO, vent connector slope and support, combustion-air flow, and HRV balance. Confirm that CO alarms are installed and operating. The final test must be done with the door closed and all exhaust fans operating, because that is the condition that caused the problem.