Carbon Monoxide Complaint in a Home With a Sealed Mechanical Room
A family of four reports headaches, fatigue, and nausea that improve on weekends away. Their home has an atmospherically vented natural gas furnace and a natural gas water heater in a utility closet with a solid-core door and no dedicated combustion air opening. The closet is on the same pressure plane as the finished basement. A new 400 CFM downdraft kitchen exhaust hood was installed three months ago, and the symptoms began around the same time. A blower door test earlier this year measured 2,100 CFM50. During a worst-case depressurization test with all exhaust fans running, the utility closet measured -6 Pa relative to the basement. The furnace flue is a single-wall galvanized pipe with a short vertical run and a draft hood. There are no CO alarms in the home. The client wants to know if the hood is the problem.
1. Assess the combustion safety risk and explain the interaction between the new exhaust hood and the vented appliances.
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This is a potential life-safety condition and should be treated as an emergency until proven otherwise.
The mechanism. The furnace and water heater are atmospherically vented with draft hoods. A draft hood is a dilution device that relies on the appliance's own buoyant plume and on the house being at or near neutral pressure relative to outdoors. A 400 CFM downdraft hood in a 2,100 CFM50 house is a large exhaust relative to the envelope's tightness and can depressurize the basement. The measured -6 Pa in the utility closet confirms the closet is being pulled negative.
Why negative pressure is dangerous here. At negative pressure, the flue becomes a competitor with the house for air. Spillage occurs when the flue gas no longer rises up the vent but instead rolls out of the draft hood into the closet. Once in the closet, the solid door and lack of a combustion air opening mean the CO has no dilution path but does have a path into the conditioned space through gaps around the door and any duct leakage.
Why the timing matters. Symptoms began with the hood installation, which is a strong temporal association and consistent with the mechanism. Improvement when away from home is also consistent with an indoor source.
What is not yet established. The -6 Pa closet reading is suggestive but the critical measurement is worst-case depressurization with the closet door closed and all exhaust devices operating, combined with a combustion analyzer reading on each appliance and a spillage test at the draft hood. A single CO alarm absence is itself a deficiency regardless of the cause.
Immediate advice. Stop using the hood, open windows, and have the appliances tested by a qualified technician before further use. Install CO alarms per manufacturer instructions outside sleeping areas and on each level.
2. Given the measured data, propose a diagnostic sequence and a durable remediation plan.
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Diagnostic sequence (in order):
Baseline CO. Place a calibrated CO monitor in the basement and in the utility closet. Record ambient CO before anything else.
Worst-case depressurization. Close all exterior doors and windows, operate every exhaust fan including the hood, the dryer, and bath fans, then measure pressure in the utility closet and the basement relative to outdoors. The -6 Pa reading should be repeated under this protocol and recorded with the door closed.
Spillage test. With the worst case established, use a smoke pencil or a combustion analyzer at the draft hood of each appliance to determine whether flue gases spill and for how long. A draft gauge reading in the flue confirms whether negative draft exists.
Combustion analysis. Measure CO, CO2, and O2 in the flue of each appliance and calculate steady-state efficiency and excess air. A cracked heat exchanger or a fouled burner shows up here.
Zone pressure mapping. Measure the pressure difference between the utility closet and the basement with the door open and closed to determine whether the closet is being isolated or fed.
Remediation plan, in priority order:
- Make the appliances safe. Replace the atmospherically vented units with direct-vent or power-vented equipment, or convert to sealed combustion. This removes the depressurization sensitivity entirely and is the most durable fix.
- Provide combustion air if the existing appliances are retained: a properly sized two-opening or one-opening combustion air duct per the appliance manufacturer and local code, terminating in a location that is not itself depressurized.
- Reduce the exhaust imbalance. Replace the 400 CFM downdraft hood with a recirculating or lower-CFM unit, or add a make-up air system interlocked with the hood.
- Air seal and insulate the utility closet only after combustion air is resolved, so the closet is not turned into a tighter trap.
- Verify. Repeat worst-case depressurization and spillage testing after remediation and document results. Install CO alarms regardless.