Spillage in a Utility Closet with a 400 CFM Range Hood

The draft hood is the key reading. A properly drafting natural-draft appliance under worst case should show a clear negative pressure at the draft hood - several pascals, not a fraction of one. At -0.9 Pa this unit is spilling, and the protocol threshold matters less than the condition: with the house at -4.5 Pa it cannot establish draft and is discharging into the closet.

The furnace tells you the chimney is not blocked. At -2.8 Pa it is drafting, so this is not a total flue obstruction. That narrows the cause to the natural-draft appliance losing the competition for combustion air, in an oversized unlined masonry chimney on an exterior wall that runs cold, with a house being pulled to -4.5 Pa by a 400 CFM hood and a bath fan in a 9 ACH50 shell.

The CO number is not the decision-maker. 320 ppm in the flue is elevated, but what drives the call is the spillage and the 18 to 22 ppm rise in the closet with the door closed. That is combustion product entering occupied space. The 1 ppm outdoor ambient is your baseline - it lets you say the closet CO is not instrument drift or outdoor air.

Rust at the connector and no cap are consistent with long-term spillage and water entry down the flue.

This is a red tag.

• Shut the water heater down: gas valve to OFF/pilot, upstream appliance valve closed, tagged, not relit. Do not leave a spilling appliance running because the family wants hot water.
• Re-test the furnace with the water heater off and confirm it still drafts within spec. Leave it running only if it tests safe.
• Verify CO alarms: at minimum one per level and near sleeping areas, tested and working; recommend combination smoke/CO. Say out loud that an alarm is a backup, not a repair.
• Measure ambient CO in the living space, not only the closet, and record it with the time and conditions.
• Written notice: the readings, the test conditions (all fans on, doors set, -4.5 Pa), exactly what was shut off, what must not be used, and who to call - a licensed gas fitter or chimney professional.
• Photographs of the draft hood, the rusted connector, the chimney top, and the manometer display.
• Explain it to the occupants in plain language, and leave a copy of the notice in the home. If anyone reports headaches, nausea, or dizziness, that is a medical referral, not an energy upgrade.

Ranked options.

1. Direct-vent or power-vent water heater. Decouples combustion from house pressure and eliminates the orphaned-flue problem. Usually the lowest total cost and the most durable fix - normally my first recommendation.
2. Reline the chimney with a liner properly sized for the appliances that will remain. Coordinate with the mechanical contractor: if the furnace is replaced with a high-efficiency unit later, the water heater is orphaned and needs a small dedicated liner.
3. Fix combustion air. Two permanent openings sized per the mechanical code (on the order of 1 in² of free area per 1,000 Btu/h of total input) or ducts run directly to the closet. Necessary, but alone it will not fix an oversized cold chimney.
4. Reduce exhaust. Interlock the 400 CFM hood with a make-up air damper, or replace it with a lower-CFM hood. A 400 CFM hood in a small, leaky 1960s shell can consume much of the available combustion air.
5. Cap the chimney and replace the rusted connector.

Refuse as a fix: a CO alarm alone, 'crack a window,' or relighting the appliance without re-testing under worst-case conditions. Re-test after any repair with all exhaust fans running and document draft and CO again.