The Combustion Appliance That Fails Under Worst-Case
A single-story ranch has a natural-draft gas furnace and a natural-draft gas water heater sharing a masonry chimney in an interior utility closet. The closet has a louvered door and a return-air grille in the adjacent hallway. During a blower door test at 50 Pa the house reads 2,900 CFM50. The technician runs a worst-case depressurization test by closing all exterior doors and turning on the dryer, both bath fans, and the kitchen hood. The water heater flue reads 1.2 Pa spillage at the draft hood, and the furnace flue reads -0.4 Pa. The combustion analyzer shows 8.2% O2 and 35 ppm CO in the furnace flue. The homeowner reports headaches in winter.
1. Interpret the test results. Is this a pass or fail, and what is the mechanism?
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This is a fail and a health concern.
- The water heater draft hood spills 1.2 Pa under worst case. A natural-draft appliance must maintain a negative draft at the draft hood (flue gases drawn up the chimney); a positive spillage reading means combustion products are entering the closet and, because of the return grille, the house. Any spillage under worst case is a fail.
- The furnace flue at -0.4 Pa is negative but marginal; a natural-draft furnace should show a clear negative draft, and -0.4 Pa is within measurement noise and indicates the chimney is barely drafting.
- The 35 ppm CO at 8.2% O2 is elevated for a furnace (typical clean readings are well under 100 ppm as-measured, but 35 ppm with marginal draft and spillage warrants shutdown and repair).
- Mechanism: the return-air grille in the hallway depressurizes the utility closet relative to the chimney, and the louvered door is undersized for the combined 2,900 CFM50 envelope leakage plus the exhaust fans. The chimney is a shared masonry flue, so the water heater and furnace compete for draft, and the house's negative pressure under worst case overcomes the chimney's buoyancy.
Conclusion: fail; the appliances are backdrafting under realistic winter conditions, which explains the headaches.
2. What immediate actions and follow-up diagnostics do you recommend?
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Immediate: shut down the furnace and tag it out; advise the homeowner not to run the water heater until the draft is corrected, or provide a CO alarm and a clear written warning. Document the readings.
Diagnostics to run:
- Zone pressure in the utility closet relative to the hallway and outdoors under worst case, to quantify how much the return grille and door are driving the depressurization.
- Spillage and draft test on each appliance separately with the other off, to see whether the shared flue is the problem or the closet pressure is.
- Chimney inspection (liner condition, size, height, termination) - a shared masonry flue with two appliances often needs a properly sized liner or a reline.
- Combustion analysis after any repair, checking CO and O2 at steady state.
Corrective options, in order of preference:
- Seal and separate the return-air path from the utility closet (remove the return grille or duct it so the closet is not depressurized).
- Enlarge combustion air openings to the closet (two permanent openings sized per code, high and low) or duct outdoor combustion air directly to each appliance.
- Reline or resize the chimney, or replace the natural-draft water heater with a sealed-combustion unit.
- Do not simply add a chimney cap or a fan; fix the pressure and the flue.