The Draft Hood That Won't Hold: CO Call on an 80% Furnace
January, 46°F outside. You are sent on a 'no heat, smells funny' call to a raised cottage in Metairie. The 80% AFUE upflow gas furnace sits in a hallway closet, and the only return path for the whole house is a 24 x 24 louvered grille in the closet door. The flue is single-wall 4 in., rising 4 ft, one elbow, then 12 ft horizontal, terminating 14 in. below a soffit vent. A mirror held at the draft hood fogs over, and the flames pull off the burners and roll when the closet door is closed. Your combustion analyzer reads 9 ppm CO in the return airstream and 180 ppm in the supply. The homeowner is 82, has an infant grandchild in the house this week, and refuses to let you shut the system down because 'we'll freeze.' You are three months out of school and the lead tech is not answering his phone.
1. What do you do first, in what order, and why that order?
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Order matters because the hazard is active while you think.
- Stop combustion, not just the thermostat. Shut the gas cock at the appliance and the service switch. A thermostat set to OFF can be turned back on by anyone; a tagged, de-energized appliance cannot.
- Notify the occupants verbally and in writing before you leave the room - an 82-year-old with an infant is the highest-risk population for CO. Tell them to open windows and not to relight or reset anything.
- Tag and photograph the gas valve, the switch, and the analyzer display. Your readings are the evidence; a red tag with no numbers is a note, not a record.
- Call the gas utility if the customer refuses. Rendering the appliance safe is not a negotiation - leaving a spilling draft hood running because the customer objects puts the death, and the liability, on you.
The 180 ppm supply reading is the deciding fact. Any CO above ambient in the supply airstream is a fail; 180 ppm is two orders of magnitude past that, and it is being distributed to every room through the ductwork.
2. Explain the failure mechanism in plain language for the homeowner, and name the defects you can defend.
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The mechanism: the furnace needs a supply of combustion air and a chimney that pulls. This closet has neither. The louvered door grille makes the closet a return-air plenum, so every time the blower runs it depressurizes the closet relative to the house. The draft hood is the path of least resistance, so combustion products spill out of the hood and get pulled into the return and distributed. The mirror fogging and the flame roll-out when the door closes are the visible proof of that pressure relationship.
Defects you can defend in writing:
- Return air drawn from a closet containing a fuel-burning appliance - prohibited, and the root cause here.
- Venting terminating under a soffit vent - the termination is inside the re-entrainment zone of the soffit, so flue gas can be drawn back into the attic or the house.
- Draft not established - documented by spillage at the hood and flame disturbance.
Do not tell the homeowner the heat exchanger is cracked unless you have actually inspected or tested it. Overstating gets you cross-examined; the three items above are enough to condemn the installation as-is.
3. The homeowner agrees to repairs. Lay out the sequence and what has to be verified before the system goes back on.
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Sequence, cheapest and most fundamental first:
- Break the closet-to-return connection. Duct the return from conditioned space, or seal the door grille and provide a properly sized return elsewhere. This alone may stop the spillage - but it must be verified, not assumed.
- Provide combustion air to the confined space per the mechanical code (two permanent openings sized to the input, or a ducted outside air supply).
- Re-terminate the flue so it discharges outside the soffit's re-entrainment zone, with correct clearances and a listed cap; replace deteriorated single-wall sections.
- Verify with the door closed and the blower running: draft at the hood, no flame disturbance, and a combustion analyzer showing CO in the supply air at or near ambient. Record the numbers.
- Leave a CO alarm as a backstop and document the recommendation.
If it were your own mother's house, you would do step 1 and 4 the same day. Steps 2 and 3 can be scheduled, but the appliance does not run until step 4 passes.