Spillage and Depressurization in a Natural-Draft Furnace Closet
A 100,000 BTU/h natural-draft furnace rated 80% AFUE sits in a closet measuring 4 ft × 2 ft × 8 ft (64 ft³) with a louvered door opening to a basement that was recently air-sealed. The homeowners added a 300 CFM bath fan and a 150 CFM dryer in the same basement. They report headaches and a sooting smell when the dryer runs. Combustion analyzer readings taken in the flue ahead of the draft hood: O2 6.0%, CO2 8.0%, CO 60 ppm, stack 380°F, ambient 70°F. Draft at the draft hood is −1.2 Pa with the closet door closed and the dryer running. A CO monitor held in the dilution air at the draft hood shows 6 ppm after two minutes and 28 ppm after four minutes. A natural-draft water heater shares the same flue. Outdoor temperature is 35°F.
1. State the immediate actions required at this call, in order.
Read the worked answer
This is a spillage event with CO present in the living space. Safety outranks diagnosis.
- Shut the appliance off at the service switch or gas valve and do not relight it. Leave the flue and draft hood undisturbed.
- Stop the exhaust fans (bath fan, dryer) and open a door or window to relieve the negative pressure - but do not create a new depressurization path that makes it worse.
- Get people out of the affected space and ventilate. If anyone has headache, nausea, or dizziness, they should be evaluated medically; advise the customer accordingly and do not minimize the symptoms.
- Red-tag the appliance and attach a notice that it must not be operated until repaired. Disable it so it cannot be re-energized casually.
- Check the water heater too - it shares the flue and is subject to the same depressurization.
- Notify the customer in writing of the hazard, the reason, and what must be corrected; follow the utility/emergency notification requirements that apply in your jurisdiction.
- Do not leave the site with the appliance operating at any point during the visit. Confirm with a personal CO monitor before and after the work area is cleared.
2. Interpret the combustion and draft readings. Calculate the air-free CO and explain what the numbers do and do not tell you.
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Air-free CO
- CO(air-free) = measured CO × 20.9 ÷ (20.9 − O2)
- = 60 × 20.9 ÷ (20.9 − 6.0) = 60 × 1.402 ≈ 84 ppm air-free.
What the numbers say
- 84 ppm air-free is below the roughly 100 ppm air-free action level used for vented natural-gas appliances. Combustion itself is not producing gross CO - this is not (yet) a classic cracked-heat-exchanger signature. O2 of 6.0% and CO2 of 8.0% are in a plausible range for a natural-draft burner at this altitude and temperature.
- Draft at the draft hood of −1.2 Pa is inadequate. Natural-draft appliances need roughly −2 to −5 Pa at the hood to reliably vent; −1.2 Pa is at the edge of, or below, the spillage threshold.
What the numbers say about the real problem
- The rising CO at the draft hood (6 → 28 ppm over four minutes) is a spillage test failure, not a combustion failure. Products of combustion are coming back into the space because the flue is not drafting.
- The combination of a 64 ft³ closet, air-sealed basement, and 450 CFM of competing exhaust explains it: the appliance cannot get combustion air and the exhaust fans are pulling the flue backwards.
What the numbers do not tell you
- They do not clear the heat exchanger. Visual inspection and, where applicable, a manufacturer-approved test are still required before the appliance is returned to service.
3. Specify the corrective work and the verification testing needed before the appliance is returned to service.
Read the worked answer
Combustion air calculation
- Indoor air from an unconfined space requires roughly 50 ft³ per 1,000 BTU/h: 100 × 50 = 5,000 ft³. The closet has 64 ft³. It is nowhere close, and the louvered door opens into a basement that has been sealed - so the "unconfined space" assumption no longer holds.
- Two permanent openings to outdoors (or to a space that freely communicates with outdoors) are required, sized by the applicable rule - a common basis is 1 in² of free area per 1,000 BTU/h, split high and low, with the actual sizing taken from the adopted code and the appliance listing.
Corrective work
- Restore combustion air to the closet with properly sized high and low openings, or convert to a direct-vent/sealed-combustion appliance, which removes the depressurization sensitivity entirely.
- Correct the flue - verify the shared flue is properly sized for the combined appliances, unobstructed, and lined as required. With a water heater sharing the flue, confirm the vent sizing is still correct for both appliances.
- Address the exhaust/makeup air balance in the basement so 450 CFM of exhaust has a planned makeup path rather than pulling through the flue.
- Inspect the heat exchanger and burners and clean the burners and pilot/orifice if sooting is found.
Verification before returning to service
- Worst-case CAZ depressurization test: all exhaust fans on, doors and windows closed as the occupants would use them, and confirm draft at the draft hood is within the manufacturer's range (roughly −2 to −5 Pa) and that the spillage test is 0 ppm with no rise over the test period.
- Repeat the combustion analysis and record O2, CO2, CO, and air-free CO; verify no spillage at the water heater either.
- Verify CO alarms are installed and functioning per the manufacturer's listing on each level and near sleeping areas.
- Document all before/after readings in writing and give the customer the report.