Depressurization and Combustion Air in a Tight House
A 1,600 ft² single-story house with 8 ft ceilings was air-sealed to 2.0 ACH50 during a weatherization job. It has two 80 CFM bath fans, a new 300 CFM range hood, and a 40,000 Btu/h natural-draft (atmospheric) water heater and a natural-draft furnace sharing a 4 ft × 6 ft × 8 ft closet with a louvered door. Within a month the owner reports a gassy smell near the closet and that the water heater's draft hood spills when the range hood runs. The hallway carbon monoxide alarm has chirped twice. The weatherization contractor says the house is 'too tight' and proposes cracking a window.
1. Quantify the pressure problem and the combustion air shortfall before doing anything else.
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Volume: 1,600 ft² × 8 ft = 12,800 ft³.
Exhaust imbalance: 300 CFM hood + 80 + 80 CFM baths = 460 CFM simultaneously. At 2.0 ACH50 the natural infiltration is only on the order of 0.1 ACH (roughly one-twentieth of ACH50, an estimating rule of thumb) ≈ 21 CFM. Running the hood and both fans therefore drives a net exhaust of roughly 440 CFM, about 2 air changes per hour, with no planned make-up air. The house becomes a large negative-pressure zone and every natural-draft appliance in it is the path of least resistance.
Combustion air: the closet is 4 × 6 × 8 = 192 ft³. The usual indoor-air rule is 50 ft³ of volume per 1,000 Btu/h of connected appliance input. With 40,000 Btu/h water heater plus the furnace, the required volume is many times the closet volume - the louvered door to a conditioned space is the only air source, and that space is at negative pressure.
Conclusion: this is a classic depressurized combustion appliance zone, not a tuning problem, and the CO alarm makes it an immediate hazard. The 'crack a window' proposal is uncontrolled, unreliable, and unacceptable as a design.
2. Outline your test protocol, including the safety order and the limits you would apply.
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Safety first: treat the CO alarm as a red-tag event. Recommend the appliance(s) be shut off and the occupants protected until testing is complete. Do not run a worst-case test with occupants present and appliances firing unattended.
Worst-case depressurization test:
- Set a calibrated manometer with its reference to outdoors at the combustion appliance zone.
- Close all exterior doors and windows, turn on every exhaust fan (hood on high, both baths, dryer if present), and close interior doors.
- Record the CAZ pressure. A commonly used action level for a natural-draft appliance is about −3 Pa (verify against the locally adopted standard); a CAZ that is at or beyond that limit with the appliance firing is a fail.
- Run a spillage test at the draft hood - smoke or a mirror at the relief opening - with the appliance firing under worst-case conditions, and measure CO in the undiluted flue gas before and after.
- Inspect the flue, vent connector, and heat exchanger; measure draft in the connector and verify the vent is properly sized, sloped, and terminated.
Combustion air calculation: for two permanent openings, size each at about 1 in² of free area per 1,000 Btu/h of total input, high and low, corrected for the louver's free area (wood louvers are typically 20–25% free, metal 60–75%). Do not size the openings off the louver's rough opening dimension.
3. What repair sequence do you recommend, and how do you verify it worked?
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Priority 1 - remove the hazard at the appliance. Replace the atmospheric water heater with a direct-vent or power-vent sealed-combustion unit, and either do the same for the furnace or duct outdoor combustion air directly to the closet and weatherstrip/air-seal the closet from the conditioned space. This decouples combustion air from house pressure.
Priority 2 - reduce the exhaust imbalance. Replace the 300 CFM hood with a recirculating hood, or provide engineered make-up air interlocked to the hood. Set the bath fans to 80 CFM or less with timers or continuous low-speed operation rather than simultaneous high-speed.
Priority 3 - give the tight house intentional ventilation. A 2.0 ACH50 house needs planned ventilation, not accidental leakage. Size a balanced HRV/ERV to the ventilation rate the adopted energy code requires - for 1,600 ft² and 3 bedrooms this is roughly 0.03 × 1,600 + 7.5 × (3+1) ≈ 78 CFM. Balanced ventilation is the point: it does not depressurize.
Verification: repeat the worst-case CAZ test with the hood and both bath fans running and confirm the pressure is inside limits; repeat spillage testing at the draft hood; document flue CO readings; verify with a blower door that the air-sealing work still meets the target; and leave the CAZ test and CO results with the homeowner in writing.