Worst-Case Depressurization and Spillage in a Townhouse Mechanical Closet
1,250 sq ft two-story townhouse, built 1994, Minneapolis. A 100,000 Btu/h atmospheric-draft furnace (80% AFUE) and a 40,000 Btu/h atmospheric water heater share a first-floor closet with a louvered bi-fold door and vent into one shared 6-in. chimney liner. The closet holds about 90 cu ft. The owner just replaced windows and the sliding door, added weatherstripping, and installed a 300 CFM vented range hood and a 110 CFM bath fan. Winter complaints: headaches, and a “smell” near the water heater. Worst-case CAZ test: closet door closed, dryer and both fans running; CAZ pressure -54 Pa with the air handler off, -38 Pa with it on. After firing the water heater, the draft hood spills for the full 4-minute observation. Undiluted flue CO at the draft hood: 130 ppm. Ambient CO in the closet: 12 ppm; 3 ppm in the adjacent kitchen.
1. Interpret the test results. What is happening, how severe is it, and what are your immediate actions?
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The numbers describe a failed combustion-air and venting situation, and the appliances are unsafe to leave running.
- CAZ at -54 Pa is an order of magnitude beyond the roughly -5 Pa limit applied to naturally drafting appliances. The 300 CFM range hood plus 110 CFM bath fan plus dryer, in a townhouse freshly tightened with windows and weatherstripping, is overpowering the only combustion-air path: the louvered door.
- Spillage for the entire 4 minutes means flue gases are entering the closet instead of the chimney. Spillage that does not clear within about a minute during a worst-case test is a failure.
- 130 ppm undiluted CO at the draft hood exceeds the ~100 ppm undiluted action level; converted to air-free it would be far higher. Add 12 ppm ambient in the closet and you have a documented exposure condition, not a theoretical one. The headaches are a symptom, not a coincidence.
- The volume math confirms there was never enough air. Using the 50 cu ft per 1,000 Btu/h indoor-air rule, 140,000 Btu/h of combined input requires roughly 7,000 cu ft. The closet is 90 cu ft. The louvered door was doing all the work, and the new exhaust fans removed the margin.
- The shared 6-in. liner is also suspect. It may be marginal with both appliances firing, and if the furnace is ever replaced, the orphaned water heater will be drastically oversized for that liner and will spill even more.
Immediate actions: shut off gas to both appliances, red-tag them, ventilate the space, advise the owner in writing not to operate them, recommend CO alarms, and refer to a licensed HVAC contractor for evaluation and repair.
2. Lay out the remediation options in priority order, with the reasoning behind the order and the verification step for each.
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Fix in this order - the cheapest and most reliable fix is to stop creating the depressurization.
- Reduce the exhaust imbalance. Interlocked make-up air for the range hood, a smaller hood, or an HRV/ERV that supplies while it exhausts. Do not tell the owner to open a window; the occupants will not do it, and it does not fix the closed-door condition that produced the failure.
- Give the closet real combustion air. Two properly sized openings to outdoors per the applicable code table for the connected input, or a ducted supply. Verify with a repeat worst-case test.
- Fix the venting. Have a chimney professional evaluate the shared liner; the water heater alone likely needs a correctly sized liner. Plan the flue configuration around the possibility that the furnace is replaced first.
- Best long-term: direct-vent, sealed-combustion appliances, which decouple combustion air from house pressure entirely and end the spillage mechanism.
- Retest worst case after any change and document the numbers - depressurization, spillage duration, and CO - in the file.
Sequencing note worth putting in the report: this house was tightened with no combustion-safety review. On every weatherization job, run the CAZ test before air sealing and after, and treat the exhaust fans the occupants actually use as the worst case.
3. What goes in the written report, and how do you deliver the news to a homeowner who is worried about cost?
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Write it as a hazardous condition, not as one recommendation among many.
Include: date and time, outdoor temperature, appliances and inputs, every exhaust device running, CAZ pressure with and without the air handler, spillage duration, undiluted and ambient CO readings with locations, instrument model and calibration date, and photos of the draft hood and the red tags.
Language: “Unsafe condition observed. The water heater draft hood spilled continuously for four minutes under worst-case conditions with a CAZ depressurization of -54 Pa. CO measured 130 ppm undiluted. Gas was shut off and the appliance red-tagged. Do not operate either appliance until they are evaluated and repaired by a licensed contractor.”
Delivery: describe what you saw, why it matters - the headaches are already a symptom - what you did, and who to call. Do not soften it to protect the relationship, and do not offer to do the repair yourself if it requires a license you do not hold.
On cost: separate the must-do from the better-later. Interlocked make-up air and correctly sized combustion-air openings are modest and address the immediate failure. Appliance replacement is the durable fix but can be phased. If a program is funding work here, notify the administrator of the failed test and the stop-work per its rules, and schedule the retest before closing the file.