Combustion Appliance Spillage After System Commissioning
An SSD system is commissioned in a 1990 two-story colonial with a basement. The basement contains a naturally aspirated 40-gallon gas water heater and a Category I gas furnace, both vented into a masonry chimney. The system uses a 1/3 hp fan, one suction point, and discharge above the roofline. During commissioning, with the basement bathroom exhaust fan and the clothes dryer running, a smoke pencil at the water heater draft hood shows flue-gas spillage that stops when the dryer stops. The homeowner mentions headaches and a CO detector on the first floor that has never alarmed. The radon system manometer reads -0.9 in WC at the fan, and all slab penetrations appear sealed.
1. What is the hazard, and what must you do immediately?
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The hazard is backdrafting of a naturally aspirated water heater: combustion products, including carbon monoxide, are spilling into the basement instead of going up the chimney. This is a life-safety condition, not a comfort issue.
Immediate steps: stop commissioning, ventilate the space, and advise the homeowner not to run the dryer and bath fan together until the problem is corrected. Perform combustion safety testing on both appliances with all exhaust devices operating: spillage at the draft hood, draft in the flue, and CO in the flue and ambient air. If CO is present in the living space, evacuate and call the utility. Do not leave the system in a condition that worsens depressurization.
Document the readings, the appliance conditions, and the exhaust devices that were running. A smoke pencil alone is a screening tool; it must be backed by CO and draft measurements.
2. How do you determine whether the radon system is contributing?
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A properly sealed sub-slab system should not depressurize the basement, because it draws from beneath the slab, not from the room. To test the contribution, measure basement pressure relative to outdoors with the radon fan on and off, and repeat the spillage tests under both conditions.
If spillage occurs with the fan off and the exhaust fans running, the cause is house tightness and exhaust capacity, not the radon system. If it only occurs with the fan on, look for leaks: unsealed pipe penetrations, a loose sump lid, or a system that is inadvertently drawing room air. Also check the discharge location relative to windows, doors, and combustion-air intakes.
A manometer reading of -0.9 in WC at the fan is normal for many installations and does not by itself indicate a room-air leak. The pressure diagnostic, not the manometer alone, answers the question.
3. What remediation and verification do you recommend?
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Provide makeup air for the combustion appliances and the exhaust devices, such as a dedicated combustion-air duct sized to the appliances, or a supply fan interlocked with the exhaust fans. Seal any radon-system air leaks. Have the chimney evaluated by a qualified technician; an oversized or blocked flue may need a liner or repair.
After repairs, repeat spillage and draft tests with the dryer and bath fan running, verify CO levels in the flue and ambient air, and confirm the radon system manometer and pressure-field extension. Provide the homeowner with a written report and recommend a post-mitigation radon test.
Do not simply tell the homeowner to stop using the dryer. The condition must be corrected at the source, and the verification must be documented.