Orphaned Water Heater on an Oversized Masonry Chimney
A 1958 bungalow has an 8 in × 12 in clay-tile masonry chimney. The original 80% efficient furnace (100,000 BTU/h input) and a 40,000 BTU/h natural-draft water heater shared a wye connector into the chimney. The furnace is replaced with a 96% condensing unit vented through 2 in PVC out the sidewall; the water heater remains connected to the chimney. The mechanical room is 9 ft × 12 ft × 7.5 ft with two 30 in × 80 in louvered doors to the basement; no dedicated combustion-air duct is installed, and a clothes dryer is in the room. On the first cold morning, the water heater draft hood spills for several minutes after startup. A consumer CO alarm on the floor above reads 65 ppm; the water heater flue connector is warm but the chimney exterior is cool. Outdoor temperature is -15°C. The homeowner reports the old furnace never spilled.
1. Explain the most likely reason the water heater now spills even though the chimney and connector were unchanged. What measurements would you take before choosing a repair?
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The furnace was the 'engine' that kept the chimney warm and diluted the water heater flue gases. Removing it leaves an orphaned 40,000 BTU/h appliance connected to a chimney sized for roughly 140,000 BTU/h. With -15°C outdoor air, the flue gases cool quickly in the oversized clay tile; draft weakens, and the draft hood spills. The clothes dryer can also depressurize the room.
Before repairing, measure:
- draft in the connector and at the draft hood with the door closed and open;
- spillage at the draft hood using a mirror or smoke;
- flue-gas temperature and CO in the connector;
- ambient CO and room pressure relative to outdoors;
- combustion air openings and dryer operation;
- chimney condition, liner size, blockages, and connector slope.
Also verify gas input and draft-hood operation. A cap or warmer weather may mask, not fix, the problem.
2. The homeowner wants the water heater left on the chimney and asks you to add a chimney cap and 'make it pass.' What code and safety issues govern your response?
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A cap does not correct an undersized flue-gas temperature or an oversized chimney. The installation must comply with the gas code and manufacturer's instructions for venting an orphaned appliance. That usually means either relining the chimney to a listed liner sized for the water heater, replacing the water heater with a direct-vent or power-vent unit, or using another listed venting system. The chimney must be sound, lined, and sized so the appliance develops draft under all operating conditions. Combustion air must be adequate; the dryer and louvered doors do not guarantee it. Because CO has already entered the living space, the appliance is unsafe until corrected. I would not sign it off with a cap. I would explain the options in writing and red-tag the water heater if it still spills.
3. Describe an interim and permanent plan that keeps the home heated and safe tonight, and what you document.
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Interim: shut off the water heater if spillage or CO persists; ventilate; keep the CO alarm active; do not run the dryer while testing; provide a safe temporary heat source that does not add CO, or arrange service. Permanent: reline/replace venting or replace the water heater with a direct-vent or power-vent model, verify combustion air, and retest draft, spillage, and CO under cold conditions.
Documentation: readings before and after, outdoor temperature, appliance input, venting configuration, CO alarm level, actions taken, customer notifications, and any refusal of repair. If the customer refuses, record that the appliance was left off or tagged.