Orphaned Natural-Draft Water Heater After Furnace Replacement
A 1950s bungalow has a natural-draft 40 gal water heater and a 20-year-old 80% furnace that formerly shared a 6 in. B-vent connector into an exterior masonry chimney with an 8 by 8 in. clay tile flue. The furnace has just been replaced with a 95% condensing unit vented through a sidewall. The water heater remains connected to the chimney. The chimney is 35 ft high, unlined, and on an outside wall. The house has new windows and spray-foam attic insulation. Within a week, the water heater pilot goes out intermittently, soot appears at the draft hood, and the homeowner reports a headache in the adjacent room. A carbon monoxide alarm has not been installed. The contractor must decide whether the existing chimney is acceptable for the orphaned water heater and what corrective action is required before leaving the site.
1. Explain why the water heater now spills combustion products and why the original chimney may have been acceptable before the furnace replacement.
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Before replacement, the chimney was handling the combined flue gases of the furnace and water heater, so its size was matched to a larger total input. After the furnace was removed from the chimney, the water heater became an orphan. The 8 × 8 in. flue is now oversized for the water heater alone. An oversized exterior chimney warms slowly, cools flue gases, reduces draft, and can cause condensation and spillage at the draft hood. The tighter house adds another factor: exhaust fans and stack effect can depressurize the utility room and reverse draft. Soot and pilot outage are signs of spillage, not just a dirty pilot. A headache in the adjacent room is a CO exposure concern. The original installation may have worked because the furnace diluted and warmed the flue; that does not make the orphaned condition acceptable. The correct judgement is to treat this as an unsafe venting condition until proven otherwise by testing.
2. Compare acceptable options for venting the water heater and state the code considerations that govern the choice.
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Options include: relining the chimney with a listed liner sized for the water heater and its vent length, with proper termination and clearances; replacing the water heater with a power-vent or direct-vent model; replacing it with a high-efficiency condensing unit vented per manufacturer; or, in limited cases, documenting by draft and spillage testing that the existing chimney works under all conditions. An unlined exterior clay flue with spillage is not acceptable as-is. Code considerations include venting sized and installed per CSA B149.1 and the manufacturer, verification of combustion air, clearances to combustibles, proper termination, CO alarms, and permit and inspection. Do not simply add a cap or extend the vent without sizing it. The choice should be based on the appliance input, vent height, chimney condition, and the tightness of the house.
3. What tests and safety measures must be completed before the water heater is placed back in service?
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Tests should include draft measurement in the flue, a spillage test at the draft hood with doors and windows closed and exhaust fans operating, CO measurement in the flue and occupied space, gas manifold pressure, combustion analysis for CO2, O2, CO, and temperature, and a visual check of the liner and termination. Safety measures include installing CO alarms, ventilating the space, shutting off the appliance if spillage or CO is present, tagging it out, and notifying the owner. After correction, repeat the tests and document the results. Also verify the new furnace's PVC vent termination clearances and confirm that it is not recirculating flue gases into the combustion air intake. The judgement point is that a water heater that lights is not necessarily safe; draft and CO testing are required before leaving.