Orphaned Water Heater in an Oversized Masonry Chimney
A contractor replaced an old 100,000 BTU/h natural-draft furnace with a 96% AFUE condensing furnace that now vents through 2-inch PVC out the sidewall. The existing 40-gallon, 40,000 BTU/h atmospheric water heater still connects to the original 8 by 12-inch clay-tile masonry chimney, which runs 25 feet up an exterior wall and has no liner. The water heater's draft hood outlet is 4 inches; the connector rises 24 inches and runs 12 inches laterally into the chimney. It is January. The homeowner reports that the water heater's pilot has gone out twice and that there is a musty smell and white staining around the cleanout in the unfinished basement. The basement is about 2,000 cubic feet, the weather-stripped door has a sweep, and there are no combustion air openings. The homeowner says the old furnace "never had a problem."
1. Explain why removing the furnace made the remaining water heater more dangerous, in terms of vent sizing and draft.
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The chimney was sized for the combined input of both appliances and now serves a single 40,000 BTU/h water heater with a 4-inch draft hood. An 8 by 12-inch flue has roughly 96 square inches of area, closer to what a 12-inch round flue would provide, and it is many times larger than this appliance needs. An oversized flue is slow to warm, so the flue gases cool against the cold masonry, drop below their dew point, and condense. That condensation is what produces the white staining at the cleanout and the musty odor, because flue gas condensate is acidic and attacks mortar. A cool, oversized flue also develops weak draft. Removing the furnace took away its heat input, which had been warming the flue and helping to carry the water heater's products up, and it also removed a large volume of dilution air. The result is an orphaned appliance that spills at its draft hood instead of venting, which is a carbon monoxide hazard rather than merely a nuisance.
2. Describe the tests you would run, in order, and what a failing result would tell you.
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Start with a worst-case condition. Close windows and exterior doors, turn on every exhaust device in the house, including bath fans, the dryer, and the range hood, and start the water heater. Use a smoke pencil or a mirror at the draft hood to check for spillage throughout the burner cycle, and watch for spillage that persists instead of clearing within a minute. Measure the pressure in the combustion appliance zone to confirm how much the house is depressurizing the basement. Take ambient CO readings at the draft hood and at breathing height near the appliance, and analyze the flue gases to see what the appliance is actually producing and how hot they are at the hood. Inspect the chimney for a blocked flue and for liner condition, and confirm the connector slope and rise. Sustained spillage, rising ambient CO, or a cold, wet flue all mean the venting arrangement is unsafe and the appliance cannot be left operating as installed.
3. What corrective options would you present, and what would you write on the work order?
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The venting has to be corrected to match the appliance. Options include relining the chimney with a listed liner sized for a single 40,000 BTU/h draft-hood appliance per the venting tables and the liner manufacturer's instructions, replacing the water heater with a power-vented or direct-vent model, or power-venting the existing tank. Combustion air also has to be addressed, because a tight 2,000-cubic-foot basement with no openings and exhaust fans running is a classic depressurization setup. Provide outdoor air per the applicable code, and install a CO alarm on each level and near the sleeping areas. On the work order, record the spillage and CO readings with the conditions under which they were taken, photograph the staining and the draft hood, state plainly that the appliance was left off or tagged because it spilled flue gases into the living space, and note that the old furnace's removal created the condition. The customer should sign an acknowledgment.