NHIE exam practice question · Analysis & Reporting
This is the classic orphaned water heater, and it is a consequence of doing something good. A chimney flue was originally sized for the combined output of a furnace and a water heater sharing it. Replace the furnace with a high-efficiency unit that vents through the sidewall, and the water heater is left alone on a flue several times larger than it needs. A small burner cannot warm that volume of masonry quickly, so the draft is weak or absent at start-up, and flue gases linger and cool. Two problems follow: spillage of combustion products into the house while the draft establishes, and condensation inside the chimney, where acidic moisture deteriorates the liner and mortar over time. The remedy is relining the flue to a size matched to the remaining appliance, which is why the inspector reports the arrangement rather than treating it as a completed upgrade.
Removing the furnace from a shared flue leaves the water heater orphaned on an oversized chimney, where weak draft can allow spillage and cooled flue gases condense inside the liner. The arrangement is reported and relining or evaluation recommended.
Chimney flues are sized for the appliances connected to them. When a high-efficiency furnace is installed and vented separately, a remaining atmospheric water heater is left on a flue sized for the original combined load, producing poor draft and condensation. Relining to the correct size is the standard remedy.
Upgrade the furnace, orphan the water heater. One small flame cannot warm a chimney built for two — expect weak draft and wet brick.
Look for the confirming evidence: staining or white deposits at the water heater draft hood, rust on the vent connector, or deteriorating mortar at the chimney crown.
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