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Most older homes, especially in the Midwest and Northeast, use masonry chimneys to vent their combustion appliances. During the past ten years, many old furnaces and boilers have been gradually replaced with higher-efficiency models that use PVC sidewall vents. When an older furnace is disconnected from a masonry chimney, it may leave behind a so-called “orphaned” water heater, attached to a flue that is now oversized. However, when it comes to sizing a flue, bigger isn’t always better. Oversized flues can contribute to at least three problems: poor draft, chimney corrosion, and freeze/thaw damage to the chimney. Anemic Draft When a gas water heater fires up, it may not have enough heat in its flue gases to push through the large