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.
When a gas water heater fires up, it
may not have enough heat in its flue
gases to push through the large