On Saturday, May 10, 2014 3:11:39 PM UTC-4, bob haller wrote:
others mentioned the water heater



flue concerns.







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Christopher A. Young





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I always wonder about those comments about a oversize flue. around here they are seperate flues.



Now take the follwing breakdown situation!



The furnace and water heater share the flue...



the temperature isnt very cold.



a hot water line breaks for any reason.....



so the water heater is coming on but the furnace has nearly no reason to come or breaks somehow so the furnace burner never comes on.



whats the difference between a direct vent furnace install, so it never vents exhaust gas in the chimney, but the water heater still does?

The difference at issue here is if it's in a climate
where it's cold during winter and at least part of the
chimney is exposed, ie not withing the heated part of
the building. Then with a cold, over-sized chimney,
the combustion gases from the orphaned water heater will
condense inside the chimney. That produces acidic condensate,
which over many years can eat away at the mortar in the chimney,
causing failure. A liner eliminates the problem. With the old
furnace venting into the chimney, it provided heat to keep the
chimney warm enough so that condensation was not possible.

Aside from that, chimneys are supposed to be sized to the
appliances, so that it will draft properly. You can probably
get away with that being oversized in most cases, eg the chimney
still works for just the water heater in summer, when the furnace
wasn't running, but it's better if it is properly sized.