"ransley" wrote in message

AFAIK, two appliances are not allowed on one chimney. That would include
two of the same type. .



Do you mean a furnace-boiler and water heater cannot share a chimney,
Its code here.
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My old house had both too, very common. I recall recently that it is no
longer allowed, but I don't have any firm information. It may have been a
local code. I know that woodstoves and oil/gas burners are not allowed.
I've not had any reason to check so I'm not up to date on that.