1 and 1/2 times the size of the largest flue opening
your flue pipe will have to be resized, if the largest apliances has a 6'' flue
then the new flue will have to be 9'' or round up to 10'', they do not make 9''
and, if the furnce is a positive air flow [fan assisted]
then it has to tied in below the water heater..see image
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Last edited by frodo; 02-15-2017 at 09:01 PM.