I have a 40 year + old Ford Oil Fired water heater and a 40 year+ williamson 5 in 1 furnace. So its time!!!. the furnace has a 6" flue and the old water heater has a 6" flue. They are both Tee'd in to an Amerivent Model FB 7" Factory Built all Fuel Chimney. The origional builder did a real nice job, see pic. I am looking at the Toyotomi OM148 instant hot water heater ( 148K BTU )and a new Generic oil furnace/AC unit of about 150K BTU/ 3.5 Ton. The million dollar question is can I Vent the new Toyo water heater ( 148K BTU ) and the new furnace ( 150K BTU ) through the existing 7" chimney. The Toyo stove has a 4" vent that has a 5" chimney adapter accessory and is a positive draft design and the furnace will be 6" natural draft. I know the rule smaller vent on top where entering main stack and also the Toyo pos draft Vent to be atached with Y pipe. Not sure how the positve draft and neutral draft work in harmony if they do at all.
The NFPA31- 6.5.19 states, Connectors serving appliances operating under natural draft shall not be connected into any portion of a mechanical draft system operating under positive preasure.
With that being said, i only have a hundred and one visions of what they mean. Any Plumber out there who really comprehend this stuff.