Venting a Oil Fired Water Heater

Hi, I am just an average Joe. I am looking to put in an oil fired hot water heater and combine the venting with my oil fired furnace. The furnace vents into a clay lined chimney. Both units have a 6" flue I know I have to increase the diameter of the vent if I join the 2 appliances, I just don’t know how much? Also, everything I have read says the smaller vent needs to be on top, what about when they are the same size? Is there anything else I need to be concerned with? Thank You!

You may or may not have to increase the flue size. There’s a complex table that is used to determine flue sizes. You should contact a mechanical contractor. That’s a job for a licensed mechanical contractor.