I just removed an old oil burner and replaced it with a natural gas boiler. For cost reasons I am also installing a natural gas water heater. Both are atmospheric vented. Can I put the 2 exhaust pipes together in a Y and vent up through the chimney using the old oil burner piping?
Your whole idea won't work. A modern condensing appliance will be vented out the side of your house through a 4-inch PVC pipe; it won't vent properly through the existing flue (the exhaust gases aren't hot enough). And no, they each need their own vent. Read the installation directions again.
A natural Gas Boiler has to have its own exhaust pipe seperate. If you join another appliance into the same flue, it can impede the exhaust gasses from the boiler escaping efficiently. When this happens the boiler will shut down automatically.
