It is ok according to Massachusetts code to run an oil or nat gas or propane fired water heater up thy same flu as thy boiler-furnace.
Provided ye shall compensate ye local Bay State building inspector. Ho-ho....
......chuckles....but it really is ok to run a hw heater and boiler into same flu.......just needs correct flu pipe set-up for it to work ok.....with no back drafts or smelly black oil smoke.