I'm the the middle of an involved HVAC remodel to a 1960s split level that started with oil-fired hydronic heat, no central air, no hot water tank (tankless coil in boiler instead) and no NG service. My goal is to lose the oil. I had a air source heat pump installed with attic ducting, sized to carry demand down to typical january temps (4 tons for 2200 sq ft in a relatively mild climate). For space heating/cooling, very pleased with system. The next step is clearly the DHW, which is killing me. I have estimated ~200 gals oil/yr (half for water, half for standby losses) to get a system with a max output of ~2.5 gpm (e.g. you can shower as long as you like, but we have to 'take turns' with the usage). In other words $600-700/yr for a lousy demand DHW service. Original plan was to pull the boiler, and sit an electric tank DHW heater in the same spot in my finished basement laundry room. But, since I might run NG service to the house for other appliances, (running NG service turned out to be much cheaper than I expected) I am now wondering if I can use a NG DHW tank for lower operating cost, faster recovery, etc. (Not interested in any demand DHW systems, btw) So, my current boiler is vented through a 25' interior tile flue, which appears to have a 5.5" SS flex liner the whole way. When I pull the boiler, this vent will be unused. So a major issue is if I can use the existing, orphaned vent for a new, high-eff DHW heater. Power venting out the wall would be a pain, given the layout of the space. Can I direct or power vent out the chimney? Will I have condensation problems in the chimney? Drop a small liner down inside? Right now, the flex is mortared in 60" off the floor, so I would to bump the height anyway, or buy a shorty heater. Am I barking up the wrong tree here? Any advice would be welcome.