To maintain a decent draft, there are limits on how far you can go...it depends on the manufacturer. I'd prefer maybe a powervent in this situation that vented out the sidewall. ALso, keep in mind, unless your BTU requirements are fairly small, a WH may not be able to supply your needs. The type of heat loops (radiators, in-floor, panels, etc) each require different temps. Some require higher than a typical WH can provide. The average WH probably won't have the reliability of a boiler, either. Today's modulating HE boilers are in the mid-90% efficiency range and can easily provide all the DHW you want more efficiently than a WH.