CHP cannot be directly and solely connected to HWS.
At this time, you cannot specify a CHP solely for HWS generation. A CHP system can only be attached to the Generator of a HVAC system. To take advantage of CHP for HWS generation, your HWS generator must be the same as the HVAC system (i.e. "Same as zone x heat source"). From the iSBEM modeling guide:
A possible work around for this limitation would be to create a new 'empty' zone in the project. Set up the empty zone (Room) with a very small volume and no occupants. You can specify a separate heating system for the zone, attach a CHP system to the heating system set to 0% Heating Supplied, 100% Water Supplied and then create you HWS system so it is powered by your empty zones heat source.