We’re not building a Nuclear Submarine here, these are pretty simple systems, a Normally open valve isn’t going to work on the city water supply to the pump, when pump is stop there isn’t anything to stop the flow of water out to bay. You need to use a Normally closed solenoid valve with a SPDT relay to take the solenoid valve load off the timer. With only 3 bays I wouldn’t mess around with a gravity fed hot water system I’d just go direct feed and be done with it. You could use this valve
http://www.kleen-ritecorp.com/p-3397-dole-dual-inlet-valve.aspx Did they get your Navien 240 installed on Friday?