Hi Barry:

I used a timer nearly identical to this:   https://www.amazon.com/ustyle-Digital-Programmable-Precision-Anti-Interference/dp/B07TC9Z45J/ref=sr_1_7?keywords=24+volt+fish+feeder+timer&qid=1564180095&s=gateway&sr=8-7 

I believe mine is just a slightly older version.   I wired it's output to a second "ganged" 24 volt relay just to make sure I didn't overload or shorten the life of the contacts of the relay in this timer.  The coil of the second relay is wired such that when the timer calls for flushing (i.e. the timer closes its "NO" (normally open) contact, it energizes the more robust relay coil.  The second relays contacts are wired into the fresh water pressurization CB (circuit breaker)  wiring.  When I leave the boat, I switch the timer from the "run" (i.e. always on) position to the timer position then move the valve in the engine room to the flush position.  Thus when flush is called for the timer, energizes the second "ganged" relay, turns on the fresh water pump that runs for the programmed length of time (in my case 2 minutes every other day).  My system goes thru about 200 liters of H20 in 6 months on the hard using this schedule.  Others may vary based on the pump etc.  I just put a bucket where the flush water comes out on the port side of the boat, ran the program and measured the through-put of the water.  The only down side is that when on the boat and the timer is set to the run position, the ganged really is continuously energized (using a small amount of electricity) and you have to leave the main battery switches on to provide 24 Vdc power to the timer/relay circuity.  Mine has been running perfectly for about 11-12 years now.

Sincerely, 

Gary S. Silver
s/v Liahona    Amel SM 2000 #335
On the hard in Puerto Del Rey - Puerto Rico