how about a timer? either a timed relay, or something more complex like http://www.dse.co.nz/cgi-bin/dse.sto...uct/View/K2807
the water temp goes down enough to supply 12v to the timer. this sets it off, which turns on a relay to turn the pump on. the timer is preset to say 20 seconds or however long it takes to fill up the main level
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