re: #194 b_sharp

I have all kinds of project boxes, electronic sensors, relays, valves and pumps around. From the lower tank I can set the return valve to shut off if the water flow from the sending pumps stops for any reason, including a power outage.

Actually you don’t need a shutoff switch with a well designed overflow system. The overflow is set so it is at the water level in the top tank (it determines the water level). You have a pump in the lower tank that pumps water up to the top tank (be sure to incorporate anti-siphon holes in the return just above the water level of the top tank). As water fills the top tank anything that goes above the level of the overflow drains out and back down to the lower tank. If the power goes out then the pump simply stops pumping water into the top tank. The water in the return line drains into the bottom tank but the anti-siphon holes break any siphon and no more water flows from either tank.