You have to run the pump for ~5 minutes after the heater is shut off. Otherwise, the remaining water in the heater boils off and leaves scale and junk in the heater, meaning it won't last terribly long.
The simplest way to arrange this would be a delay to off relay triggered by the pilot controls for the heater. If you have a pool control system (ie aqualink) it takes care of this for youat all times.
mike
