Let me try one more description. When some one in the house turns on the hot water, cold potable water starts travelling through the plate HX to feed the water heater. My question is this: what is available at reasonable cost to monitor the potable water flow so the boiler side circ pump can be turned on at that time (or zone valve, I haven't decided yet). I have a back ground in industrial controls, so I'm familiar with many different types of flow monitoring sensors. But everything is industrial and very expensive.