Jessey,

If you can accurately control the temperature of the water, then you don't need a flow sensor. Just maintain the water at a slightly elevated temperature, under no flow conditions. Once the tap is turned on, the water temperature will suddenly decrease. Detect the change to turn on the heaters. When the tap is closed the heaters will turn off, due to normal temp control. If they are off for a certain amount of time, revert to standby mode, which allows it to slowly cool off again.

That seems to be how our's work. It has a relay that clicks on for about 10-15 seconds every 20-30 minutes, just to keep it warmed up.

.