>Just detect the water temperature, using your sensor
>mounted very close to the heater and switch the heater
>when temp drops below the setpoint. You will probably
>get a few seconds thermal lag anyway, but if not try
>and switch the heater with at least 5 seconds on,
>if your period is too short you risk element failure
>from the continual thermal stresses. It should regulate
>within 0.1'C with no probs.

I used to do this with my spa pool using a CA3059 to read the sensor and
drive a Triac to control the heater. Worked extremely well.

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