Hi Al,

I agree, phase angle control is nice but IMO overkill for a system like this.

The heater itself and the system as whole is most likely quite "slow". Creating a low frequency PWM with a period of 1 or even 10 seconds will likely work just fine.

That way the slight error introduced by the solid state relay (or triac) not shutting off until next zero-crossing can be disregarded. (Max 1% with 1s period or 0.1% with a 10 second PWM period.) Depending on if the SSR is "random firing" or not it'll be a little bit different but still good enough IMHO.

Of course it depends on the power of the heater, the amount of water in the tank and the amount of "temperature ripple" you can tolerate but I don't think the tank will "respond" fast enough that a 1 or even 10 second PWM period won't work.

/Henrik.