Hey wait a minute...the solid state devices you all mentioned are generally used for rapid switching, right? So by varying the firing angle, you will get continuous control of the heating element...whereas what Chris had in mind is simple ON-OFF control.

Of course continuous control can be more optimal, it will also be more complex and probably not suitable in this context. For one the control algorithm will be more complex too. Secondly the signal should to be analog, rather than simple digital inputs, to do any justice to the algorithm. I don't think all that is warranted for a simple home water heating system.

Of course, if Chris wants to make this project more challenging, he could go for continuous control ;-)

kundi