As I want to control the power been consumed by the heater to limit to the surplus power produced by the solar system and minimize RF noise im using the zero crossing switching of the Triac. So I will attempt to control the power by using burst mode control where the PWM period will be long enough to turn the heater element on for a few cycles and off for a few cycles. After a few attempts I settled on 10Hz as the pwm frequency. This covers 5 complete cycles. giving me 10 "power level" steps each step being 1/2 a cycle or 10ms.

As the PWM frequency is not syncronised and not exactly matched to the mains frequency there is a bit of drift in the actual numbe rof cycles that could be on in a given PWM period.

The esp8266 can signal the triac to turn on at any point in the cycle but the Zero crossing detector prevents the triac turning on untill mains voltage crosses zero. The minimum trigger needs to be greater than 10ms to guarantee that the triac will fire and hold for each half cycle it aslo then runs the risk of firing for more than 2 periods this "jitter" in this application I dont believe it to be a problem though testing with a light globe the extra cycle switched was visually evident - 10hz though can be easily seen as pulses and as such this is not suitable for light control. phase control running at the mains frequency would produce a smoother response but as a water heatre there is no need to introduce extra complexity in the circuit to reduce unwanted RF inteference for no significant gain in performance.