Dear Steve:

I don't think I got what you mean from thoes question marks but if you mean why not converting 220v to DC by a bridge then my answer is there is no need for that because even half wave 220v Ac can be more than enough for say, a 100w heater.

I also thought about Burst mode control(It is much simpler, no need for pwm), but I'm not sure if it works(i have no experience in this regard).

I have no idea about exact thermal capacity of the material but roughly it is about thermal capacity of water.

and would you use ideal PID algorithm for control of this in a commercial product?

thank you very much

PS: sorry for delay