How do I design a high powered micro-controlled PID with triacs
If you are using heater, then a better solution is to use PWM technique with zero cross SSR / triac, rather than dimming with phase control. The entire process won't "see" the difference if you choose for example a 10s PWM period and turn the opto triac with zero cross with a timer controlled output. The phase control also produces EMI and has nonlinear output vs. control phase angle, additionaly you need a zero cross detector.
It makes sense to use phase control for dimming lights or for motor control.

EDIT:
Difference between phase angle control and PWM control of a triac/SSR