Need help with understanding PID control

[richard]

with a little bit of experimenting you should be able to find the approximate amount of drive needed to maintain your chosen temperature . then use the p_error * kp _value to vary your pwm drive about this point. its best to start with a small kp and increase it till the temp oscillates about your set point . if the load conditions vary substancially however or you wish to vary the set point over a large range you may need use a full pid filter .
ps look up pid in Wikipedia for a good insight