hi
there are some points to be noted for the pid control;
1) what will be your final control element, which will allow you to use it in the active region most of the time.
2)whether it is really required a pid control for a slow process like temperature
3)what is the algorithm you are planning to use for the pid control and whether it is possible for the 8051 to implement it.
(((I DON'T MEAN 8051 IS SOO WEAK IN THESE AREAS, BUT I AM)))
best of luck
saji