it doesn't send the signal to heat up.
And what does that mean? That uitsturing is not pwm'd on pin 3? If that's what you mean, how do you know that? Have you stuck a meter or a 'scope on the pin? If you serial print uitsturing is it the correct value? Is pin 3 a pwm pin on your board?- I know it is on a Uno but I don't know if it is on all boards, and you didn't say it's a Uno or whatever.
(btw, you assign the value 3 to heaterpin, and make heaterpin an output, but the analogWrite refers to the pin by number. Bit dangerous if you later make it pin 5 in the declare, but forgot to change the analogWrite. That's the whole point of using names ;) )
(Plus, I see that Anshu_Raj didn't get a compile, nor did I. That could explain why it's not working but I somehow think you would have noticed if it hadn't compiled. But it obviously means you haven't psted the last compilable version.....)