Part pseudo code… but should help stimulate the imagination and start the learning curve.
int variable1; // or float if required
int variable2; // or float if required
void DoSomethingNow() {
// check temperature, store value in variable1
timer.setTimeout(30000, DoSomethingLater); // will run this function in 30 seconds
}
void DoSomethingLater() {
// check temperature, store in variable2
// if variable 2 < variable1 then activate relay until next check
// else deactivate relay
}