That's it, Andreas! Outsider is completely right. tempC will never have a value less than or equal 20 AND greater than 30 at the same time (this is for the first if statement on your code). It will be either less than 32 OR greater than 30. If you want to check out whether the tempC ranges from 20 to 30 then you need to code:
if (tempC >=20 && tempC <30){
.
.
.
}