As I understand it, なら (and all ば-form verbs, because なら originally came from ならば) have a nuance of, “if and only if” to them. So, following that logic, this sentence is simultaneously implying, “Please don’t turn on the fan if it isn’t hot.”
I saw this illustrated with these examples:
お金を持ったらいい。。。
お金を持てばいい。。。
The first sentence sounds like, “It’d be good if I had money, but other things (maybe food/clothes) would be nice to have, too.” The second sentence sounds more like, “Money is the only thing I’m missing!! If only I had money…”
That being said, I don’t think “If and only if” necessarily works for all cases of なら… and this is all stuff I’m recalling from quite a while ago. I’m not confident that this perfectly portrays the nuance, but it’s helped me understand things like ～ばよかった as, “Things would’ve been good if only I’d done [verb], but I didn’t…” (In other words, “I wish I’d done [verb].”)
EDIT:
We can also compare/contrast どうしたらいい？ with どうすればいい？
I remember reading that どうすればいい？ is used more for things that have a well-defined answer, like maybe you’re asking about the next step in a set of instructions when you’re assembling furniture. It sounds to me like, “What must I do?” or, “What do I have to do?”
どうしたらいい？ is a more open-ended question, more likely used when there is no clear answer, but you want to do something to help the situation. どうしたらいい？ sounds closer to, “What can I do to try to make things better?” or, “What do you think I should do?”
