There's no middle ground.
You can get a hot room with a freezing shower for 250 baht. If you want it the other way around, it'll cost you twice as much.
Fan rooms are miserable. If you get a ceiling fan, it won't do much at all. If you get an oscillating wall-mounted fan, it's actually quite bearable for the five seconds out of every thirty that it's pointing at you.
Suffering through this means that, when you finally do get an air-conditioned room through accident or economic insanity, you go completely berserk.
Your hostel has planned for this. You can expect an individual remote-controlled split-system air conditioner unit that's completely out of proportion to the size of the room. The thermostat will go all the way down to 16 degrees, which is the only setting worth considering.
In some guesthouses you'll also get a Freezer Death Ray. This is a very large Mitsubishi fan bolted to a sturdy wall at right angles to the AC unit and aimed directly at the bed. It serves to collect the cool breeze gently wafting from the AC unit, focus it into a firehose of arctic air, and launch it directly at your now-whimpering form.
To start with this seems like a good idea, but around 1am you'll wake up shivering, and lie paralysed by tiredness and cold for several hours. At this point you'll remember that you own a high-tech synthetic sleeping bag that's rated to five degrees below zero. It's hard to miss; it takes up half your pack.
Obviously you have to get into the sleeping bag. You paid for that A/C. You're not turning it off.
You'll wake up early and book yourself into a fan room for the following night.