I don't have air conditioning...
I've been having some of the same worries lately, particularly since I just moved into a new apartment with no air conditioning (they have their own room for their cage which, luckily, does not have a hot air vent in it and is windowless which seems to suit them just fine as long as I let them run around in my living room when I'm home- the one likes to sit on the sill of my balcony door). I find my chins seem to be okay if I keep the lights off (it keeps the temp. down), and keep the door open to allow air to circulate. I also keep a thermometer in the room so I can ensure that the temperature never spikes over about 22 celsuis. One of my friends found some tiles in her basement left over from rennovations that the landlord did so I rotate those in the freezer and the cage so they can lay on those to keep themselves cool, and I also bought a couple of small ceramic hamster-sized food bowls which I fill with water and freeze into essentially big ice cubes, which I then put into my chin's cage one at a time. They like to lick the ice as it cools them down and will also sometimes curl up right beside the bowl as it cools them off. I also have been giving them refridgerated water lately, as well as occasionally cooling their pellet food off in the fridge.
Unfortunately I can't install an air conditioner (it's against the terms of my lease), but I do have a back up plan were it to get too hot because I knew as soon as I moved into my current place that it would be imperative. I have several friends (as well as my boyfriend) who have central air conditioning and have volunteered to temporarily take the chins in, and if worse comes to worse I'll move them back into the house that I grew up in for the summer (only 2 hours away, I moved away for university), and my sister (their half-mommy) will take care of them while I have to work during the week and I can go home and take care of them Friday through Sunday.