This is an extract from about.com.
"If the temperature is too low or too high, the zooxanthellea, which most corals require for survival, will die or vacate the polyps. When the algae leaves the coral, it exposes the white base calcium carbonate of the coral, it is sometimes called Coral Bleaching."
A general guideline is to adjust the temperature of your reef to the same temperature range that your corals and livestock are naturally found in.
It sounds to me that you are putting the servicing of your chiller off due to finances? If that's the reason, I would suggest to use a fan as an interim solution till you have saved up enough to fix the chiller. Losing corals and livestock due to temperature can be more expensive than servicing your chiller.