I think some people may be missing AndrewH's very valid points about the particularly high temperatures and also later about looking good in respect of your reputation and other bookings.Hello lucywho. The A/C was missing for almost the whole time in just one bedroom out of three. With only 4 people staying you wonder how much use that bedroom got. However, to my mind that is not really the point. If you book fully air conditioned accommodation then that is what you are paying for, and that is what you can expect. Summer temperatures in Turkey must be very high, so A/C is a 'must' if you want to be comfortable.
How much to refund to these guests is subjective and opinions will vary. If it were me in these circumstances, I would be offering a painful 25% refund.
I would also be inclined to refund more than the actual cost of the AC, but then I do not want any disgruntled or unhappy guests!
Another factor is also how "high end" the property is, as then expectations are higher.
If the same happened in a hotel and the AC was not able to be quickly fixed, guests would probably demand a full refund! (Not that I think that is right).
Perhaps Lucywho could come back to us and let up us know what she offered and what guests were expecting??