Horse Summer Sheet

Where I live, it's about 100 degrees in the summer, and in the winter the highs are in the 30-40's. My horses have lived here all their lives and are well adapted to this weather. At what temperature should i not hose down my horses when they are really sweaty? I do not have heated water or a heated barn, but i put coolers and sheets on them and put them in the barn out of the wind.