Are ACD's Heat tolerant?

Yes!! ACD/Heelers are made for the heat. 

 Aussie Heelers are said to be among the most heat tolerant of all dogs, and is one of the few breeds capable of working in the blisteringly hot temperatures common to the Australian Outback.  They are also famous for their relentless drive and energy, and are capable of tirelessly working for hour after hour.  

This breed is perfectly suited for the desert region of the southwest, but don't be surprised if they trick you into thinking they will die if you don't let them come in and sleep on your couch!!!