 But the city, for the first time this year, is keeping one of its cooling centers open 24-7 since officials found about a third of heat-related 911 calls were actually happening overnight or in the early morning. The city is also trying to respond to heat emergencies quicker. The Phoenix Fire Department this summer adopted a new method for immersing heat stroke patients in ice-filled bags to try to bring body temperatures down much faster in the ambulance on the way to the hospital.