Senior Airmen David McMillen (front) and Hugh Blacketor, 28th Civil Engineer Squadron heating, ventilation, and air conditioning technicians, inspect their completed work after adding refrigerant to a chiller during maintenance at the base air traffic control tower on Ellsworth Air Force Base, S.D., June 28, 2012. HVAC specialists are responsible for maintaining heating, ventilation and air conditioning systems in facilities on base. (U.S. Air Force photo by Airman 1st Class Kate Thornton/Released)