Senior Airman David McMillen, 28th Civil Engineer Squadron heating, ventilation and air conditioning technician, installs an exhaust stack into a new boiler system in the base Exchange at Ellsworth Air Force Base, S.D., Aug. 15, 2013. McMillen is responsible for installing and maintaining heating, ventilation and air conditioning systems on all base facilities. (U.S. Air Force photo by Airman 1st Class Zachary Hada/Released)