The DVFD was alerted on a Box Assignment for smoke in the structure at the Millsboro Buffet. E731 under the command of Lt. C. Hudson responded with a crew of (5) five. E832 arrived to find light smoke through out & from the roof area of a 1 story restruant. E731 arrived as the second due Engine & secured a water supply at the hydrant on the Bravo side of the structure. The crew of E731 reported to IC & was assigned to division 1 w/ their Thermal Imager to check the kitchen area. E732 & Rescue Co. 73 were staged & advised to hold their manpower. After investigating it was determined to be a malfunctioning HVAC unit on the roof. IC placed the incident under control releasing units as ready. The DVFD operated for approx. 45 mins.