AURORA, Colo. -- A maintenance man with Metro Brokers was severely burned Saturday morning when he switched on the furnace in a vacant house and the home exploded, 7NEWS reported.

Juan Quintanilla, 42, was taken to University Medical Center with second and third degree burns on 50 percent of his body, including his face. He is listed in critical condition.

The house was for sale and Quintanilla was at the home on behalf of the real estate company to prepare it for an open house. Firefighters say the victim was trying to light the furnace when the house blew up, just after 9 a.m.

"There was one big bang. I never heard anything like it," said neighbor Rhonda Matsumoto. "Part of the house was in the street ... And I saw the guy run out, holding his head like he was delirious, and then ran to his neighbor's ... and just knelt down on the grass."

When firefighters arrived, the first level of the home -- from the front door to the back patio -- was blown out. The two-story home is considered a total loss.

Firefighters said a natural gas leak caused the explosion and have ruled the blast accidental.