Carbaugh said the fire took off when the doors were opened, allowing air to get inside the house and feed oxygen to the fire.
Minnie Carbaugh died of carbon monoxide poisoning due to smoke inhalation, Conner said. No autopsy is planned, he said.
A malfunctioning natural gas space heater was the cause of the fire, said Trooper Jeffrey Sarver, a Pennsylvania State Police Fire Marshal. He said two distribution fans in the unit had not been working "for some time," causing heat to build up in the heater.
The fire inside the unit spread through a vent pipe connected to a chimney and into the walls. He said the old, two-story, wood-frame house was of "balloon construction" with no fire stops in the walls to limit the fire from spreading.
Carbaugh was found at the kitchen table, Sarver said. Witnesses told him they had initially seen smoke coming from the chimney and then from under the eaves of the house, said Sarver, who added there were no working smoke detectors in the house.
Martin Carbaugh said he and other family members looked after Minnie Carbaugh, who had lived in the house for about 40 years.
"She lived alone. She was very independent. She wouldn't take anything from anybody," he said.
Minnie Carbaugh is survived by two daughters, five sons and a number of grandchildren, family members said.
Chambersburg Assistant Fire Chief Norman Reitz said the fire was called in at 9:22 a.m. with a report of someone possibly trapped inside. The Franklin Volunteer Fire Co. and the Medic unit from Chambersburg Hospital also were at the scene.
It was the second fatal fire in Franklin County in less than three months. On Dec. 10, Joe and Joan Bowser and their 6-year-old grandson, Elijah J. Virago of Catonsville, Md., died in a fire at the Bowsers' East Franklin Street home in Greencastle, Pa.