Police in South Carolina found an elderly couple dead inside their home where the heater had been reached over 1,000 degrees Fahrenheit (537C), according to a report.
Officials in Spartanburg, South Carolina, said they were sent to conduct a welfare check on Saturday, where they found the couple dead in the "extremely hot" home, according to WYFF.
The couple's family called the police after not seeing or hearing from them in several days.
The doors to the home were locked when police arrived, but a window was accessible. They entered through a window leading to the bedroom, where they found the elderly couple dead.
Firefighters at the scene found the heater in the home's basement was running extremely hot. The temperature reading on the heater was reportedly over 1,000 degrees, WSPA reported.
"They then measured the temperature of the heater itself at more than 1,000 degrees," the police report said.
One firefighter described the scene, saying "the heater was so hot it looked as if the basement was currently on fire,” according to The Messenger.
The heater did not catch fire, which allowed officials to deactivate the appliance.
Fire officials opened the home's windows to the cold outside air. After 20 minutes with the windows open, the house's internal temperature was still in excess of 120 degrees (48C), the officials said.
The elderly couple were eventually identified as Joan Littlejohn, 84, and Glennwood Fowler, 82. Mr Fowler lived at the home and Ms Littlejohn was staying with him.
The Spartanburg County Coroner said that no foul play was involved in their death.
The coroner is still working to determine the cause of death.
The family told officials that they had helped the couple with their heater, which the couple said was running too cold. They said they "fiddled" with a wire until a pilot light turned on.
