An everyday household appliance sparked flames last month in Lake County that grew into California’s largest wildfire this year, state authorities said Wednesday, detailing for the first time the suspected cause of the Rocky fire, which blackened nearly 70,000 acres, destroyed 43 homes and put thousands of residents under evacuation orders and alerts.

The initial flames in the massive blaze, which erupted July 29 southeast of Clearlake, were traced back to a faulty gas-powered water heater inside an outbuilding in southern Lake County, Cal Fire officials said.

They spread from the structure off Morgan Valley Road to the surrounding wildland and merged later with a second fire that erupted a few miles away and remains under investigation, officials said. The combined blaze then raced across rural properties and public forestland, encroaching on 1,200 residents of Lower Lake and hopping a major highway through the area before firefighters brought it under control last week.

It remains unclear how the water heater actually ignited the fire, Cal Fire officials said.

“We need to figure out why that water heater failed,” said Cal Fire spokesman Mike Mohler.

Mohler said he did not know whether a residence was associated with the water heater or who owned the property on which it was located.

He declined to elaborate on how fire investigators pinpointed the source of the flames. Several Cal Fire investigators have been working this month on determining the cause of the Rocky fire and the adjacent Jerusalem fire, which has burned an additional 25,118 acres since it ignited Aug. 9. That newer fire, which was 95 percent contained Wednesday, also remains under investigation.

Water heaters spark thousands of fires annually in the United States, according to the National Fire Protection Association.

Anana Bliss, whose Morgan Valley home of 35 years narrowly escaped the flames that singed her land, said she had her own experience with a faulty water heater and flames some years ago. She was trying to a repair a leak in her water heater at the time.

“It caught on fire,” Bliss said.

She guessed that the water heater that triggered the Rocky fire was propane, like hers, because there are no natural gas or electric utility services in the Morgan Valley area.

In 2011, the last year listed in National Fire Protection Association statistics online, water heaters sparked some 5,900 home fires. Of those, about 56 percent were started by gas-fueled water heaters.

Water heaters accounted for 11 percent of fires started by home heating devices from 2007 to 2011, according to the association’s website.

In 2011, water heater-triggered fires resulted in 20 civilian deaths, 170 injuries and $139 million in direct property damage, according to the fire protection association.

You can reach Staff Writer Glenda Anderson at 462-6473 or glenda.anderson@pressdemocrat.com. On Twitter @MendoReporter.