It is the phenomenon every firefighter worries about. Flashovers happen when hot smoke fills a room and causes everything to suddenly combust. If someone is caught in it, with or without protective gear, the result often times is death.
Thursday, smoke billowed out of the four story house on Berkeley Way in Diamond Heights. The home sits a hillside, which made it that much harder for firefighters to handle.
"The houses are sort of upside down. The front door is on top of the house and it is built into the side of the hill. So they went in with a hose line on top of the fire so all the heat is coming up at them," said SF Firefighters Union president Tom O'Connor.
What happened next, according to firefighters inside the house was a flashover. It's when all the smoke in the room rises to the ceiling, creating a thick dark layer. That's a hot, combustible cloud that can ignite the entire room. Abid Kemal is fire and combustion expert.
"It gets very hot very quickly. You're talking about thousands of degrees Fahrenheit. Firefighters' personal protective equipment can help save them, but only very briefly," said fire and combustion expert Abid Kemal from Exponent Failure Analysis Associates.
"You're in a furnace," said Menlo Park Fire Chief Harold Schapelhouman.
Schapelhouman has firsthand experience with flashovers. Now he trains fire fighters in a fire flashover simulator. It's the only one in the peninsula. Menlo Park bought it specifically after two Contra Costa County firefighters died in 2007, during a flashover. Schapelhouman's teams train in it annually.
"I want them to be exposed to the heat, I want them to see the smoke conditions, I want it to be as real as possible," said Schapelhouman.
The key to combating flashovers is ventilation. That's why often times, you'll see fire fighters cutting massive holes into the roofs of burning homes.
"The heat is being accumulated at the ceiling level. If you create ventilation, you can have that hot layer go out of the compartment," said Kemal.
A flashover can occur, less than five minutes after a fire starts.