On Monday, fire tore through Villa Capri, an assisted-living and memory-care facility on Fountaingrove Parkway across from Keysight Technologies in Santa Rosa.

A main building next to the parking lot was obliterated, but Varenna residential structures remained intact. Four fire engines from San Francisco’s fire department were on site Tuesday morning, one day after the flames. They arrived at about 5 a.m. just after fire raced through Fountaingrove. San Francisco sent another five engines to fight fire near Glen Ellen.

The Fountaingrove business district was patrolled Tuesday by police and firefighters from San Francisco, Oakland, Contra Costa and San Mateo Counties, plus many others. Firefighters from Los Angeles stopped into the REI store on Santa Rosa Avenue to buy needed gear.

A wheelchair draped with a blanket sat abandoned in the parking lot in front of a structure in ruins, apparent signs of hasty evacuation by residents, many of whom suffer from Alzheimer’s or other forms of dementia.

“We’re standing by,” said Ian, a San Francisco firefighter who commutes to the city from his home in Cotati. He declined to give his last name. “We do 24-hour shifts,” he said. He has been fighting fire for 19 years.

“We had a structure fire up there last night,” he said, nearly a day after the giant flames whipped through, powered by wind that gusted beyond 50 miles an hour. “It was a big two-story apartment complex,” Ian said.

“Amazing,” he said of the fire’s remains. He relieved a crew of San Francisco firefighters who had come even earlier. “They went over to the Kmart” on Cleveland Avenue across Mendocino Avenue, he said.

“When they got there it was totally involved. They saved a bunch of homes on that side. They stopped it. It could have kept going. I do a lot of road cycling out there (west of Santa Rosa). I’m pretty familiar with all those roads. It would have been incredible, gone all the way to the coast.”

When gargantuan fires burn out of control, firefighters are sometimes overwhelmed and can do nothing to stop the inferno, Ian said.

“I imagine they will keep us all around,” Ian said, as he sat in the driver’s seat of a fire engine.

On Monday his crew worked at the Fountaingrove Golf and Athletic Club up the hill.

His buddy who lives in Larkfield lost his home on Pacific Heights Drive near Molsberry Market in the Larkfield Shopping Center on Old Redwood Highway. “It looks like something from a movie set,” Ian said.

His San Francisco firefighting crew worked to defend some of the structures on the Keysight Technologies campus on the south side of Fountaingrove Parkway.

“Our guys actually saved the roof of that building yesterday,” Ian said, likely Building 1 next to modular buildings Vista East and Vista West, which both burned completely. “It started catching on fire. They are very lucky,” he said. “It’s weird when you drive into the parking lot and see the car sitting there” that was incinerated. “Then the one next to it is fine.”

In nearly two decades battling blazes, Ian studied fire behavior, noting that it has become a distinct scientific discipline. “Lately they have been doing a lot of studies in structure fires where firefighters have died,” he said. “They’re relating it to fire flow path. That has to do not just with the way the fire moves through the building because of what’s there to burn,” he said, “but with the construction of the building, how different pressures (exist) from having a door open or closed. It’s pushing the fire and affecting it.”

Wind outside a burning building applies a rapidly shifting set of pressures inside the structure, he said, and the status of doors inside can have a significant effect on where flames go. “We recently had two guys die in a fire,” Ian said. “It was a below-grade fire. It had some wind down there. It was like an inferno” that blew up and killed the men before they could escape.

Fire departments are revising policies and procedures “to handle those fires differently,” Ian said.

Los Angeles County Fire Department created a training video on fire behavior and flow-path door control, with suggested tactical considerations for differing fire environments.

“There’s a big difference between tactics and theory,” said Steve Kerber, a fire-research engineer at Underwriters Laboratories. He studies ways to improve firefighter safety, fire-service ventilation, lightweight construction and smoke-management fire modeling. “Practical applications must be developed by your department.”

Kerber spent 13 years with the fire service in Maryland. He earned bachelor’s and master’s degrees in fire-protection engineering then pursued a Ph.D. in fire-safety engineering at Lund University in Sweden.

Whether a firefighting rig has two or six responders affects how they should proceed in fighting active flames, Kerber said. “You can’t test every possible configuration,” he said, noting how many variables impinge on fire behavior. “There aren’t enough fires to make you an expert so we need research to bridge the gap.”

Tactics might suggest entering a burning building through the front door in certain types of blazes, Kerber said, then opening a window and cutting a hole in the roof. “We want to make sure you guys aren’t guessing on the fireground.”

Deciding whether or not to ventilate a burning structure is part of firefighting strategy. Some firefighters adopted strategies of only applying water from outside a flaming structure, or never working over a basement fire because it might explode upward. But that’s not necessarily the best way to fight the blaze, Kerber said.

Larger homes, more open spaces, expanding fuel loads and different building materials make today’s fires behave differently than they did in older structures, Kerber said. Typically fires propagate much faster and burn hotter today, he said, with more rapid flashover.

A flashover occurs when combustible material in the structure ignites nearly simultaneously. For example, a burning couch might generate hot smoke that spreads across the ceiling of a living room. Radiated heat from the smoke heats all the combustible elements in the room, which emit flammable gases. Those gases can explode in a flashover that instantly involves the entire structure. Flashover typically happens at nearly 1,000 degrees Fahrenheit.

A rich flashover happens when flammable gases ignite when their temperatures reach the upper region of the flammability range, such as in rooms where fire slowed due to lack of oxygen. A smoldering item can ignite the rich flashover, also called a backdraft.

A lean flashover refers to ignition of flammable gases under a ceiling, sending the fire rapidly into the entire structure. In a lean flashover, gas temperatures are at the lower end of their flammability range.

A delayed flashover or smoke explosion happens when cool gray smoke moves from one room to another then explodes violently.

A hot-rich flashover occurs when smoke’s temperatures rises above the upper limit of its flammability range then leaves the building, exposing it to fresh air and causing spontaneous ignition. The burst of flame can then turn and go back into the burning building, sometimes causing a rich flashover.

“They are creating problems we don’t even understand yet,” Kerber said of current fire environment.

“They were analyzing a few fires that were fatal,” Ian said of the research, “and how to prevent it.”

James Dunn covers technology, biotech, law, the food industry, and banking and finance. Reach him at: james.dunn@busjrnl.com or 707-521-4257