A fire alarm connected to a home security system alerted firefighters to a blaze at a multimillion-dollar Mill Neck estate that erupted while its owners weren’t home — saving much of the house from being engulfed, a fire official said Saturday morning.

Kevin Barry, an assistant fire chief with the Locust Valley Fire Department, said more than 200 firefighters from a dozen local departments brought the fire Friday at the home on Heather Lane under control, including extinguishing all its hot spots, by about 12:30 a.m. Saturday. The blaze started just before 8:50 p.m.

“It was originally picked up by the fire alarm in the house, which allowed us to get to it sooner because no one was home to call it in,” Barry said. “The house would have had to been halfway up in flames before someone, a neighbor, could see it. The house is so remote, it would have burned a lot longer before someone else saw it.”

The owners showed up to the home shortly before the departments and their apparatus left the scene about 3:30 a.m., Barry said.

The fire damage was contained to the upper floor of the house, Barry said, but other portions sustained heavy water damage.

“The amount of fire wasn’t large but the house was tremendous,” Barry said. “Its layout and sheer size just made it difficult to operate and get inside up in the ceilings and walls. It was very labor intensive.”

In addition to all the departments at the scene, extra units were kept waiting at the Locust Valley headquarters as backup.

“Just based on the size of the house, we wanted to make sure we had resources ready if things turned south,” Barry said. “But thankfully, it didn’t.”

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Water problems, along with small access roads, thwarted efforts Friday night to put out the flames quickly, prompting fire officials to call for water tankers from as far as Yaphank.

A Locust Valley firefighter was taken to Nassau University Medical Center for exhaustion, Mike Uttaro, Nassau assistant chief fire marshal, said from the scene early Saturday morning. The injured firefighter was in stable condition Saturday morning, Barry said, and would likely return home within the day. No other injuries were reported, Barry said.

“It was a difficult fire,” Uttaro said.

Since no one was home at the time, the first firefighters forced their way inside, he said.

The fire tore through the entire 100-foot-long roof, he said, and it started to collapse.

On top of that, the water pressure was low but the cause of the problem was not immediately clear, Uttaro said.

Nassau County police arson squad were at the scene. A police spokeswoman Saturday morning did not have any further information about the investigation.