Boston Fire Department

A three-alarm blaze sparked by a heater caused an estimated $5 million in damage at a condo building in the heart of downtown Boston this morning, fire officials said.

No one was injured. Resident John Walsh, the owner of the Elizabeth Grady spas, said he was taking a shower in his master bedroom when a fire alarm went off. He said he called 911, threw on some sweats, and then left the building. Officials said he was the only one home at the time.

The fire was reported at 9:18 a.m. at 17 Commonwealth Ave., a five-story brownstone, the Boston Fire Department said in an official tweet. Just before 10 a.m., Fire Department commanders declared a third alarm.

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Firefighters opened up the fourth-floor ceiling to get at the fire and chased fire in the walls, the department said.

Investigators determined that the cause of the fire was a radiant heater, mounted in the fourth-floor ceiling to heat a fifth-floor room, the department said.

Smoke poured out of the fourth and fifth floors, each of which constitute a single unit. Walsh said the occupants were on vacation. He said a total of about five people live in the building, which is in the upscale Back Bay area just down the street from the Public Garden and the Taj Boston hotel.

A major renovation is underway on the building next door, where a green tarp covered the facade, with about 30 workers on the site. Workers saw the smoke and also called authorities.