Firefighters responded to a three-story commercial building fire near the Healdsburg downtown plaza Thursday afternoon, shutting down a portion of the city’s main thoroughfare for two hours. The Thanksgiving Day fire started just before 4:30 p.m. in the Brandt Insurance building in the 200 block of Healdsburg Avenue. A malfunctioning heater in the third-floor ceiling caused the smoke and flames, which set off the building’s internal sprinkler system, Healdsburg Fire Marshal Linda Collister said. Crews with Cal Fire, Cloverdale, Dry Creek Rancheria, Geyserville, Rincon Valley, Santa Rosa and Windsor assisted the Healdsburg Fire Department in extinguishing the blaze within 40 minutes. The southbound lanes of Healdsburg Avenue were reopened just after 6 p.m. Thursday. The building, which houses the insurance company, as well as retail stores on the first and second floors, was saved by its sprinkler system and had no structural, Collister said. She estimated the damage at $20,000-$30,000.