A three-alarm fire sparked by a heater and reported around 9:20 a.m. at 17 Commonwealth Ave.  caused no injuries but did an estimated $5 million in damage, according to the Boston Fire Department .
The fire spread through ductwork to the fourth floor of the five-story brownstone, the department reports.
Around 10:20 a.m., after the bulk of the fire had been knocked down, the third-floor ceiling collapsed, likely because of the weight of water used to combat the fire; no firefighters were underneath it.
The department says the one resident home at the time escaped without injury. The department adds:
Investigators determined cause to be an in-floor radiant heater mounted in the 4th floor ceiling to heat a 5th floor room.