Investigators say that in-floor radiant heating caused the three-alarm fire in a Commonwealth Avenue brownstone Wednesday morning that resulted in $5 million of damage but no injuries. 

Firefighters responded to the five-story, three-condo building at 9:18 a.m. and quickly called for backup. 

"They were here in a matter seconds," John Walsh, who lives on the third floor of 17 Commonwealth Ave. and was the only one of the building's occupants who was home when alarms sounded, said of the Boston Fire Department.

"When they came into the building the assessed that there was some smoke coming. Without any hesitation, they called for additional backup and they went right into the building, up the stairs," he said. 

Walsh said he was in the shower when he heard the alarms go off. He got out, dried off, got dressed and went to check the panel downstairs, where he saw that the smoke detectors on the fourth and fifth floors went off. As he went to call the building management, the fire department arrived, he said. He then evacuated the building.

Firefighters had to pull down the ceiling in the upper condo, which is on the fourth and fifth floors of the brownstone. The resident of that condo lives alone and was away on business, according to Walsh. 

Not only did the fire destroy the top condo, Walsh's condo suffered extensive water damage. 

"Water is just pouring down," into the third floor, fire spokesman Steve MacDonald told reporters at the scene. 

Firefighters put out the fire within 35 minutes, MacDonald said. 

The adjacent buildings didn't appear to suffer damage. There was a brick firewall between 17 Commonwealth Ave. and the building next door.