A space heater connected to an extension cord apparently sparked a fire Monday evening that badly damaged a three-story house in Butler’s Institute Hill section.

Two tenants at the home at 408 Second St. managed to get out safely after the fire ignited shortly before 6:30 p.m.

Three Butler firefighters suffered minor injuries battling the blaze that caused about $250,000 in damage, fire officials said.

“It was a fast-moving fire but the guys did a good job to knock it down,” said Butler Fire Chief Nick Ban. ”They made a very aggressive attack.”

The home’s balloon frame, common in houses built in the late 1800s and early 1900s, has no fire stops, which made it easy for the fire to quickly spread to the upper floors.

The house, owned by Dane and Agnes Aveard, is about 150 years old. It has been converted into three apartments.

The couple live on the second floor. Their grand daughter, Katelynn Trombetto, lives on the third floor.

The Aveards were eating out and Trombetto was at work when the fire hit.

The couple returned home and saw smoke coming from the rear of the house. They were the first to notify 911, Ban said.

Fire officials suspect the blaze started in the back stairwell between the first and second floors.

A space heater had been placed in that stairwell, just below a bathroom. The heater was to make sure the pipes didn’t freeze.

“The outlet in the stairwell didn’t work,” Ban said, “and an extension cord was being used from the kitchen, down the stairs.”

The overloaded extension cord possibly ignited the space heater fire, authorities said. Trooper Dave Jungling, a state police deputy fire marshal, is leading the investigation,

The Aveards’ immediate concern when they noticed the house was on fire was the safety of their tenants, Susan Schuler and her roommate, who live on the first floor.

The Aveards hurried to the rear of the house and tried to get in the door.

“It’s a steel door,” Ban said, “and it was too hot to open.”

The couple went to the front and opened the door. Almost simultaneously, the tenants came out unharmed, authorities said.

Dane Aveard made his way up the steps to the second floor in hopes of rescuing his pet dog, a white Siberian Husky.

“He opened the door to the apartment but the heat pushed him back,” Ban said. He got of the house without the dog.

However, he left the door open, which helped save the animal. Firefighters found it hiding under a bed in the first-floor bedroom.

The dog was taken to Butler Veterinary Associates and Emergency Center in Center Township for treatment of smoke inhalation.

“She was fine,” Aveard said this morning.

A dozen Butler firefighters responded. The VA Butler Healthcare’s rapid intervention team assisted.

Three Butler firefighters were hurt. Ban described the injuries as a strained or sprained knee, ankle and back.

Damage to the house was estimated at $150,000, with an additional $100,000 in damage to its contents.

The Aveards, who have lived there for 15 years, had insurance.