PORTLAND — Investigators have concluded that an improperly installed gas water heater caused a fire in the Old Port on Sept. 29.

According to the fire investigation report released Wednesday to the Press Herald, the fire started in the basement of 420 Fore St.

Lt. Robert Pinder, the city’s fire and explosion investigator, said the fire was deemed accidental. A gas water heater in the basement was not properly vented and was not equipped with the necessary heat shields.

Pinder said that the heater — a Bosch Aquastar manufactured in 2002 that has since been discontinued — had a makeshift heat shield built from a 12-inch-by-24-inch piece of sheet metal and an aluminum pot cover. The shield had fallen off before or during the fire.

The heater should also have had a double-walled vent pipe vented outside. Instead there were two single-walled pipes, Pinder said. Single-walled pipes must be at least six inches from combustible material, but here the vent pipe was touching a wooden joist, he said.

“The improper installation of the gas, on-demand water heater and the failure of a homemade heat shield combined to expose the wood joist and adjacent electrical wires to hot exhaust gases every time the unit fired,” Pinder wrote.

The report does not indicate who installed the water heater. City Hall spokeswoman Nicole Clegg said that is for insurance companies to sort out.

Investigators list Michael Lionette as the maintenance person for the building. On Wednesday, Lionette, who has been doing light maintenance for about a year, said he didn’t think the building owner installed the gas water heater.

“We don’t know who installed it,” Lionette said. “We think one of the restaurant owners installed it.”

According to the report, nearby business owners said they had smelled “burning plastic” a month before the fire, but didn’t report it at the time. The smell returned the night before the fire.

Firefighters on the scene reported heavy smoke in the building, but little heat. It took two hours for firefighters to determine “the seat of the fire,” he said.

The fire, which occurred around 1:30 a.m., temporarily closed seven business. Three of them — Joe’s New York Pizza, Mark’s Place and the Dancing Elephant — located at 416 and 420 Fore St. remain closed.

Attempts to reach those tenants Wednesday were unsuccessful.

The building is owned by 420 Fore LLC, which is controlled by Joseph Soley, who couldn’t be reached for comment Wednesday. Property manager P.J. Roberts did not return a call Wednesday.

Earlier this year, Soley and Roberts were cited for 11 fire-safety violations in that building. Those violations had not been addressed prior to fire, but Clegg said even if they had, it would not have stopped the blaze.

Soley is a prominent downtown property owner who has had previous legal disputes with the city. In 2009, the city deemed his property at 10 Exchange St. unsafe, and temporarily evicted businesses and tenants. Code violations at that property were discovered after a small fire.

More than a decade ago, Soley was successfully sued by tenants over the conditions of one of his apartments. The four tenants refused to pay rent after the problems went unfixed, so Soley seized their belongings. They were awarded $1 million in punitive damages.