BANGOR – A federal jury will decide who is responsible for a fire at a Deer Isle boatyard nearly 14 months ago.

Testimony began Monday in U.S. District Court in a civil lawsuit that pits insurance companies and boat owners against the father-son boatyard owners and an area heating firm.

Standard Fire Insurance Co. of Hartford, Conn., and One Beacon Insurance of Boston are suing owners of the Jericho Bay Boatyard in Sunshine and New England Heating Inc. in Dedham to recoup losses paid to owners of boats destroyed in a Dec. 31, 2001, fire.

Six boat owners, whose combined losses equaled about $500,000, are plaintiffs in the case, along with the two insurance companies. The defendants are the boatyard owners: Reginald Thompson, his son, Jeffery Thompson, and Leanna Jones; and Marvin Lovely, the owner of the heating firm.

In opening arguments Monday, lawyers for boat owners and the insurers said they would prove that the fire started because the Thompsons improperly installed an oil furnace at the back of the 55-by-100-foot building. The Thompsons bought the furnace from Lovely in November 2001 to heat the building so winter repairs on boats could be done more comfortably, attorney Leonard Langer told the jury of five men and three women.

The Thompsons’ attorney, William “Sandy” Welte of Camden, said in his opening statement that his clients followed the advice Lovely gave them via telephone over the several days it took to install the furnace.

Attorney Michael Savasuk, representing Lovely, said the evidence would prove that Lovely sold the Thompsons the furnace, but the only advice he offered was that they get the town code enforcement officer to inspect it and hire someone to install it according to code.

The December fire was the second at the boatyard in eight months. In April 2001, a fire broke out at the back of a larger boat storage building across the road. That 200-by-80-foot wooden building was destroyed in the blaze, along with more than a dozen powerboats and sailboats inside.

Damage to the building and boats in that fire was estimated at close to $10 million.

Jurors will not hear testimony about the April fire, however. A motion to exclude testimony and evidence from that fire was granted without objection.

The trial concerning the December fire is scheduled to continue all week. The case is being tried in federal court because the storage of boats, on and off the water, comes under maritime law.