A 52-year-old Londonderry plumber has been committed for trial in the Crown Court accused of causing the death of two teenagers three years ago.
The judge, sitting in Coleraine, said he was satisfied there was a prima facie case against George Brown.
Aaron Davidson and Neil McFerran, both 18, died in August 2010 when there was a gas leak in their holiday apartment in Castlerock, County Londonderry.
Mr Brown, of Ballygawley Road, Aghadowey, was granted continuing bail.
Mr Brown, is accused of the manslaughter of the two friends as well as 19 other charges.
The judge said he was "well aware of the nature of the tragedy" that had befallen the teenagers' families but that his duty was not to perform a "dry run" of a trial but to decide if Mr Brown had a case to answer.
"Two young men died from carbon monoxide poisoning," the judge said.
"The source of that carbon monoxide was from an incorrectly-fitted flue which had dislocated at the elbow joint.
"That flue was part and parcel of the boiler installation which the apartment owner had arranged with the defendant whose fingerprints were found on one section of it."
Their friend Matthew Gaw, who was also in the seaside apartment at Tunnellbrae at the time, survived.
The friends, who were all prefects at Glengormley High School, had gone for a weekend away before they received exam results but their parents became alarmed when they did not come home.
They rushed to the apartment, finding Aaron and Neil unconscious having been overcome with carbon monoxide fumes, while Matthew was groggy but alive.
Releasing Mr Brown on continuing bail, the judge adjourned the case until 15 November when he will be arraigned at Laganside Courts in Belfast.