Neil McFerran and Aaron Davidson, both 18, were overcome by noxious fumes at a holiday apartment in Castlerock, Co Derry, in 2010.
Part of the flue from a new gas boiler fitted by George Brown’s company became dislodged and filled the flat with the odourless toxin.
At Belfast Crown Court Mr Justice Weir sentenced Brown, 52, from Aghadowey, Co Derry, to four years for two counts of manslaughter and fined him £19,000 for a range of health and safety offences.
The judge said his “cavalier attitude” to the dangers presented by carbon monoxide was “reprehensible”.
Mr McFerran and Mr Davidson, from Newtownabbey near Belfast, had been on a break in the seaside town after doing their A-levels.
Their friend, Mathew Gaw, survived the incident. He was rescued when family members broke into the property when they realised something had gone wrong. By that stage the other two men were already dead.
Brown, from Ballygawley Road in Aghadowey, ran a gas shop in Coleraine and had been asked to investigate a problem at the apartment earlier in 2010.
He recommended replacing the boiler and flue and converting the burner from natural gas to liquefied petroleum gas (LPG).
Experts concluded that part of the flue, used to carry emissions from the boiler to the outside through a void in the apartment ceiling, had separated because sections were not secured properly at a right-angle bend.
At least two screws were missing and the metal sections did not overlap or slot into each other far enough, allowing them to come loose.
Brown initially denied any offending but later pleaded guilty to two charges of manslaughter and a further 19 counts involving failures to comply with health and safety legislation.