George Brown, 52, from Ballygawley Road in Aghadowey pleaded guilty in January to unlawfully killing Neil McFerran and Aaron Davidson on 3 August 2010.
He also pleaded guilty to 19 further charges of failures of health and safety legislation.
On Thursday, he received two years in jail and two years on licence. He was also fined £19k for a range of health and safety offences.
The 18-year-old friends died of carbon monoxide poisoning after they were overcome by the deadly gas in a holiday apartment in Castlerock. Another friend Matthew Gaw was seriously injured.
The trio had been on a break in the seaside town after doing their A-levels.
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.
Brown, who ran a gas shop in Coleraine, 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.
The case should serve as a warning to anyone working on gas appliances and systems that by law they must be on the Gas Safe Register - and have the correct qualifications and competence for the work they are carrying out.
Louis Burns, HSENI
The judge said Brown's "cavalier attitude" to the dangers presented by carbon monoxide was "reprehensible".
In court last Friday, prosecution barrister told Belfast Crown Court that Brown had carried out work he was not competent to do.
Judge Justice Weir said he had presided over "shoddy, dangerous workmanship".
He said that more people could have died as Brown had carried out many other gas installations in the area.
"At least it has brought to light that this man and his business were going around this area carrying out substandard and dangerous gas installations."
Following the sentencing, Detective Chief Inspector Rachel Shields commented: "While the judge in giving the sentence today acknowledged all of the circumstances surrounding the incident; no sentence will ever bring Aaron or Neil back.
"It is an extremely difficult day for Aaron and Neil's families and I would like to thank them for their relentless bravery and the unequivocal support that they gave to all the various stages of this extensive investigation.
"The families of Aaron and Neil have and continue to, highlight the issue of carbon monoxide poisoning. It is an issue that all households should be aware of and I would remind service providers who work with installing heating systems or maintaining heating systems of the legal requirements and responsibilities that govern this area of business."
Louis Burns, from the Health and Safety Executive added that the case was one of the biggest ever undertaken by the body.
"The tragic and untimely deaths of Neil and Aaron from carbon monoxide poisoning serve as a potent reminder of the dangers this deadly gas poses," he said.
"Our deepest sympathies remain with the family and friends of Aaron and Neil at this very difficult time. Our thoughts are also with Mathew Gaw who was seriously injured in this terrible incident."
He concluded: "Gas Safe registered engineers must carry ID cards which provide proof of their technical expertise for all the appliances they are legally qualified to work on. It is also essential that when installing gas appliances, systems or flues that they follow the manufacturer's instructions."
Pictured below: the gas flue incorrectly installed by George Brown.