Richard Copp, of Hillside Avenue, Mutley, pleaded guilty to three breaches of the Gas Safety (Installation and Use) Regulations 1998.
He was given an eight month prison sentence suspended for 12 months and ordered to pay costs of £500.
Plymouth Magistrates Court heard how Mr Copp was asked to undertake a large two-storey extension to a house in Bowers Park Drive, Woolwell in June 2013.
Concerns were raised by a registered gas engineer when it was found the boiler flue had opened into the extension with the boiler still in operation, which had then been extended with an inappropriate piece of piping.
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This potentially meant poisonous carbon monoxide gas could have escaped and affected the family living inside the house.
Mr Copp's work was classified as "Immediately Dangerous" by a registered Gas Safe engineer and HSE was alerted.
An investigation found Mr Copp was not Gas Safe registered, a legal requirement to carry out work on gas appliances and gas appliance flues.
HSE Inspector Simon Jones, said: "The work that Richard Copp carried out posed a real risk to homeowners from carbon monoxide poisoning as the flue, which should have carried waste gases outside the house, actually ended inside the building.
"Builders should not undertake building work that causes gas appliances to operate incorrectly or dangerously, such as enclosing a gas boiler flue.
"Only Gas Safe registered engineers are legally able to work with gas and Richard Copp put lives at risk by ignoring the law."
(LM)
