Simon Dale was jointly prosecuted by the Health & Safety Executive (HSE) and Wiltshire Council's trading standards.
Salisbury Crown Court heard that Mr Dale, aged 39, was registered as a plumber under the CORGI scheme, now the Gas Safe Register, but not as a gas engineer.
An HSE investigation found that working under the company name Cathedral Plumbing Services, he carried out work on combination gas central heating and water boilers at six properties across Wiltshire between November 2007 and October 2009. He was not registered to do that work.
Trading standards officers found that Mr Dale, who also traded under the company name Wiltshire Plumbing & Heating Ltd, was found to have misled customers on 13 occasions between October 2007 and December 2010 regarding his professional accreditation.
While quoting for boiler installations he committed fraud by telling customers he was CORGI registered or qualified to do gas work. He used official logos on paperwork and his work van giving the impression that he was approved by CORGI and the Oil Firing Technical Association (OFTEC).
On one occasion he claimed to be Part P qualified to undertake and self-certify electrical work. On all of these occasions he was not appropriately qualified or registered to undertake the work and never had been.
Speaking after the prosecution, HSE inspector Andy Shaw said: "Carrying out work on gas installations without being a member of the approved registration body, which is now the Gas Safe Register, is both dangerous and illegal.
"The law is there to ensure work is carried out safely to minimise any risk of death or serious injury resulting from gas or fume leaks or explosions.
"This is a classic case of an unqualified tradesman carrying out illegal gas work and making fraudulent claims about his accreditation. We are pleased to have worked together with Wiltshire Council's trading standards to reveal Mr Dale's failings and illegal practices."
Mr Dale, of Walworth Road, Andover, Hampshire, pleaded guilty to six offences under section 33(1)(c) of the Health and Safety at Work etc Act 1974 for breaching Regulation 3(1) of the Gas Safety (Installation and Use) Regulations 1998.
He also pleaded guilty to 10 offences of fraud by misrepresentation, breaching Section 2 of the Fraud Act 2006, and a further three breaches of Regulation 12 and Schedule 1 Paragraph 4 of the Consumer Protection from Unfair Trading Regulations 2008, for engaging in unfair commercial practice.
He was sentenced to serve 18 months in prison for eight of the fraud offences and a further 12 months for the gas safety and unfair trading offences, to run concurrently. For the two additional fraud offences, that he committed whilst on bail for the other offences, he was sentenced to a further 12 months to run consecutively. This comes to 30 months in total. No prosecution costs were awarded.