Tommy Smith pleaded guilty to a total of five safety, business and consumer protection breaches at the Margate Magistrates' Court today after a series of jobs he undertook in January 2007.
The court heard Mr Smith had reduced the height of two chimneys, which contained gas boiler flues, giving rise to a risk of the build up of carbon monoxide in the properties affected.
Health and Safety Executive inspector Mike Walters said it was fortunate nobody was harmed.
Mr Smith - who was handed the fine of £13,998 plus ordered to pay a total of £12,212 in costs - was prosecuted following a joint investigation by the HSE and Kent County Council's Trading Standards board.
Mr Walters said: "We are pleased that justice has been done today.
"It is illegal to carry out gas safety work if you are not registered to do so. Mr Smith took advantage of these vulnerable, elderly residents and as a result, not only broke the law, but caused them deep distress, and potentially put their lives at risk."