Margaret Eileen Powell, 72, and Thomas Gwyn Morgan, 74, were discovered dead in Mrs Powell’s home in Brecon, Wales, in December 2006.
Cardiff Crown Court heard gas engineer Peter Tongue, 60, of Llanspyddid, had carried out work on the woman’s Powell’s ducted warm air central heating boiler only weeks earlier. When the boiler was examined, part of it was found to be congested with soot and subsequently leaking out carbon monoxide.
Mr Tongue was found guilty of two counts of manslaughter and six offences under the Health and Safety Act in relation others works he had carried out.
He has been released on bail to be sentenced on 30 July.