Leamington Magistrates fined gas fire installer Cecil Boliver Crane from Shipston £1,000, with £3,000 costs, for a gas fire installed without adequate ventilation.
Crane, who trades as Peter Crane Plumbing & Electrical, installed a gas fire in house in September 2009.
The occupants of the house fell ill with flu-like symptoms including tiredness, headaches and vomiting after using the fire for around four hours a day.
The Health & Safety Executive (HSE) found the heating appliance produced high levels of carbon monoxide, although a catalytic converter within the appliance reduced these to safe levels.
HSE inspector, Peter Snelgrove, said Crane had failed in his duty of care by installing a fire that was “immediately dangerous”.