Craig Hall left a gas pipe liable to come loose in Robin and Marion Cunningham’s home in Callander, Perthshire, by crucially failing to solder a vital 90 degree joint where it attached to the new equipment.
For eight months the dodgy fitting was held together by just a little paste used to prepare the joint for connection.
But eventually it separated, pouring gas at up to nine cubic metres an hour into the couple’s utility room.
Around 5.45 am on March 28, 2013, the flammable atmosphere ignited, possibly sparked by a light switch, or the compressor on a fridge freezer, “totally demolishing” the property.
Mr Cunningham was taken by air ambulance to Glasgow Royal Infirmary with burns to his head, face and hands, and spent a week in hospital.
Mrs Cunningham, then 74, had less serious injuries, but Stirling Sheriff Court was told she has been left “frail and very anxious” by what happened, while Mr Cunningham’s Parkinson’s Disease had “obviously progressed”.
Hall, of Tullibody, Clackmannanshire, was told to pay the £3000 fine at a rate of £150 per month at Stirling Sheriff Court on Wednesday.
He had denied causing the blast by carrying out the installation of the boiler dangerously, but he was found guilty by Sheriff William Gilchrist.
Passing sentence, he said he was satisfied Hall hadn’t meant to cause the explosion but found that he had failed to take reasonable care.
The court was told Hall remains employed but has had to undergo further training.