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”.

After a five-day-trial, Sheriff William Gilchrist took less than 40 minutes to find Hall, of Tullibody, Clackmannanshire, guilty of installing the combination boiler “otherwise than in accordance with appropriate standards and in such a way as to prevent danger”.

He said: “I’m satisfied beyond reasonable doubt that the failure to ensure that the gas supply pipe was adequately joined to the gas inlet pipe had the consequence of allowing gas to escape, thereby causing the explosion.”

Hall in evidence, insisted that he had applied heat with his blow-lamp to solder the so-called “Yorkshire fitting” between the inlet pipe and the boiler, but accepted that the solder had not “run” and the joint had not been properly made.

Sheriff Gilchrist said heat could only have been applied “very fleetingly”, because not only had the solder in the joint not run, but even some of the flux paste, applied as part of the process, had not melted.

The court heard that it was this sticky paste that held the joint together and allowed it to pass a “gas tightness” test that Hall carried out before firing up the system.

Sheriff Gilchrist, who deferred sentence until February 7 for background reports, said he was not considering jail, but Hall could face a fine or a community-based sentence.