A plumber who installed a new boiler at a house which was destroyed in an explosion told a court he "had to accept" a joint had not been properly soldered.
Craig Hall said he double-checked the new system was "gas-tight" in the home of Marion and Robin Cunningham.
But he said restricted space behind the fitting-plate of the boiler made it difficult to visually inspect his work.
Mr Hall, 35, denies failing to install the boiler properly.
Mr Hall, who was promoted to plumbing manager at Stirling-based TRS Plumbing and Heating Services in the years since the 2013 blast, said: "It passed on two different tests, everything was fine, no problems at all.
"There was no smell of gas, no leaks, the boiler was working and the customer was happy."
He said he could remember soldering all the joints on the boiler, which he put in on 2 August 2012.
'Smelled no gas'
But he said he had to accept expert evidence that the solder had not in fact "run" in a so-called "Yorkshire fitting", joining a vertical gas supply pipe to the boiler itself.
This was found after the explosion to have separated completely, allowing gas at full domestic pressure to flood from its open end.
Mr Hall told his lawyer, advocate Susan Duff, that because his work had passed gas tightness tests using both a traditional manometer and an electronic pressure gauge, he "didn't appreciate" that the joint hadn't been made.
He told Stirling Sheriff Court he returned to the Cunninghams' home in Callander, Perthshire, in February 2013 - just weeks before the explosion - to adjust a pipe, and again smelled no gas.
Mr Hall said: "You've got a kind of nose for it, having been in the game for many years."
He agreed the British Standards Institution technical standard advice for soldering a Yorkshire fitting stated "the finished joint shall be visibly examined to confirm that solder has run".
He said he accepted that on all the other pipes he had attached to the new boiler, a "ring" of solder was visible.
But referring to the gas pipe joint, he said: "It's in a tight space, you've got a boiler in front of you, a jig plate in front of you, and that's why testing methods are in place as well."
Shona McJannett, prosecuting, challenged him: "I don't think you soldered that joint at all, did you, Mr Hall?"
Mr Hall replied: "There was heat applied to every joint."
Mr Hall is accused of failing to ensure a gas pipe was properly supported when he fitted the equipment eight months earlier.
As a consequence, it is alleged, the supply pipe separated from an inlet pipe, to which it should have been joined, allowing gas to escape and ignite.
The explosion caused extensive damage to the Cunninghams' two-bedroomed home and injuries to them both.
The trial continues.