Asked by Leslie Thomas QC, representing the family, whether this was most likely when the safety device was short- circuited, Mr Magner said:"'It's the only conclusion I came to on the evidence available to me."
A brother and sister, aged six and seven, died from carbon monoxide poisoning because of 'bodged' work on a hotel boiler during a holiday in Corfu, an inquest was told.
Christi Shepherd and her younger brother Bobby died in their sleep at their family chalet at the four-star Corcyra Beach Hotel on the Greek island in October 2006.
Their father, Neil Shepherd, and his girlfriend Ruth Beatson, were also put into a coma by the fumes during the family's half-term break.
Wakefield Coroner's Court heard that there had been at least three blunders in the installation and maintenance of the boiler supplying hot water to the hotel chalets, including a safety feature deliberately being short-circuited.
The hearing was told that this may have been done the day before the family began feeling unwell, after neighbouring holidaymakers complained they did not have hot water.
Coroner David Hinchliff said: "What should have been a very happy and relaxed half-term break became the most appalling tragedy."
He described how the children, from Horbury, West Yorkshire, had been feeling unwell in their holiday bungalow the day before they were found dead by a cleaner.
Mr Shepherd and Ms Beatson – who have since married - were both overcome by the fumes but were saved by emergency treatment.
A post-mortem examination in Greece concluded that Bobby and Christi died from carbon monoxide poisoning.
Mr and Mrs Shepherd were both at the hearing along with the children's mother, Sharon Wood.
Mr Hinchliff said: "The family of these children have waited a long, long time for this day to come."
The jury was told that the family arrived at their hotel on October 23 2006, and they were initially offered a room in the main block because there was a problem with the accommodation they had booked, before eventually being allocated a semi-detached two-bedroom bungalow in the grounds.
Two days later, the children started to feel unwell, with Bobby tripping on the floor and appearing to be dizzy.
He had recovered slightly by bedtime, but his sister started to cry and was being sick after Mrs Shepherd, who was also feeling unwell, went to bed herself.
Mr Hinchliff told the inquest that the adults had gone into the children's room, but could not remember what happened after that.
When a chambermaid let herself into the chalet to clean the next morning at around 11am, she found Christi dead on the floor and Bobby dead in the bed.
The two adults were close by, both unconscious.
Heating engineering expert, Thomas Magner, explained to the inquest how carbon carbon monoxide from the boiler which supplied hot water to the bungalow had got into the building.
Mr Magner, who was instructed to examine the scene by tour operator Thomas Cook after the deaths, said the boiler was housed in an outbuilding and supplied water to two adjoining bungalows.
He said the boiler had no flue to connect it to the outside, meaning that fumes from the burners built up inside the outhouse.
The engineer said there were gaps in the walls where air conditioning pipes went into the building and this had enabled the lethal carbon monoxide into the ceiling space above the children's beds.
Mr Magner agreed with the coroner that, by British standards, this work had been 'bodged and botched'.
He said a third problem was that a water leak meant the boiler was working more than it should have been.
But the engineer said a crucial problem was that a safety cut-off device had been deliberately short-circuited, meaning the boiler would not turn itself off.
The jury was told that the day before the Shepherd family started to feel unwell, holidaymakers staying in the adjacent bungalow had complained of not having any hot water - which prompted hotel staff to examine the boiler.
The hearing continues.