Robert Schenker (31) was overcome by the odourless gas – known as the "silent killer" – after builder David Johnson allowed the chimney of his Peterborough home to become blocked during work to cure a damp problem.

Speaking after Johnson was jailed yesterday following his conviction for manslaughter, Max Schenker (35) said: "My feeling is that three-and-a-half years isn't particularly long for the life of a guy who was only 31 when he died, although I am led to believe that it's quite a stiff sentence."

He added: "Devastation is the only word for the effect on our parents. You never imagine that you will lose your child, but that's what they are going through every day.

"From my point-of-view, Robert was desperate to be an uncle but missed seeing my little daughter born, and she will never have any cousins from his side. It's going to affect generations."

Johnson (56), formerly of Delph Street, Whittlesey, but now living in Ipswich, was hired by Mr Schenker to fix a damp problem in a bedroom at his home in Meynell Walk, Netherton, Peterborough.

The self-employed builder and roofer traced the damp to the chimney, which he repaired in March 2006.

READ ET COMMENT: Sentence must be warning to every builder

A two-week trial this year heard his work was "grossly negligent", in that while repairing the stack he allowed old debris and fresh mortar to fall down the chimney.

The material blocked the chimney at the point a flue pipe from the home's gas boiler ran into it, preventing carbon monoxide produced by the appliance from escaping safely upwards into the air.

Mr Schenker, an engineering graduate who had launched his own business, switched the boiler back on after checking with Johnson it was safe to do so.

But he was overcome by carbon monoxide fumes and was found dead in his bedroom after his worried girlfriend raised the alarm.

Tests afterwards showed it took just seven minutes for the lethal gas to reach the room.

At Norwich Crown Court yesterday, Judge Peter Jacobs jailed Johnson for three-and-a-half years for manslaughter and imposed no separate penalty for exposing Mr Schenker to risk to his health and safety.

Mr Schenker, a mechanical engineer, said he was continuing his brother's Porsche car spares business – FrazerPart – as a tribute to him.

He said: "The last thing he said to me was, 'Come and work for me', because I had received notification that I would be made redundant in a year.

"He was very popular and is missed by all. He had everything going for him at 31."

Mr Schenker urged people to fit a carbon monoxide alarm in their home conforming to British Standards, to have gas boilers regularly serviced, and to consult CORGI if they have any doubts about gas safety.