Wates Construction pleaded guilty to breaching Section 3(1) of the Health and Safety at Work etc Act 1974. It has been fined £640,000 with more than £20,862 in costs.
R J Fitters also pleaded guilty to the same charge and was fined £30,000 with over £10,431 in costs.
Cambridge Crown Court heard how a mistake regarding the correct floor level resulted in a live flue being blocked on 19 December 2014.
Scaffolding was erected outside a 13-storey block of flats without marked lift levels and the external wall of the building had no markings to identify floor levels or flat numbers. Operatives from from R J Fitters were given a diagram marked with the redundant flues and were expected to find the redundant flues amongst live flues.
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However, the problem was only discovered when a CO monitor activated inside a property and the homeowner and her son investigated. The damaged boiler was switched off before potentially any serious ill-health could occur.
A Health and Safety Executive (HSE) investigation into the incident found both Wates and R J Fitters failed to manage the risk involved with the project.
Investigators found the companies could have marked the levels on the scaffold and the levels/flat numbers on the external wall of the building. A supervisor could have marked the redundant flue pipes to ensure the correct cowls were removed and flue pipes blocked up.
In addition, the companies could have instead of blocking the redundant flue pipes put a cage around the cowls to ensure they did not fall.
HSE inspector Sandra Dias said: "It is the responsibility of both the principal contractor and subcontractor to ensure that safe systems of work have been identified and adopted. When there is risk of death to members of the public, the safe systems should be well thought through and robust.
"The risks associated with blocking a live flue could result in carbon monoxide entering properties and potentially killing all occupants."
(LM/MH)
