Cambridge Crown Court heard how, on 19 December 2014, a mistake regarding the correct floor level resulted in a live flue being blocked.
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 subcontractor R J Fitters were given a diagram marked with the redundant flues and were expected to find the redundant flues amongst live flues.
The problem was only identified when a CO monitor activated and the homeowner and her son investigated. The damaged boiler was switched off before potentially any serious ill-health could occur.
An HSE investigation found that main contractor Wates and R J Fitters Ltd failed to manage the risk involved with the project.
Inspectors found that they 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.
The companies could have instead of blocking the redundant flue pipes put a cage around the cowls to ensure they did not fall.
Wates guilty to a safety breach and was fined £640,000 and ordered to pay costs of £20,862.52.
R J Fitters Ltd also pleaded guilty and was fined £30,000 and ordered to pay costs of £10,431.28
Speaking after the hearing 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”.
A Wates spokesperson said: “We very much regret this incident. Safety is our highest priority and we are committed to ensuring that no individual is ever harmed or put at risk as a result of our work.
“We fully support all efforts by the HSE to make construction sites a safer place to work, and we have co-operated fully with the HSE’s investigation into this incident.
“On this occasion, however, despite our best efforts, we accept we fell short of the required standard.
“We immediately reviewed and enhanced our existing controls to help prevent a reoccurrence, and we remain committed to achieving the highest standard of health and safety moving forward.
“As part of our ‘We’re Safer Together’ Zero Harm campaign launched in 2016, we strive to achieve a working environment free from incident, ensuring that health and safety remains uppermost in the minds of both our employees and sub-contractors.”