Apollo Property Services Group, now part of Keepmoat Group, was sentenced in relation to an incident at the Abbey Road Estate in Camden between 18 February and 18 March 2008.
The Old Bailey heard during a two-week trial that seven residents, none of whom want to be named, were exposed to carbon monoxide because boiler flues servicing the flats were obstructed during roof refurbishment work.
An investigation by the Health & Safety Executive (HSE) found that Apollo knew that some flues may still be serving boilers in the properties, but did not have an adequate system for inspecting them. So it just carried on working without making any checks.
The company also failed to ensure that the work was adequately supervised, and did not ensure that workers were familiar with safe working practices in relation to gas flues or were aware of the risks.
Apollo Property Services Group Ltd, of Church Street, Waltham Abbey, was fined £165,000, ordered to pay £117,582 in costs and a total of £19,000 in compensation after being found guilty of breaching section 3(1) of the Health and Safety etc. at Work Act 1974.
After sentencing, HSE inspector Helen Donnelly said: "Despite the client's adviser warning the company to consider the risk of blocking or covering flues at the start of the refurbishment project, the company did not assess the risks to residents or have a safe system to ensure flues serving live boilers were not obstructed.
"Moreover, as part of one of the UK's largest property service organisations, Apollo Property Services Group Limited is very experienced in refurbishing housing estates so there is no reason for this incident to have happened."