Stirling University student Thomas Hill, 18, died after being exposed to the deadly gas in the bathroom of remote Glenmark Cottage, at Tarfside, by Edzell, in October 2015.
The cottage is owned by Burghill Farms, a partnership which at the time was run by the Earl and Countess of Dalhousie with their son Simon Ramsay, Lord Ramsay.
Dundee Sheriff Court heard that no annual checks were carried out on the cabinet heaters, which were also placed in small rooms without enough ventilation.
Burghill Farms, trading as Dalhousie Estates, let the cottage to Piers Le Cheminant, 76, from March 2008 and sub-let it to holidaymakers from across the UK. Both parties pleaded guilty to breaches of The Gas Safety (Installation and Use) Regulations 1998 and the Health and Safety at Work etc Act 1974. Burghill Farms were fined £120,000 and Le Cheminant £2000.
The breaches came to light as part of the investigation into the death of Mr Hill, although his death was not caused by the failings. The 18-year-old died of carbon monoxide poisoning on October 28, 2015. He was found unconscious behind a bathroom door after carbon monoxide escaped from a faulty heater and he was pronounced dead en route to Ninewells Hospital. Ten days before, a family staying at the same cottage experienced problems with the bathroom heater making “putt-putt” noises and causing a woman’s eyes to sting.
Le Cheminant subsequently instructed an unqualified gas engineer to replace the gas bottle and he reported finding no other issues with the heater. There was also no maintenance programme for the cabinet heaters in place. While an engineer visited yearly, no tests were carried out and he was not qualified to work on that type of heating.
Alistair Duncan, head of the Health and Safety Investigation Unit, said: "By failing to ensure the safety of visitors to Glenmark Cottage, Burghill Farms and Piers Le Cheminant have put them at unacceptable risk. This prosecution should remind landlords that they have a responsibility to those who visit their properties and will be held accountable for any failure to do so.”