Self-employed contractor Mark Bullock, 50, of Milton, Stoke on Trent, was carrying out repairs inside the cooker at John Pointon & Sons when the incident happened on 5 November 2011.
While he was inside, steam from elsewhere in the system fed into the area where he was working. He was badly scalded and died in hospital the following day from his injuries.
An investigation by the Health and Safety Executive (HSE) found Mr Bullock was allowed to enter the cooker without the proper precautions being taken. The company had not properly considered the risks of entering the cooker, had failed to put in place a safe system of work, and did not competently manage the work as it was taking place.
Stafford Crown Court heard that in 2004 another employee was killed at the same site when he entered a confined space without proper precautions being taken.
John Pointon & Sons, of Bones Lane, Cheddleton, Leek, pleaded guilty to breaching Section 3(1) of the Health and Safety at Work etc Act 1974.
HSE inspector Wayne Owen said: "The cookers in operation at the company form the core part of the business. Steam and hot vapours getting into the cookers from other connected pieces of equipment is foreseeable, and precautions should have been taken to ensure all avenues which had the potential to allow steam to be fed back into the cooker had been suitably isolated.
"John Pointon and Sons Ltd failed to do this and it cost Mark Bullock his life.
"Work in confined spaces can be extremely dangerous, which John Pointon & Sons Ltd were fully aware of having already had a fatality at the site. Companies must identify what measures should be taken to ensure the safety of their workforce. I would urge any company that carries out work in confined spaces to double check their procedures."
Mr Bullock's partner of 27 years, Christine Knowles, said: "Mark had a great passion for life. In some ways he never grew up. He loved fairgrounds and holidays and loved to sing and dance. He had an extremely generous nature and a wicked sense of humour.
"To die that young is a tragedy. He was so fit and healthy. In 2009 we moved to a beautiful house on the canal. He built a balcony and bought a boat and had hoped to retire early. We had started to really look forward to retirement and lazy sunny days on or near the water.
"Mark's friends put some money together and have had a tribute put up at the site – a tree and a stone with the inscription "How difficult can it be?" He was a practical man and used to say that a lot.
"The company should have made sure that Mark was safe. Every company should do the same for their workers. Mark was a great man. He touched many people's hearts and broke mine when he died."
