Almost like a boiler but it does not use the heat of burning gas directly but the heat of the hot water from the central boiler. This one has a broken actuator and the tenant was left without hot water. As in most of the cases this fault could have been prevented. The blue scale marks on the valve were visible before the actuator got broken and they mean an issue with the valve. During a maintenance visit the early sign of breakdown of the valve would have been spotted. unfortunately, there was no maintenance made on this hence the issue got worse and the valve got stuck and broke the actuator.
The HIUs are just as important parts of a heating system as a boiler and as such they need to be checked, maintained at least annually, preferably quarterly as they are providing the hot water as well.