I clearly remember the days of hot water boilers as a way to heat a house or building. When I was very small, we had heat registers against the wall. We kids, were reminded constantly not to get close to the heater registers.
Once in while, we would hear stories about water heating systems blowing up. Then, there weren't the safety features like valves to shut off if the water level became too low or if the pressure became too high.
In one of the old elementary schools I went to, we had a boiler heating system. Everyone knew where the boiler room was. We always made jokes about the "boiler man" and what he was doing in his little room!