Hot water Heating and Storage
Hot water tank: insolated storage container made of metal
Boiler: Uses a small gas flame to make steam
Pump: used to send hot water from the boiler to the hot water tank
Circulating Pump: used to keep the boiler feed
Exhaust: product of gas combustion escapes
There are two closed systems and one open system at work here. The open being the first, when the gas burns and escapes through the exhaust. The first closed system is the loop feeding the coil inside of the tank. It is used to transfer the steam heat from the boiler to the hot water tank. The last closed system is the input and hot water product of the tank.
There are a few control mechanisms not mentioned in the diagram. The amount of gas burned is controlled. The about of heat transferred to the tank from the boiler is controlled. The amount of water in the tank is controlled. The feedback I would describe as the hot water, gas and drain outputs.
The purpose of this system is to describe the process of converting gas energy to hot water at the tap.