The diagram illustrates the operation of a central heating system in a house. Most notably, the system consists of two main stages, one of which uses cold water and the other with hot water.
In the first stage, cold water enters the house through a system of pipes installed right below the roof. Part of it flows down to the boiler, which is gas- or oil-driven to become heated, while the rest goes around the house through pipes placed alongside the house walls.
As for the hot water, it takes two different directions. It is either pumped to hot water radiators or goes to the water taps. Following the first route, hot water travels around the house to the radiators which are made of a set of small tubes. From these tubes, it radiates heat to warm up the house. Alternatively, hot water from the boiler flows through the storage tank to the water taps for direct use. Simultaneously, part of the unused water can be sent back to the water tank through the overflow pipe.