Gas back burners use a boiler behind the room fire to heat water in a closed loop with the cylinder. This means that the hot water is transferred into a loop of pipe inside the hot water cylinder to transfer heat to the water you would use to wash up.
Solid fuel systems are the same, but there is no separate boiler behind the main coal or wood burner.
Circulation Through Radiators
The hot water is circulated through a network of pipes to radiators throughout the home. As the water flows through the radiators, it transfers its heat to them, warming up the rooms in which they are located.
Return for Reheating
After the water has passed through the radiators, it will have cooled down. It returns to the back boiler to be reheated by the ongoing fire, and the cycle repeats.
Hot Water Supply
In addition to heating the home, the hot water produced by the back boiler can also be used for domestic hot water needs. This usually involves a separate circuit that directs water to a hot water cylinder where it is stored for use in taps and showers.
A heat exchanger is used in the cylinder, for both fuel types, it is usually a coiled pipe inside the hot water cylinder that transfers the heat.
The combustion process produces exhaust gases that contain carbon dioxide and water vapour, among other things. These gases are typically vented out through a flue that leads up the chimney.