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the cylinder (known as Bottom Dead Centre or BDC) the exhaust valve starts to open. As the piston now moves up the cylinder, the hot gases (consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen) are expelled from the cylinder. As the Piston approaches TDC again the inlet valve starts to open and the cycle repeats itself.
The cylinder-is the main part of the open vented hot water system; this is where the hot water is heated. The heat can come from an immersion heater, directly from a boiler or indirectly from a boiler using a coil fitted inside the cylinder.he water is heated via a coil (heat exchanger) from a boiler. This is done because there is a set of radiators connected to the boiler, in this case water in the central heating system slowly becomes contaminated by iron residues from the radiators which would make the hot water unusable for washing.