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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.
DIRECT VENT GAS MODELS Direct Vent water heaters utilize a co-axial (pipe inside a pipe) vent pipe that takes air in from the outside (outer vent) and vents combustion gases directly back to the outside (outer vent). It is the perfect choice for situations with insufficient combustion air. No electricity required.