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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.
How does a Direct Vent Fireplace work? Combustion air is drawn from outside the building. Cool room air is pulled into the lower chamber; it will circulate around the firebox and is released into the room as warm air via the fan. The Terminal is mounted outside, connecting to the fireplace indoors.