A natural gas or propane high-efficiency furnace works like this:
The furnace starts up when the thermostat senses that the temperature had dropped below a pre-set level.
The ventor motor does a safety check to ensure the venting is clear, as determined by the pressure switch. The ignitor then ignites the burners inside the furnace combustion chamber and starts creating heat. The heat is transferred to the primary heat exchanger, which heats the air as it flows through the furnace. The combustion gases are then passed through a secondary heat exchanger to further extract heat.
The blower motor pushes the warmed air through the plenum and out into the house via the registers.
The combustion gases, including carbon monoxide, are safely vented to the outdoors from the furnace.
Outside fresh air is pulled into the burn chamber to provide a more efficient burn
Any cold air left in the home gets circulated into the furnace to be heated up.