Today I want to give you a basic Sequence Of Operation for a standard gas furnace aka 80% Efficiency furnaces. This will not apply to every single one out there but typically will cover about 90% of what you will find.
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The Furnace is in standby mode waiting for a call for heat, the Thermostat has a call for heat, which closes a switch inside the thermostat and sends 24 Volts to the W (Heat) Terminal on the control board.
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The Inducer draft/combustion blower motor then starts.
