The Building Thermostat senses temperature, turns on the furnace burner: As building temperatures drop a wall-mounted thermostat in the occupied space senses the temperature drop and in response, calls for heat at the furnace, causing the furnace oil or gas burner to turn “on”.
The Gas or oil burner: The heat source such as a gas or oil-fired burner will turn on in response to the thermostat, and it will continue to run until either the high temperature or “FAN LIMIT” temperature is reached inside the warm air plenum or until the building thermostat inside the occupied space senses that the desired temperature there has been reached.
At that time the burner is turned off but the blower will usually continue to run for a bit longer for reasons we’ll explain in more detail below.
Heat exchanger: Hot combustion gases produced by the oil or gas burner circulate inside of the furnace’s metal heat exchanger causing it to get hot. Combustion gases leave the inside of the heat exchanger and flow through a flue vent connector to a building chimney where they are vented safely outside.
The furnace blower inside the furnace blower compartment draws returning cool air from the living area and blows it across the outside of a steel “box” called the heat exchanger, sending the now-warmed air onwards into the furnace’s output side or “supply air” plenum where it is sent into the building warm air duct system for delivery to the occupied space.
Air ducts connect and permit movement of cool air from occupied space through furnace and deliver warm air back to occupied space: The building air duct system includes return or cool air ducts and warm air ducts. Cool air ducts carry air from the occupied space into the furnace return air plenum, possibly through an air filter, and into the furnace blower compartment. Warm air supply ducts connect to the supply air plenum and carry warm air into the occupied space where it flows out of floor, wall, or ceiling warm air registers or diffusers.
Combination Fan & Limit Control: This control turns the furnace blower on and off at the proper times. The FAN ON setting on this control makes sure that the blower fan does not turn on (even though the building thermostat has asked for heat) so that the furnace wont’ blow cool air into the occupied space.
Furnace FAN ON: When an adequate warm temperature has been reached inside of the furnace warm air plenum chamber the FAN ON switch turns on the furnace blower to deliver warm air to the occupied space.
Furnace FAN OFF: The furnace combination fan and limit control FAN OFF setting lets the furnace blower continue to run for an interval after the furnace burner has turned off, but will shut the blower off after the heat exchanger has been cooled down and the heat it contained has been sent to the occupied space.
Furnace LIMIT: The LIMIT indicator setting is a safety control that will turn off the oil or gas burner if temperatures inside the warm air plenum exceed a safe level.