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How a Gas or Oil Furnace Works

Forced air or warm air heating systems use a gas or oil fueled furnace to warm a home. There are four key components to a forced air heater; a thermostat, the furnace, the air ducts and the combustion gases vent.

The system works in the following way. The thermostat is set with a temperature that is the lower limit at which the home should be maintained. When the ambient temperature at the thermostat drops below the preset, a circuit closes (effectively a switch) allowing current to flow to the furnace controls and thus signalling it to begin heating. The furnace ignites and begins heating the plenum in the heat exchanger. After a preset time, to allow for the heat exchange to heat up, the blower begins and it draws air in from the "cold air return", passes the air over the heat exchanger and then forces it out through the home's heating ducts. Once the ambient temperature at the thermostat rises enough, the switch in the thermostat opens, cutting power to the furnace, which then shuts off the flame. However, the blower continues for a short time to take advantage of the remaining in the heat exchanger. The combustion gases resulting from the burning of the fuel are contained by the exchanger and vented out through the roof of the home.

How the Thermostat works

The thermostat is essentially a thermometer with a switch. The thermometer in older mechanical thermostats use a coiled bimetallic strip that uses the movement of the metal in reaction to temperature changes to activate a mercury switch. In modern digital thermostats, a thermistor measures the temperature by the change in electrical resistance with the change in temperature.

A programmable thermostat has added computing power to the digital thermostat and allows the entry of temperature preferences by time of day and day of the week.

Whenever a preset condition is reached the thermostat allows low voltage current to flow through the heating or cooling equipment allowing it to operate. When the desired temperatures are reached at the location of the thermostat, the switch opens and cuts power to the equipment controls. The equipment still has line power and so the blower may continue to operate even when the thermostat is set to "off".

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