The gas furnace is the simplest to operate and understand. Therefore, we will use it here to look at a typicalheating system. This type of natural-gas furnace is used to heat millions of homes in the United States. Note the location of theblower switch and the limit switch. The transformer provides low voltage for control of the gas solenoid. Ifth

e limit switch opens (it is shown in a closed position), there is no power to the transformer and the gas solenoidcannot energize. This is a safety precaution, because the limit switch will open if the furnace gets toohot. When the thermostat closes, it provides 24 volts to the gas solenoid, which energizes and turns on thegas. The gas is ignited by the pilot light and provides heat to the plenum of the furnace. When the air in theplenum reaches 120 degrees F, the fan switch closes and the fan starts. The fan switch provides the necessary120 volts to the fan motor for it to operate. Once the room has heated up to the desired thermostat setting, the thermostat opens. When it opens, thegas solenoid is de-energized, and the spring action of the solenoid causes it to close off the gas supply, thereby turning off the source of heat. When the plenum on top of the furnace reaches 90 degrees F, the blower switch opens and turns off theblower. As the room cools down, causing the thermostat to once again close, the cycle starts over again. Thegas solenoid opens to let in the gas, and the pilot light ignites it. The heat causes the temperature to rise inthe plenum above the limit switch`s setting, and the switch closes to start the blower. Once the thermostatsetting has been reached, it opens and causes the gas solenoid to turn off the gas supply. The blower continuesto run until the temperature in the plenum reaches 90 degrees F, and...

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