From FOUR SEASONS HEATING & AIR CONDITIONING – CHICAGO.
How does a furnace work? The heating cycle begins as soon as the thermostat determines that the temperature in the room is below the setpoint at which the consumer has set on the thermostat.
***(As a side note, a programmable thermostat can save you a great deal of money in the long run as you can easily program the thermostat with a lower setpoint for all the hours that you know you will not be home. Why keep the house any warmer than you need to when no one is there? If you know that you usually arrive home from work at 6:00pm, for example, you can program the thermostat to warm your house up for you starting at 5:30 rather than keeping the house unnecessarily warm all day long).
On most recently manufactured furnace units, this causes an inducer fan to pull air through the unit establishing adequate combustion airflow. After proper airflow is established, the gas valve opens, which causes gas to flow through the burners where it is ignited by a spark or the heat from the ignitor. (Older units have a “standing pilot” which is a small flame that burns constantly to light the gas.)
After the furnace unit senses that the flame is lit, the spark or ignitor is disengaged. Normally, the gas will burn for approximately two minutes before the blower is started. This prevents blowing cold air out through your vents at the beginning of the operating cycle.
After the preset time or at a set temperature, the blower motor is energized and air is blown over the heat exchanger. The air is then heated and supplied to the duct system and begins flowing through your registers.
When the thermostat senses that no more heat is required, the gas valve is de-energized and no more gas is provided to the furnace. The blower motor will then run for a short period of time to cool off the heat exchanger. This process is controlled by a timer or a temperature switch. Want more info see www.fshac.com for more heating & cooling information.
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Tags: Furnace, heat exchanger, heating cooling, HVAC, programmable thermostat