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How A Home Furnace Works Essay
8 years ago
Alright it's time for you guys to help me with another essay, now this one I REALLY don't want to do lol so hopefully your guy's powers will help me out.
Operation Of A Home Furnace:
==========================
The operation of a home furnace is actually quite simple when you look at each process in steps. The modern furnace system has numerous built in safety systems that must all check out successfully before a furnace will even fire. Such safety systems are necessary when dealing with open flame, gas, and carbon monoxide. The following is a detailed overview of exactly how a typical 80%-90% furnace functions at start-up.
Much like your typical Air Conditioning system, which is often part of the same system as the furnace if a home has both A/C and heat, the Thermostat sets everything to follow in motion. Power is supplied to the Thermostat from the furnace, which in turn gets power from the building's electrical system, the Thermostat is more simply described as a sophisticated switch cutting off and supplying power to different subsystems as they're called for. As the temperature in the building lowers, a bi-metal coil within the thermostat subtly changes shape due to thermal expansion. Older thermostats used a glass tube with a droplet of mercury within it as part of the call-for-heat switch. The bi-metal coil would warp with the change in temperature changing the orientation of the glass mercury tube attached to it. This would cause the mercury to shift from one end of the tube to the other completing a circuit on either end of the tube as it moved to either side. Modern Thermostats no longer use mercury tubes and instead use medal contacts assisted with magnets.
As the temperature in the room falls below the preset temperature, the contacts close and send a 24 volt G and W call down to the furnace's integrated furnace control board (IFC). The IFC board is the brain of the furnace, every part of the furnace is connected to it and it's the IFC board's job to run the following sequence of operation every time the furnace turns on, as well as turn it off if any safety conditions suddenly fail to be met.
The first step of the start up sequence is to start the inducer fan motor. The inducer fan is linked to the heater coil unit where return air is blown through and heated by the furnace. The inducer fan's job is to make sure that any un-burnt gas or other harmful fumes are separated from the heated air going into the living space and instead vented out through the flue pipe, which is a dedicated exhaust duct that vents these harmful fumes outside of the building. It's worth noting that the type of flue duct is an easy way to distinguish an 80% efficiency furnace from a 90% furnace as %80 furnaces will have a larger steal flue and %90 furnaces will have a smaller PVC pipe flue.
Assuming that the inducer is running the furnace goes through a 30 second pre-purge cycle, simply running the inducer fan and pumping out any gases or fumes that may be floating within the furnace from it's last operation. The IFC board has no way of actually knowing weather or not the inducer fan is running once it's been energized, such as if the motor has burned out or for whatever other reason has ceased working. This is where the pressure switch comes in, think of the pressure switch as the IFC's nerve-endings, 'feeling' that everything is working properly. Attached to the suction end of the inducer fan housing via a small rubber hose, the pressure switch is quite simply a rubber diaphragm connected to an electrical switch. If the Inducer fan is indeed operating as it should be, the fan will be drawing in air to the flue, this will cause a small vacuum within the suction area of the fan housing which will pull in the diaphragm of the pressure switch and close the electrical sensor switch inside. This sends the okay voltage to the IFC board letting it know that the fan is running and pulling air,
After the pre-purge cycle, if the inducer fan is shown to be running, the IFC board will attempt to light the furnace. A 24 volt signal is sent to the gas valve. The gas valve is exactly what the name implies, an on/off electrically controlled valve on the main gas-line coming into the furnace, it either allows gas to flow into the system or it doesn't. With the 24 volts sent to the gas valve, the valve opens and allows gas to enter the furnace manifold and out the burner ports, after allowing gas to flow for a moment, the furnace will send 24 volt power to the gas igniters. There are several different types of gas igniters found in modern furnaces, Hot Surface Igniters which are like the glow-plugs found in diesel motors, direct spark igniters like the spark igniters you find on gas stoves. Older furnaces also used intermittent spark igniters which are the same as direct spark except that they light a pilot flame rather then the burner directly. The furnace will activate the igniter and if gas is flowing like normal, the gas will ignite and light the burners of the furnace.
Much like the inducer fan, the IFC board doesn't have any direct way of knowing weather or not the burners have ignited, this is where the flame rectifier comes into play, usually situated on the furthest burner down the line, the rectifier is a sensor situated next to the burner with a probe sticking into the path of the burner flame. This metal probe sends out a small electrical current, a little known fact is that fire can actually conduct electricity. If the flames are lit like they're supposed to be, this probe will sense a connection to ground through the flame. This flame rectification process is important, because without it the system would open the gas-valve, the burners would never light, and the furnace would instead just pump the building full of gas causing a highly dangerous explosion hazard. That or it could potentially pump lethal levels of carbon monoxide into the building. If the burners do not ignite on the first try, the IFC board will attempt to light the burners a total of three times before locking out and giving an error code.
Once the burners light and the heat pipes begin rising in temperature internal temperature sensors will since this raising heat and once a predetermined temperature is met the main blower fan at the base of the furnace is activated and starts pumping heated air into the living-space. You may be asking why the blower fan doesn't activate the moment that thermostat calls for heat. There are two reasons, the first is that it would blow unburned gas and fumes into the living space before the inducer fan has a chance to vent it out. The second reason is that delaying the blower-fan's activation allows the furnace to pre-heat the air inside before it's sent into the living-space. Without this pre-heating phase, the furnace would initially blow cold air into the living-space and then slowly heat-up afterwards.
Once the temperature within the living-space raises to the set maximum heat temperature, the thermostat ends it's 24 volt G and W call to the furnace, this tells the IFC board to deactivate the system. The gas valve is closed, stopping gas flow and extinguishing the burner flames. Many thermostats have what are called heat anticipaters which will actually shut off the furnace a few degrees below the set temperature so that the furnace doesn't overheat the building by the time it shuts off completely. The inducer fan then runs for an additional 35 seconds doing a post-purge process, pumping as much gas and fumes as possible out the flue of the unit. Once this post-purge process is completed the main blower fan shortly afterwords turns off and the furnace goes into stand-by.
Operation Of A Home Furnace:
==========================
The operation of a home furnace is actually quite simple when you look at each process in steps. The modern furnace system has numerous built in safety systems that must all check out successfully before a furnace will even fire. Such safety systems are necessary when dealing with open flame, gas, and carbon monoxide. The following is a detailed overview of exactly how a typical 80%-90% furnace functions at start-up.
Much like your typical Air Conditioning system, which is often part of the same system as the furnace if a home has both A/C and heat, the Thermostat sets everything to follow in motion. Power is supplied to the Thermostat from the furnace, which in turn gets power from the building's electrical system, the Thermostat is more simply described as a sophisticated switch cutting off and supplying power to different subsystems as they're called for. As the temperature in the building lowers, a bi-metal coil within the thermostat subtly changes shape due to thermal expansion. Older thermostats used a glass tube with a droplet of mercury within it as part of the call-for-heat switch. The bi-metal coil would warp with the change in temperature changing the orientation of the glass mercury tube attached to it. This would cause the mercury to shift from one end of the tube to the other completing a circuit on either end of the tube as it moved to either side. Modern Thermostats no longer use mercury tubes and instead use medal contacts assisted with magnets.
As the temperature in the room falls below the preset temperature, the contacts close and send a 24 volt G and W call down to the furnace's integrated furnace control board (IFC). The IFC board is the brain of the furnace, every part of the furnace is connected to it and it's the IFC board's job to run the following sequence of operation every time the furnace turns on, as well as turn it off if any safety conditions suddenly fail to be met.
The first step of the start up sequence is to start the inducer fan motor. The inducer fan is linked to the heater coil unit where return air is blown through and heated by the furnace. The inducer fan's job is to make sure that any un-burnt gas or other harmful fumes are separated from the heated air going into the living space and instead vented out through the flue pipe, which is a dedicated exhaust duct that vents these harmful fumes outside of the building. It's worth noting that the type of flue duct is an easy way to distinguish an 80% efficiency furnace from a 90% furnace as %80 furnaces will have a larger steal flue and %90 furnaces will have a smaller PVC pipe flue.
Assuming that the inducer is running the furnace goes through a 30 second pre-purge cycle, simply running the inducer fan and pumping out any gases or fumes that may be floating within the furnace from it's last operation. The IFC board has no way of actually knowing weather or not the inducer fan is running once it's been energized, such as if the motor has burned out or for whatever other reason has ceased working. This is where the pressure switch comes in, think of the pressure switch as the IFC's nerve-endings, 'feeling' that everything is working properly. Attached to the suction end of the inducer fan housing via a small rubber hose, the pressure switch is quite simply a rubber diaphragm connected to an electrical switch. If the Inducer fan is indeed operating as it should be, the fan will be drawing in air to the flue, this will cause a small vacuum within the suction area of the fan housing which will pull in the diaphragm of the pressure switch and close the electrical sensor switch inside. This sends the okay voltage to the IFC board letting it know that the fan is running and pulling air,
After the pre-purge cycle, if the inducer fan is shown to be running, the IFC board will attempt to light the furnace. A 24 volt signal is sent to the gas valve. The gas valve is exactly what the name implies, an on/off electrically controlled valve on the main gas-line coming into the furnace, it either allows gas to flow into the system or it doesn't. With the 24 volts sent to the gas valve, the valve opens and allows gas to enter the furnace manifold and out the burner ports, after allowing gas to flow for a moment, the furnace will send 24 volt power to the gas igniters. There are several different types of gas igniters found in modern furnaces, Hot Surface Igniters which are like the glow-plugs found in diesel motors, direct spark igniters like the spark igniters you find on gas stoves. Older furnaces also used intermittent spark igniters which are the same as direct spark except that they light a pilot flame rather then the burner directly. The furnace will activate the igniter and if gas is flowing like normal, the gas will ignite and light the burners of the furnace.
Much like the inducer fan, the IFC board doesn't have any direct way of knowing weather or not the burners have ignited, this is where the flame rectifier comes into play, usually situated on the furthest burner down the line, the rectifier is a sensor situated next to the burner with a probe sticking into the path of the burner flame. This metal probe sends out a small electrical current, a little known fact is that fire can actually conduct electricity. If the flames are lit like they're supposed to be, this probe will sense a connection to ground through the flame. This flame rectification process is important, because without it the system would open the gas-valve, the burners would never light, and the furnace would instead just pump the building full of gas causing a highly dangerous explosion hazard. That or it could potentially pump lethal levels of carbon monoxide into the building. If the burners do not ignite on the first try, the IFC board will attempt to light the burners a total of three times before locking out and giving an error code.
Once the burners light and the heat pipes begin rising in temperature internal temperature sensors will since this raising heat and once a predetermined temperature is met the main blower fan at the base of the furnace is activated and starts pumping heated air into the living-space. You may be asking why the blower fan doesn't activate the moment that thermostat calls for heat. There are two reasons, the first is that it would blow unburned gas and fumes into the living space before the inducer fan has a chance to vent it out. The second reason is that delaying the blower-fan's activation allows the furnace to pre-heat the air inside before it's sent into the living-space. Without this pre-heating phase, the furnace would initially blow cold air into the living-space and then slowly heat-up afterwards.
Once the temperature within the living-space raises to the set maximum heat temperature, the thermostat ends it's 24 volt G and W call to the furnace, this tells the IFC board to deactivate the system. The gas valve is closed, stopping gas flow and extinguishing the burner flames. Many thermostats have what are called heat anticipaters which will actually shut off the furnace a few degrees below the set temperature so that the furnace doesn't overheat the building by the time it shuts off completely. The inducer fan then runs for an additional 35 seconds doing a post-purge process, pumping as much gas and fumes as possible out the flue of the unit. Once this post-purge process is completed the main blower fan shortly afterwords turns off and the furnace goes into stand-by.
I also feel like I've somehow learned the magic of how heating knows when to turn on or not. What a time to be alive.
Anyway glad you enjoyed. I believe my essay from a few months back about how an Air Conditioner works is still up too.. you might like it it's even more incredible.
Here's a visual aid you might like, cars' AC units operate exactly the same. https://www.youtube.com/watch?v=NSUeRlJ2P0g
I do find this stuff kind of interesting though.