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FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
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THERMOSTATS, HEATING / COOLING
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FAN ON OFF AUTO switch on thermostats, guide to setting, troubleshooting, repair: this article explains the use, setting, and adjustment of the FAN ON- AUTO button or switch or the FAN ON-MAN switch on room thermostats. This website answers most questions about central heating system thermostats & controls, troubleshooting, installation, inspection, diagnosis, and repair.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Guide to Air Conditioning or Heating System Blower FAN ON-AUTO and FAN MAN-AUTO Settings on Wall Thermostats
How does the FAN switch work on a digital heating or cooling thermostat and how should I use the FAN switch? Details are just below.
On most digital thermostats, pressing the "FAN" button repeatedly will cycle the fan control between two positions. Here we are using a Carrier™ digital thermostat for our examples. Our pencil points to the "FAN" switch or button in this case.
FAN switch set to: "AUTO" run let the fan be turned on or off automatically under control of the heater or air conditioner itself - when the thermostat asks for heat and the furnace warms up the fan will turn on. At the end of a heating cycle when the thermostat is satisfied, the fan will automatically turn off.
On a digital room thermostat when you press the "FAN" button, look closely at the thermostat's digital display - you should see the word AUTO (photo above left) or MAN (Photo above right) appear somewhere in the display to let you know which setting you have selected.
Typically in the AUTO mode in a heating cycle the fan won't actually turn on until the furnace has warmed-up and the fan will turn off shortly after the furnace turns off at the end of a heating cycle.
Running the fan "ON" all the time, which we like to do in lots of cases, distributes air and temperatures more evenly all the time in the home and produces less of a surge of heat or cool air when the heater or air conditioner actually turns on.
The air conditioning or heating wall-mounted thermostat may have a fan or blower control switch with (usually) two set positions "ON" and "AUTO" or sometimes "MAN" or "AUTO"
On these switches located usually on the top, side, or bottom of air conditioning or heating thermostats, the "ON" position is not what you might think. But it's simple, as you'll see.
On some room thermostats the FAN switch may show positions labeled MAN and AUTO instead of ON and AUTO.
On these thermostats "MAN" or "manual" is the same as "ON" in this situation. There are advantages of running an air conditioning or heating blower fan continuously, and "AUTO" is the same as "AUTO which we discussed just above.
Don't mix up the digital thermostat's display of the word AUTO regarding fan control with the display's use of the same word AUTO elsewhere in the thermostat's display window to describe the overall thermostat MODE setting which we discuss at SWITCH FUNCTIONS on a Room Thermostat.
Some readers have written to say that their air conditioning blower was running continuously and they didn't know why.
If the blower or fan switch is set to MAN or ON, It could be that the FAN switch had simply been set to force the fan to run all of the time. Change the setting to AUTO and see what happens.
Other causes for an air conditioning blower fan that does not turn off could be a problem with the control circuit board for the air conditioning system, or there could be other operating system problems that are preventing the system from cooling air to the desired temperature.
If the blower or FAN switch is set to AUTO and the fan never turns off, call your air conditioning or heating repair company for diagnostic help, or if you've checked the switches we describe above and you want to diagnose the fan yourself, go to our blower fan diagnostic home page at FAN, AIR HANDLER BLOWER UNIT.
Frequently Asked Questions (FAQs)About the FAN switch on Thermostats
Questions & answers or comments about using and wiring the thermostat FAN AUTO, ON, OFF switch control for heating, heat pump, and air conditioning systems
Question: Where does the white wire go?
On a york roof mount heat and air system thermostat wire white go to the com terminal or the white terminal - Gary
Not sure for your system which wire goes to which terminal - take a look at the wiring instructions on the appliance or in the installation manual to be sure. If you give me the product name and model number we can help find the installation and wiring instructions for a certain answer. I didn't find a customer service telephone number for York but the company's website includes consumer help and is found at http://www.york.com.
Also see THERMOSTAT WIRE CONNECTIONS where we give common and standard thermostat wire color coding and hookups.
Question: the A/C blower is not working properly in response to the thermostat
A friend's a/c unit has the blower fan that will only work in auto, it will not work on manual. On the other hand, even when you turn the unit from cool to off, the blower still remains on. The only way the blower will turn off, is to turn the fan to manual. He changed the thermostat, and the unit will cool to the desired temp and kick off, but you can't turn the fan off on your own unless you turn it to manual. Anyone ran into this before? - John
Reply: check thermostat wiring connections against the schematic for your unit and see these standard wire color codes
The wiring to the unit depends on if you have a heat pump, or straight a/c unit. It also depends on how the installer has wired the thermostat. With the heat pump, the red is power, the yellow is for cooling, the white is for heat, the green is for the fan, the orange is for the reversing valve, and the blue is usually always common.
Again, you will have to check to see how the installer has ran the wires from the thermostat. If you have a a/c only unit outside, you will only have a red wire, and a white wire coming form the inside unit, to the outside unit. These will go on the contactor to send 24 volts to the coil to pull the contactor in. These are the traditional wiring schematics, again, check the thermostat to see if this is the case with yours. - Bryan
Also see THERMOSTAT WIRE CONNECTIONS where we provide lists and tables of color codes and wire connections for thermostats in various uses.
Question: where does the white wire go?
on a york roof mount heat and air system thermostat wire white go to the com terminel or the white termanel - Gary 7/6/11
Gary, please see THERMOSTAT WIRE CONNECTIONS
Question: proper control wiring depends on heat pump vs. A/C unit
A friend's a/c unit has the blower fan that will only work in auto, it will not work on manual. On the other hand, even when you turn the unit from cool to off, the blower still remains on. The only way the blower will turn off, is to turn the fan to manual. He changed the thermostat, and the unit will cool to the desired temp and kick off, but you can't turn the fan off on your own unless you turn it to manual. Anyone ran into this before? - John 8/17/11
Reply from reader Bryan:
The wiring to the unit depends on if you have a heat pump, or straight a/c unit. It also depends on how the installer has wired the thermostat. With the heat pump, the red is power, the yellow is for cooling, the white is for heat, the green is for the fan, the orange is for the reversing valve, and the blue is usually always common. Again, you will have to check to see how the installer has ran the wires from the thermostat. If you have a a/c only unit outside, you will only have a red wire, and a white wire coming form the inside unit, to the outside unit. These will go on the contactor to send 24 volts to the coil to pull the contactor in. These are the traditional wiring schematics, again, check the thermostat to see if this is the case with yours.
Bryan thanks for that succinct summary of usual thermostat wiring details and color codes. As it will help other readers I'll add it at "INSTALL & WIRE Thermostats" - if you want to be credited along with contact information let me know by using our website CONTACT link that displays our email addresses. Best, Dan. Also, readers with thermostat wiring questions should see THERMOSTAT WIRE CONNECTIONS - Ed.
Question: family dispute over how to operate a programmable thermostat
I am having a dispute with my family regarding the operation of our programmable thermostat. When the actual temp in the house is for example 20 degrees C., and the fan is turned to the ON position, the fan will start blowing 20 degree air. We all agree on that.
HOwever, when the option to use the TEMP UP button to raise the temperature is used, and the temperature is upped to 24 degrees for example(and the current temp is still at 20 degrees at the time that the TEMP UP is pressed), I maintain the IF the fan is again changed to the ON setting, the furnace will initially blow 20 degrees air which will increase gradually to 24 degrees as the furnace heats up. My family maintains that the furnace will immediately blow 24 degrees air (even though the current temp reading on the thermostat is 20 degrees). I believe that it will only blow the temp of air associated with the current temp. Can someone explain how it works? Thank you. - Betty 12/12/11
Reply: clarifying what the FAN ON does and the relation between thermostat control settings and temperature of air that's delivered
Let's try to clear up a few thermostat basics that may add fire to the family argument or may quench it a bit:
A thermostat is an -on-off switch, not an accelerator. More about this is at How Mercury Bulb Thermostat Switches Work & Why a Thermostat is Just an On-Off Switch.
But in sum, SETTING a temperature on the thermostat simply turns the heater or air conditioner ON. The heater or air conditioner will continue to run until the thermostat senses that the room temperature has reached the SET temperature, then the thermostat will turn that equipment OFF.
When you turn the fan to ON instead of AUTO, that tells the fan to run continuously. The temperature of air that the fan delivers will vary depending on whether or not the thermostat has ALSO turned the heater or air conditioner on or off - a separate function when you leave the fan always on. See FAN ON: Why Run an Air Conditioner or Heating Blower Fan Continuously? for details.
Questions & answers or comments about using and wiring the thermostat FAN AUTO, ON, OFF switch control for heating, heat pump, and air conditioning systems.
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