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Guide to Using the Fan AUTO ON Switch on Digital Thermostats
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• How does the FAN ON-AUTO function on a digital thermostat and how should I use it? How does the FAN ON-MAN switch work?
• What are the Other Little Switches For on a Room Thermostat: Heat, Air Conditioning like COOL OFF HEAT and FAN ON AUTO ?
• What to do if the air conditioner blower fan runs continuously and won't turn off
• How & why to force an air conditioning or heating fan to run continuously

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 troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards.

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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.

FAN Button AUTO and ON functions on Digital Thermostats

On most digital thermosats, 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: "ON" will set the blower fan on your heating or air conditioning system run all the time, no matter what. See CONTINUOUS BLOWER FAN OPERATION for reasons people may want to run the fan continuously.

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 therostat is satisfied, the fan will automatically turn off.

Fan operation in an air conditioning system or heat pump operating in cooling cycle works similarly

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.

How Should I Set the FAN ON-AUTO or FAN MAN-AUTO Switch on My Thermostat?

"AUTO" on the fan switch on your thermostat is the normal switch position for the fan control. Setting the fan control to "AUTO" will allow the fan to turn on when the air conditioning system (or heating system) are ready to blow cool (or warm) air into the building, and to turn off automatically when cooling or heating are not needed.

"ON" or "MAN" on the fan switch: there are reasons to leave the fan in the ON position on a heating or cooling system, but we do not recommend that you do this without first asking for advice from your heating and service technician. SWITCH FUNCTIONS on a Room Thermostat - and also FAN ON AUTO Thermostat Switch discuss how to set the HVAC blower fan to continuous operation, and what to do if the blower fan won't turn on or won't turn off

Also see the Fan Limit Switch Control for another method to cause a blower fan to run continually.

If your air conditioner or heating system fan blower unit was designed to permit the fan to run all of the time that's great and you can consider the benefits of continuous fan operation which are described in detail in the articles we list just below.

You might not want to keep the fan in "ON" mode if

Your furnace or air conditioner air handler is not designed for continuous fan duty - it might wear out the fan motor or bearings a bit early (ask your system's manufacturer or your HVAC tech about your system's capability)

Your building's heating system and return air duct design results in blowing uncomfortably cool air out of heating supply ducts when the furnace is not actually in heating mode.

See at CONTINUOUS BLOWER FAN OPERATION for details of continuous blower fan operation.

What to do if the Blower Fan Runs Continuously and Won't Shut Off?

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.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

THERMOSTATS
  Types of Building & Room Thermostats
  How Thermostats Work
  Detailed Guide to Room Thermostats
  How to Set the Thermostat
  COOL OFF HEAT Thermostat Switch
  FAN ON AUTO Thermostat Switch
  HEAT ANTICIPATOR Adjustment
  HEAT ANTICIPATOR Mini Ammeter to Check
  HEAT PUMP Thermostats - Outdoors
  INSTALL & WIRE Thermostats
  TEMPERATURE RESPONSE of Room Thermostats
  SWITCH FUNCTIONS on a Room Thermostat

• Thanks to reader Harvey Tyler for suggesting clarification about how the heating or cooling thermostat FAN ON OFF switch actually functions on a digital thermostat. 02/03/2009

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