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Guide to Using the Fan AUTO ON Switch on Digital Thermostats
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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.
"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.
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
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02/25/2010 - 09/05/2006 - InspectAPedia.com/heat/Thermostat_FAN_Switch.htm © 2009 - 1997 Copyright Daniel Friedman All Rights Reserved - InspectAPedia® is a Registered U.S. Trademark