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AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE
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CIRCULATOR PUMPS & RELAYS
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COOL OFF HEAT, Thermostat Switch
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FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN, COMPRESSOR/CONDENSER UNIT
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FLOODED HEATING EQUIPMENT REPAIR
HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES
HEATING SYSTEM INSPECT DIAGNOSE REPAIR
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MANUALS & PARTS GUIDES - HVAC
NO HEAT - BOILER
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ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
PULSE COMBUSTION HEATERS
Reset Switch - Heater Primary Control
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RESET SWITCH - ELECTRIC MOTOR
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STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
THERMOSTATS, WATER HEATER
THERMOSTATIC EXPANSION VALVES
ZONE VALVES, HEATING
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 article series answers most questions about central heating system thermostats & controls, troubleshooting, installation, inspection, diagnosis, and repair.
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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.
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.
"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
If your heating system does not use a fan, for example if you have forced hot water baseboard or radiator heat, this switch can be left alone as it's not doing anything for your heating system.
Our photo (above left) shows the Fan Auto-On selector switch found on the Honeywell RTH2300/RTH 221 room thermostat.
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.
Our photo (left) illustrates the FAN ON AUTO control switch (as well as the HEAT OFF COOL switch) on a 3M Filtrete 7-Day programmable room thermostat. Watch out: the Filtrete™ 3M-22 is a battery-operated thermostat. If the batteries fail the thermostat may fail to provide heating or cooling and the building may suffer accordingly.
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, 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.
Reader Question: family dispute over how to operate a programmable thermostat & the FAN ON Switch
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 BLOWER FAN CONTINUOUS OPERATION for details.
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