How to set the room thermostat switches: this article explains the use, setting, and adjustment of the FAN On-Off and MAN-AUTO switches on room thermostats.

We explain why you might want to leave the fan ON continuously, and we review use of the MODE switch to change between COOL, OFF or HEAT.

We also explain what to do if your air handler blower fan won't turn off and keeps running continuously.

Switches on Room Thermostats: what they do, how they work, how to set them

Article Contents

• THERMOSTAT SWITCHES, EXTERNAL
  • COOL OFF HEAT, Thermostat Switch
  • FAN ON-AUTO or FAN ON-MAN
  • FAN WONT STOP - THERMOSTAT SWITCH
  • MODE SWITCH

 

 

MODE SWITCH: Guide to Air Conditioning, Heat Pump, or Heating System COOL-OFF-HEAT Settings on Room Thermostat Switches

How does the COOL-OFF-HEAT Switch work on a heating or cooling thermostat? Details are just below.

The thing that confuses some building owners is that a thermostat that is used to control air conditioning as well as heating may have a "mode" switch that allows the occupant to change the thermostat between "Cooling" or "Air" or "Air Conditioning" mode and "Heat" or "Heating" mode.

There may be a third position labeled "Off" which instructs the heating or air conditioning system not to run at all.

How to Use the "COOL" thermostat switch setting position

During the cooling season when you need air conditioning, set this switch to the "COOL" position. This tells the thermostat what to do as temperature rises above the thermostat's setting (turn on the air conditioning system) or when room temperature falls below the thermostat's set temperature (turn off the air conditioner).

It's a good idea not to turn your air conditioning system on in cooling mode when it has been cold outside in the last 24hours. Unless your system is a heat pump designed to switch back and forth in all seasons, or unless a special heater has been installed on your air conditioning compressor, suddenly turning the air conditioning system (and the compressor component of it) on when it has been cold can damage the compressor.

How to Use the "HEAT" thermostat switch setting position

During the heating season, when you will need warmth, set this switch to the "HEAT" position. This tells the thermostat what to do when temperatures fall (turn on the heating system) or when temperatures rise to the thermostat setting (turn off the heating system).

How to Use the "OFF" Switch Position on a Room Thermostat

"OFF" switch on the thermostat or on the fan switch, if there is one, will tell the thermostat to keep the equipment it is controlling turned off regardless of whether we're in heating or cooling season. On thermostats usually the "OFF" position is in the middle between "COOL" and "HEAT" positions.

Why do you think this might be? It's not smart to switch a heat pump or a combination heating and cooling system right from "COOL" over quickly to "HEAT" or vice-versa.

Watch out: Imagine driving down the highway with the automatic transmission on your car set to the "D" for "drive" - would you switch the transmission into "R" for "Reverse" at 60 MPH? (No "R" does not stand for "Race"). Well you would only do it once - after which your transmission would perhaps be in the road behind the car.

DON'T keep switching your air conditioner, heat pump, or heating system on and off rapidly.

Taking photos for these articles I kept switching my digital thermostat between OFF HEAT COOL AUTO - which led to having no heat at all.

I screwed up the control. The "fix": I turned everything off at the service switch at my heater for five minutes to reset the system, turned things back on, and re-set the thermostat to MODE=AUTO and FAN=AUTO.

MODE BUTTON: Guide to Air Conditioning, Heat Pump, or Heating System

COOL-OFF-HEAT-AUTO Settings on Room Thermostat Switches that use a MODE Button

Unlike the round or square wall thermostat shown above, your room thermostat might look like this Carrier wall thermostat shown below (photo at left). Notice the buttons in the closeup (below right)?

The MODE button is at the upper right on this Carrier programmable digital thermostat. Pressing the MODE button repeatedly will cycle the thermostat among these overall thermostat control settings:

OFF - turn everything off - no heating, no cooling, no fan, no nothing. Also
see

HEAT WON'T TURN OFF - Stop Unwanted Heat explanation of why heat may continue to come out of radiators or baseboards even though you have turned down the thermostat. Stop wasted heat by making sure you are not delivering heat when it is not needed or not wanted.

HEAT - put the thermostat in HEATING mode

COOL - put the thermostat in COOLING mode

AUTO - put the thermostat into AUTOMATIC mode - the thermostat will call for the heat to turn on or the air conditioning to come on depending on the temperature set on the thermostat.

For example if your thermostat is set to 68 °F and the room temperature drops below 68 °F the heat will come on. If temperature rises above 68 °F the cooling system will come on - presuming your building HVAC system includes both heating and cooling.

In our lab we leave this thermostat MODE set to AUTO.

WARNING: as we explained just above, don't keep cycling among these modes - you might make your thermostat get nauseous.

FAN SWITCH: Guide to Air Conditioning, Heat Pump, or Heating System Blower FAN ON-AUTO or FAN ON-MAN Settings on Wall Thermostats

How does the FAN switch work on a heating or cooling thermostat and how should I use the FAN switch?

Details about this switch are

at FAN ON AUTO MAN Thermostat Switch

Here we explain how to use it and how to determine how the switch has been set on a digital thermostat. The basics are just below.

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" on the fan switch will cause the heating or air conditioning fan or blower to run continuously. "MAN" or "manual" is the same as "ON" in this situation. There are advantages of running an air conditioning or heating blower fan continuously - which we discuss

at CONTINUOUS BLOWER FAN OPERATION.

"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.

Why Run an Air Conditioner or Heating Blower Fan Continuously?

There are reasons to leave the fan in the ON position on a heating or cooling system, such as for more even indoor temperature distribution or improved indoor air filtration. But we do not recommend that you do this without first asking for advice from your heating and service technician. Not all blower fan motors are designed for continuous operation.

Details about continuous operation of heating or air conditioning blower fans are

at CONTINUOUS BLOWER FAN OPERATION .

What to do if the Air Conditioning or Heating System 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 on the Room Thermostat 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. Details about this switch are

at FAN ON AUTO MAN Thermostat Switch

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 at the Limit Switch in the Air Handler Unit is set to MAN or ON, that switch will cause the air handler unit's blower to operate continuously.

More details about the fan control on the limit switch in the air handler are |

at FAN WONT STOP - LIMIT SWITCH.

At CONTINUOUS BLOWER FAN OPERATION we explain how at either the wall thermostat or at the switch in the air handler switches can be set to keep the blower fan on. Sometimes we want to do this for improved air filtration or more even temperature control in a building.

Is it Time to Call the HVAC Repairman to fix a Fan Stuck On problem?

If after checking the switches above 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.

Other snafus such as a shorted thermostat wire can also cause the A/C or warm air furnace fan to run continuously. Those problems are explained

at THERMOSTAT WON'T TURN OFF.

For hot water heat or steam heat that won't stop,

see HEAT WON'T TURN OFF for an explanation of why heat may continue to come out of radiators or baseboards even though you have turned down the thermostat. Stop wasted heat by making sure you are not delivering heat when it is not needed or not wanted.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Sandee Furnace or Air Conditioner Blower Fan Runs Intermittently - why?

I didn't see anything about when a fan sporadically turns on and off when the heating and a/c are off.

My thermostat does not have an off selection for the fan, just on/auto. I have turned the heat and a/c to off but the fan still cuts on several times a day.

I want to have my windows open to enjoy the good weather but I'm nervous to with the fan coming on.

I've checked the vents, nothing is coming out of them. Any ideas? -On 2021-05-26 by

Reply by (mod) -

@Sandee,

Thank you that's a very helpful question about the causes of the blower fan running intermittently during the day when there is no call and has been no call for heat and no call for air conditioning for a quarter of an hour or longer.

You say no air is coming out of the vents but your blower fan is running. That's quite odd if you do see air flow when there is a call for heating or cooling.

To have room for a detailed reply I repeat your question and make some diagnostic suggestions

at BLOWER FAN CYCLES ON INTERMITTENTLY

Please take a look at that article and let me know what you find or ask further questions as needed.

Furnace blower will keep running even after heat shuts off

My furnace blower will keep running even after heat shuts off however this only happens when the furnace is set at 68 degrees what do you think is bad? relay,limit swtch,or thermostat. thanks - On 2020-12-07 by Stu

Reply by (mod) - normal cool-down fan run time after call for heat is satisfied and furnace burner shuts off

Stu

It is normal and proper for the blower fan to run for a minute or two after the heat shut's off; that's a safety feature to avoid overheating and cracking the heat exchanger - such a crack would be unsafe.

Blower will keep running even after heat shuts off - is this a bad thermostat switch?

My furnace blower will keep running even after heat shuts off however this only happens when the furnace is set at 68 degrees what do you think is bad? relay, limit swtch, or thermostat. thanks - On 2020-12-07 by Stu

Reply by (mod) -

Stu

It is normal and proper for the blower fan to run for a minute or two after the heat shut's off; that's a safety feature to avoid overheating and cracking the heat exchanger - such a crack would be unsafe.

How can I get the fan to hook up to come on when I turn on my heat or air ?

I purchased and had installed a Goodman heat/ac pump . It connects to a mobile home , in which is a furnace with a single speed blower fan . The furnace coils are unhooked and the blower fan has to be turned on/off from the fan switch at the base of the housing in the furnace . How can I get the fan to hook up to come on when I turn on my heat or air ?

I’ve tried a couple of different ways I saw in online diagrams but neither were successful . The last attempt I tried to do it using a 4 prong relay and that failed as well . I have no idea how or were to hook up the green fan wire so my fan works accordingly ? Please help if you can . On 2020-09-21 by Michael Trent

Reply by (mod) -

Michael:

A wall thermostat is chosen that can control both heating and cooling modes, and the appropriate wires are run between the thermostat and the control board in the air handler and/or heat pump.

Starting at THERMOSTAT WIRE CONNECTIONS we give detailed as well as generic wiring instructions for most thermostats and thermostat applications

 

Can I remove the thermostat cover and clean it with canned air?

Can I take the cover off the thermostat and blow it out with caned air On 2019-11-09 by Paul

Reply by (mod) -

Sure Paul, though take care

1. don't blow dust into your eyes

2. don't blow so close and hard as to blow off any small components

Show us a photo of the dust-clogged thermostat before you try blowing it out

On 2019-11-02 - by (mod) -

Sun

Check the diagnostic steps starting at FAN WONT STOP - THERMOSTAT SWITCH inspectapedia.com/heat/Thermostat_Switch_Settings.php#FanStuckOn

and

Check to see if there is a manual FAN-ON switch on the fan limit control right in the air handler discussed at FAN WONT STOP - LIMIT SWITCH inspectapedia.com/heat/Furnace_Fan_Wont_Stop.php

If you've checked and there IS no manual-ON switch on your fan limit control at (or in) the air handler or furnace itself,

and

if you're dead certain that your thermostat is not calling for heat

and

you made sure that there is no manual FAN-ON switch set on the thermostat itself to force the fan to run

Then there may be shorted thermostat wires (happens but not common) or a defective thermostat (happens but not common) or a defective fan relay in the air handler

You can turn the entire furnace or air handler OFF but you'll have no heat; so you might want to let the fan run so you can continue to have heat (or cooling) but call for repair immediately as whenever a control isn't working, before it's diagnosed we can't be sure the system is safe.

Blower fan won't stop - I've tried everything - is this a thermostat FAN ON switch problem?

I've tried everything n still Blower fan won't shut off..do I disconnect it n leave it alone? On 2019-11-02 by Sunshine

Reply by (mod) -

Sunshine

Please see the diagnostics at

• BLOWER FAN WONT STOP - LIMIT SWITCH
• BLOWER FAN WONT STOP - THERMOSTAT SWITCH

and let me know if you are still having trouble

 

Reader Question: Blower Fan Only Works on AUTO: 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.

...

Continue reading at COOL OFF HEAT, THERMOSTAT SWITCH or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see THERMOSTAT SWITCH FAQs - questions and answers posted originally on this page.

Or see these

Recommended Articles

• BACKUP HEAT for HEAT PUMPS - back up heat for a heat pump system
• BLOWER FAN SPEED SETTINGS - how to set the air handler fan speed
• CONTINUOUS BLOWER FAN OPERATION
• COOL OFF HEAT, THERMOSTAT SWITCH
• FAN ON AUTO MAN THERMOSTAT SWITCH
• FAN ON SWITCH ADDITION
• NO HEAT - BOILER - hot water heat, baseboards, radiators, steam heat, hot water radiant heat
• NO HEAT - FURNACE - forced hot air heat
• NO HEAT - HEAT PUMP
• THERMOSTAT SETBACK ADVICE
• THERMOSTAT SWITCHES, INTERNAL
• THERMOSTAT SWITCHES, EXTERNAL
  • COOL OFF HEAT, Thermostat Switch
  • FAN ON-AUTO or FAN ON-MAN
  • FAN WONT STOP - THERMOSTAT SWITCH
  • MODE SWITCH
• THERMOSTAT WON'T TURN OFF
• THERMOSTAT WON'T TURN ON

Suggested citation for this web page

THERMOSTAT SWITCHES, EXTERNAL at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC THERMOSTATS

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