Thermostat Switch Settings
How to Set the AUTO ON & COOL OFF HEAT Switches on a Room Thermostat

THERMOSTAT SWITCHES, EXTERNAL - How to Set Switches on a Room Thermostat: Heat, Air Conditioning
COOL OFF HEAT, Thermostat Switch
- FAN ON / AUTO / OFF: Why Run an Air Conditioner or Heating Blower Fan Continuously?
- MODE SWITCH: Guide to Air Conditioning, Heat Pump, or Heating System COOL-OFF-HEAT Settings on Room Thermostat Switches
- FAN WON'T STOP: What to do if the Air Conditioning or Heating System Blower Fan Runs Continuously and Won't Shut Off?
LOW VOLTAGE TRANSFORMER TEST - separate article
FAN ON-AUTO or ON-MAN - how to use it? - separate article
CONTINUOUS BLOWER FAN OPERATION - separate article
HEAT ANTICIPATOR Adjustment - separate article
THERMOSTAT SWITCHES, INTERNAL- separate article
Questions & Answers about switch settings on room wall thermostats, continuous fan operation, & low voltage transformer diagnosis & repair
References

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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. This article also explains how to troubleshoot low-voltage transformers used in HVAC equipment.

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 Thermostat Switch wHere we explain how to 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, but we do not recommend that you do this without first asking for advice from your heating and service technician. Details about continuous operation of heating or air conditioning blower fans are at CONTINUOUS BLOWER FAN OPERATION .

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)?

Carrier programmable thermostat (C) Daniel Friedman

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 degF and the room temperature drops below 68 degF the heat will come on. If temperature rises above 68 degF 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.

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 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.

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

How to Troubleshoot Low Voltage Transformers for Heating & Air Conditioning Thermostats

Question: How do you tell if a transformer is bad? How do you test a low voltage transformer?

How do i tell if a transformer for thermostat is bad ?- Erwin 5/12/12

How do you test a low voltage transformer? - Den 7/16/12

Reply: how to test a low voltage transformer for heating or air conditioning

Erwin if you disconnect the low voltage wires connected to the transformer terminals and use a VOM you should find voltage, usually 14V. If the device looks burned, hot, or smells, don't even bother testing. If it has been buzzing that's another clue.

Den,

Remove all of the the low voltage wires from the two (or more) screws on the top of the transformer. Usually you'll see a small gauge red and white wire simply connected under two screws on the upper surface of the transformer itself.

Some heating transformers for thermostats that use more than two wires will have more than two screws and more than two low voltage wires connected or there may even be multiple sets of thermostat wires connected if the transformer is powering more than one room thermostat. (photo above left).

Check for output voltage: with power to the transformer "on", using the VOM on a low voltage scale (say 0-24VAC ) check for voltage between the two screw terminals. If the transformer itself is receiving 120V power but there is no voltage between the two low voltage output screws, then it is not working.

And thank you for the question - we have answered your question more extensively and have added details about low voltage transformers used on heaters and air conditioners and how the transformer is located, wired, and tested at LOW VOLTAGE TRANSFORMER TEST.

To test for a bad low voltage transformer you simply need a volt-ohm meter or a digital VOM. See USING DMMs VOMs SAFELY for advice on how to use a volt-ohm meter, and see Guide to Electrical Test Equipment for advice on how to electrical test equipment safely.

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. Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: see BOILER CONTROLS & SWITCHES, and also see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators. If your building uses warm air heat, see FURNACE CONTROLS & SWITCHES. If your building uses steam heat see STEAM HEATING SYSTEMS. Also see Heat Won't Turn Off - Stop Unwanted Heat.

Frequently Asked Questions (FAQs) About the COOL OFF ON, FAN OFF, FAN ON, AUTO, MAN switches on thermostats

Question: old Carrier Nova air conditioner that stopped working.

We have an old Carrier Nova air conditioner that stopped working.
An aircond mechanic fiddled about for ages and ended up changing the control switch to
an LE75 Temperature Control.

It now works but only heats or cools by about plus or minus 2 degrees Celsius.
As well there is only one fan speed (low) The others won't pull in but I can hear them trying.

The mechanic won't come back and others throw their hands in the air as they don't know where to start.

Any suggestions about how to tackle this problem?

Many thanks,
Peter
Sydney, Australia - 6/10/11

Reply:

Peter, I'd start with the diagnostic suggestions found at DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP

Sorry to read about such aggravation but at this point you need a service call from an experienced tech. Call someone else, talk about your concerns with the service manager, don't bad-mouth the prior tech or the new ones may figure you're hard to work for. Just explain that prior attempts to get things running didn't work and that tech gave up. Keep us posted.

Reader Follow-Up:

Hi,
Thanks for your reply,

To refine my question. ..where is the best place to start to look to get the
fan operating at full speed?

Cheers and thanks.
Peter

Reply:

For a single-speed fan, Peter, at the Thermostat turn the FAN ON switch to ON to see the fan run; Or on some air handlers that combine heating and cooling there should be a fan limit switch that may include a white knob that can be pulled our pushed to turn on the fan.

On a system that has a dual speed fan, I find that if the thermostat SET temperature is significantly below the current room temperature the control board in the HVAC air handler will step up the fan to its faster speed.

Question:

We have a new LG aircond 8month old it have runs only 5 minutes it becomes overload and compressor disconnected internally Jitu 8/11

Reply:

Jitu

Overloaded AC Compressor going off on reset? Don't keep trying to run it or you risk further damage or even a fire. You certainly need a service call from a professional.

Question: fan will not turn on and air conditioner won't turn on

I have central air and heat and the heat works fine and blows perfectly but the fan will not turn on and the air conditioner wont turn on. I recently replaced the thermostat. do you think it could be faulty? - Warren Wirth 7/12/11

Reply:

Warren: first be sure that your new TT is in COOL mode and then try the fan in ON position to see if you can force it to run - if so move the switch to AUTO. Set the TT below room temp; If nothing is happening I'd double check the thermostat wiring and confirm that there is voltage on the TT wires too - if the wires were shorted during installation you could have blown the transformer.

Question: Blower fan comes on for "no reason"

I have a Goodman AC and gas furnace. Lately I noticed that the fan will come on without the temperature on the thermostat calling for AC
The fan will come on for about 2mins and shut off.Then about 15-20mins later do the same thing. No AC just fan. And it will do the same without the thermostat being connected
Any clues as to what might be the issue?
Thanks for your help - Al at alsblackvw@yahoo.com, 8/27/12

Reply:

Al, while some systems include a defrost cycle that can run a fan without a thermostat calling for heat, I wouldn't expect that to happen every 15-20 minutes when the system is not in use. I suspect a wiring or control board problem.

Question: heat won't turn off

i set the lever to "off", but still the heating wont go off.. before the heating was on and at auto and it was not turning off after a while and then getting on again automatically like it should. - Sarah 2/9/2012

Reply:

Sarah,

If your home has more than one heating zone, another thermostat could be calling for heat;

If your home has only a single heating zone, then I suspect thermostat wiring has shorted together or the thermostat is not working properly. For now, turn off power at the switch for your heater and call a service technician.

Question: Heat won't turn on

My heat won't turn on when its on AUTO mode. - John 10/30/11

I have a 2 zone oil fired boiler, central heat/air. The 1st floor stat operates heat/air, fan comes on, everything ok. 2nd fl, when turning stat to heat, fan comes on but boiler doesn't come on. - Walt, Tunkhannock, PA, 5/13/2012

Reply:

John

Depending on your type of heat, see the diagnostic "no heat" procedure starting at

DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE

Walt,

Your question mentions a fan and air but a boiler - boilers are hot water heating systems, so I'm not quite sure what you've got installed; are there two heaters, one a furnace and one a boiler? do you have a water-to-air heating system? What? We need to know what that second upstairs thermostat thinks it's controlling. Also see the two links I gave to John, just above.

Question: How do I set the thermostat to make the air coming out of registers cooler?

How do u change the Thermostat for it too blow cooler Air. - Christie 7/5/12

Reply:

Christie, you've asked a helpful question - let's clear up some confusion about wall thermostats: the thermostat is not an accelerator - it doesn't make the air conditioner run faster or slower, nor warmer or cooler. Nope. The thermostat is basically an "ON-OFF" switch that turns the air conditioner ON or OFF in response to room temperature.

First make sure your thermostat is the one that controls your air conditioner. In some homes different or separate thermostats control heating and cooling. If you are at the air conditioner thermostat be sure it is set in COOL mode; typically the fan switch is set to AUTO. Then set the desired temperature to a number lower than room temperature and your A/C should operate. If at that point equipment does not turn on there is a problem to troubleshoot - but first let's summarize:

So if you SET the thermostat to call for a temperature lower than the actual room temperature, the A/C will run until the room temperature drops to the SET number.

Now if the A/C runs forever and never cools the room down to the SET temperature then either the SET number is lower than the capability of the air conditioning system (say setting the temp to 60 degF when it's 110 outside at a home that has a lot of heat gain)
OR
The air conditioner needs repair.

Before calling the repairman check for
- a dirty air filter or closed air supply registers - both of which reduce the cool air flow

And if you click on "Air Conditioning" in the links across the top of this page, then on the left side of that web page you'll see an article titled "LOST COOLING CAPACITY" that offers some homeowner diagnostic steps that can help figure out why the air conditioning system is not cooling as well as you'd like.

Question: test for a bad low voltage transformer

How do I test for a bad low voltage transformer - Den 7/2012

Reply:

Den,

To test for a bad low voltage transformer you simply need a volt-ohm meter or a digital VOM.

Details of exactly how to test the transformer are at LOW VOLTAGE TRANSFORMER TEST - separate article.

Question: the blower fan doesn't seem to work - it keeps trying to start

For cooling:
The compressor outside of the house is running and is cooling.
The blower start but after less than a minute the blower stop and the compressor still run outside of the house.
The blower always try to start but stop all the time.
After a while the pipe is frozen.
Why does the blower stops after a few seconds - Carol 8/10/12

Reply:

Carol, in the links listed at Related Topics check the article titled

BLOWER FAN OPERATION & TESTING

I suspect you have a bad start/run capacitor on the fan or a bad control switch or board

Question: blower fan runs continuously - it won't turn off

If coils cleaned, good charge, proper air flow and no wires touching where it's not suppose to.... what will cause the blower to continuously run? - Jerry 8/16/12

Reply:

Jerry, see the diagnostics at Fan won't stop: What to do if the Air Conditioning or Heating System Blower Fan Runs Continuously and Won't Shut Off?

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SWITCH FUNCTIONS on a Room Thermostat - see detailed links at page top & left

Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)

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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

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

Why Run an Air Conditioner or Heating Blower Fan Continuously?

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

How to Use the "COOL" thermostat switch setting position

How to Use the "HEAT" thermostat switch setting position

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

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

What to do if the Air Conditioning or Heating System Blower Fan Runs Continuously and Won't Shut Off?

How to Troubleshoot Low Voltage Transformers for Heating & Air Conditioning Thermostats

Question: How do you tell if a transformer is bad? How do you test a low voltage transformer?

Reply: how to test a low voltage transformer for heating or air conditioning

Frequently Asked Questions (FAQs) About the COOL OFF ON, FAN OFF, FAN ON, AUTO, MAN switches on thermostats

Question: old Carrier Nova air conditioner that stopped working.

Reply:

Question:

Reply:

Question: fan will not turn on and air conditioner won't turn on

Reply:

Question: Blower fan comes on for "no reason"

Reply:

Question: heat won't turn off

Reply:

Question: Heat won't turn on

Reply:

Question: How do I set the thermostat to make the air coming out of registers cooler?

Reply:

Question: test for a bad low voltage transformer

Reply:

Question: the blower fan doesn't seem to work - it keeps trying to start

Reply:

Question: blower fan runs continuously - it won't turn off

Reply:

Ask a Question or Search InspectApedia

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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair