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Traditional wall thermostat Thermostat Switch Setting FAQs
Room Thermostat Switches

  • THERMOSTAT SWITCH FAQs - CONTENTS: How to Set the External Cool-Off-Heat or Fan On Auto OFf Switches on a Room Thermostat: Heat, Air Conditioning
  • POST a QUESTION or READ FAQs about switch settings on room wall thermostats, continuous fan operation, & low voltage transformer diagnosis & repair
  • REFERENCES
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Questions & answers on how to set the room thermostat switches:

Frequently-asked questions & answers about how to set the various switches and controls on a room heating or air conditioning thermostat.

This article series explains the use, setting, and adjustment of the FAN On-Off and MAN-AUTO switches on room thermostats.



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FAQs about Switches on Room Thermostats

Thermostat COOL OFF HEAT settings explained

Recently-posted questions & answers about the settings of external switches on a room thermostat, posted originally at THERMOSTAT SWITCHES, EXTERNAL

On 2017-02-26 17:04:49.286965 by (mod)

If your system is a heat pump and is in HEAT mode and if the outdoor temperature is not so low that the heat pump can't produce heat without switching to your backup heat, then, yes the outdoor compressor/condenser unit including the compressor motor and the fan motor should run.

On 2017-02-26 08:05:42.615580 by Anonymous

is my ac condenser blower supposed to run when my thermostat is in heat mobe

On 2017-02-22 19:16:50.969393 by (mod)

Susann,

No. Air conditioners do not "run out of coolant" - refrigerant used in air conditioners and heat pumps is a sealed system and never needs replacement unless

1. there is a leak (that must be found and fixed)
or
2. the system refrigerant piping has to be cut open for other reasons, such as replacement of a compressor or expansion valve.

On 2017-02-22 07:05:38.340034 by Susann

Hello, my AC was switched on to the run side, instead of auto side.... it was on like that for maybe 1 week or more... will that cause the unit to run out of coolant faster, and cause my electric bill to increase tremendously?

On 2017-02-20 15:47:13.389344 by (mod)

Manny

Search InspectApedia for NO HEAT to see a sequence of easy diagnostic steps

On 2017-02-20 15:45:35.892619 by (mod)

Jennie

Probably not dangerous as long as the fan is running because that's how a control is set, such as a FAN ON switch on the thermostat or a FAN ON switch on the fan limit switch on the air handler.

Search InspectApedia.com for BLOWER FAN WON'T STOP RUNNING to read details of diagnosis and repair of this problem.

On 2017-02-20 12:59:57.403140 by Jennie

When I switch off my aircon and heater , the fan is still running the fan. Is it this is danger the fan is running all the time when I switch off?

On 2017-02-20 08:33:50.490266 by Manny

I kept my thermostat o and off when using the heating system but now that its on and way past 30=40 minutes already that it does not pick up running what shall I do?

On 2017-02-11 23:33:34.029799 by (mod)

Anon: check at the heater - is it running?

On 2017-02-11 23:26:31.733198 by Anonymous

I turned it up but no click as normal does heat comessage up few minutes after but now nothing

On 2017-02-07 13:45:21.375233 by Sara Williams

I was if it continuous running could it mean that I need to change my filter

On 2016-11-28 16:12:23.258437 by Landon

Moved into a new home, and wanted to know how to switch the hvac system on my thermostat ( identical to the square style shown ) to stop running on electricity and use the gas furnace only

On 2016-11-28 00:58:43.244300 by Rick

The unit is electric and the filter is changed regularly

On 2016-11-27 00:57:56.410216 by Anonymous

It's an electric unit

On 2016-11-26 21:44:22.375833 by (mod)

Rick:

Your blower is probably running in response to a re-heat of air in the supply plenum by residual heat in the heat exchanger after the burner turns off.

I'd look for a dirty air filter, dirty blower fan, or a limit switch that's not running the fan long-enough at the end of a heat-on cycle.

On 2016-11-26 15:45:01.717201 by Rick

Our furnace brings the house to the requested temp and shuts off. In about 30 seconds, the fan comes back on and will not shut off unless I throw the breaker

On 2016-10-23 03:50:07.244088 by Anonymous

I lite my France then turned on the heat switch set it to auto nothing happened then I clicked it over to fan on then it started to blow air but no heat yet

On 2016-10-22 18:59:22.768911 by (mod)

Anon I can't guess very usefully just why your heat is not working, but over at DIAGNOSE & FIX HEATING PROBLEMS-BOILER http://inspectapedia.com/heat/Diagnose_Heating_Boiler.php we give a detailed series of steps you can go through to troubleshoot the problem and fix it.

On 2016-10-22 14:55:10.077654 by Anonymous

Just put the switch on termostat to heat and nothing happened why ? And should it be on auto ?

On 2016-10-18 17:42:17.610493 by Thomas

Just moved into a new home in April and seemingly everything with the CA unit was working fine but now since it is getting cooler we turned off the cool air and my upstairs gets super hot. Even if the unit is in off mode the baseboard heat turns on? I checked the attic and the attic is cooler then the 2nd floor. Do I need to contact an HVAC company?

On 2016-09-28 18:02:51.357946 by (mod)

TJ it sounds to me as if the thermostat wiring is incorrect, and perhaps also the three thermostats on the second AC unit may all be trying to turn on the A/C but the A/C system lacks zone control. If all three thermostats turn on an air handler that blows air into a single, undivided duct system, with no zone dampers in the ductwork , they cannot work as you wish.

On 2016-09-28 17:30:35.627881 by TJ Queen Creek, AZ

We just moved in to brand new build home. We have two A/C units in attic. One half of home is okay and has one thermostat running it. The other half of home has three thermostats running it. We have had a tech out and they cannot find a problem.. Here is what happens. All units in off mode and fan position in auto mode. So essentially nothing on. When we turn on the master bedroom the air kicks in for cool and stays until temp drops to designated temp and air goes off.

When all units are off and the master bedroom off when we turn on cool for two other areas of home other than the master bedroom, that area of home goes on as well as the A/c for the masterbedroom.. If either one of the other two thermostats are turned on the air in the master bedroom goes on as well. We do not think thus is right. Any suggestions on what to do as a thermostat will kick on master when master is in off mode and fan in auto position. Thank you, TJ

On 2016-09-19 20:21:34.546906 by (mod)

How odd. You're not sleep-thermostat-adjusting, right?

I'd replace the TT batteries.
If that makes no difference, I'd be looking for
- a bad or loose wiring connection - possible
- a failing control board, switch, or relay in the air conditioning system
- a bad thermostat itself

But more details would be helpful: is the TT display going blank or what?
Is there any evidence that your system is not working properly: such as reduced air flow from a blocked or iced up cooling coil?

On 2016-09-19 16:27:51.672143 by Jason

My Thermostat jumps from cool to off in the middle of the night. What's the problem?

On 2016-09-05 15:12:54.305737 by Anonymous

We turned our unit off when we went out of town. Today I hear the fan trying to kick in, we have it set on automatic but the unit is on off. should the fan still work with the unit off?

On 2016-08-15 23:27:45.969840 by (mod)

Yes Uni, the AC compressor will turn off when the thermostat is satisfied, but the fan will keep running.

On 2016-08-15 10:11:49.659068 by uni

If the fan is set to "on" and the ac is set to cool, does the AC turn on and off automatically at the right temperature?

On 2016-08-14 18:41:53.640156 by (mod)

Sounds as if a control board or relay in your unit is defective; but first double check the thermostat settings;

On 2016-08-14 18:24:49.207980 by vicki roby

turn heat on for a short time now i can't get it to cool.switch it to cool and still heat is on and outside unit fan is not blowing.

On 2016-07-31 14:15:20.260927 by (mod)

I suspect that cool air from upper floors is falling to the lower floor and cooling beyond what that thermostat calls-for. The underlying issue may be inadequate control of air flow within the individual zones and/or the location and size of return duct inlets.

On 2016-07-30 15:01:15.928606 by Kathrine

My unit cools 3 floors. On floor 1 and 2 they each have their own thermostat. Floor 1 has this issue:
I set the cool temp to 80 and it cools to 70. I move the switch to 87 and it still cools to 72.
The electric company came out and replaced the control unit but it still does the same.

On 2016-06-14 14:50:31.761579 by (mod)

I am a bit confused by the question, Beth but it sounds to me as if either

The thermostat is not properly set to COOL mode
or
The A/C system is not working, perhaps because of a bad compressor.

Your HVAC service tech can sort that out but before calling for repair review the thermostat settings to be sure they're correct, as per the article above.

On 2016-06-14 12:44:40.775325 by Beth

I moved into apt heat/AC worked fine March-May. We had a frost early May. If I set it at 70* it would heat or cool to there & cut off until needed again. Mostly warmer weather so was AC mode. Last week I started to notice it was hot inside. I'm disabled, in bed mostly & need it slightly cooler than most so 70* is as warm as I can take it but it was 77* inside set at 70*. I first thought problem with AC then looked at weather report to find it was near 100* outside so it's been running high 90s each day.

Everything is closed but parts of late day & eve, it won't cool to 70* set at that. Is it the heat outside? I thought AC did a job regardless & went to the setting or stayed on. It stays on but often is going too high so I lowered it to 60* & it finally went to 70*. I can't keep it set that low obviously for unit or bill. Is this due to the heat & normal or is it another problem? I know very little about this unit as is an Apt & not used to this type.

On 2016-05-27 01:18:43.687271 by shavonne

HI. My outside ac compressor unit will only turn on if I keep my thermostat on the heat setting, but lower the temp. If I switch it to cool, the furnace fan goes, but it does not switch on the outdoor unit.

On 2015-11-26 04:39:25.642926 by pat

air conditioning not coming on we had heater on and switched it

On 2015-11-14 21:17:28.157085 by Anonymous

Switch in wrong position or thermostat wire damage or bad limit control, or heat is not coming on at all

On 2015-11-12 22:49:33.841351 by Ricky

The auto setting for my digital thermostat works for the air conditioning, but it won't turn the fan on when on the heating setting. The craziest thing is that the auto setting worked for the heating setting last winter, but now it won't. Any ideas?

On 2015-10-17 02:04:13.970334 by Julie

Just installed a digital thermostat. The heat button now turns the air conditioner on and the cool button now turns the furnace on. We checked all of the wiring and it looks correct. What could we have done wrong?

On 2015-10-11 04:58:35.152632 by Benoit

When it is really cold my heating system should change from heat pump to my heating coil system, but it is not. How can it change by itself? Do I need an thermometer connected to the control box?

On 2015-09-30 15:33:53.518731 by John

Ladies apartment heat was blowing out..thermostat was set auto cool,I disconnected white to white in Goodman handler..cool is back to normal.. What do I do when cold weather is back?

On 2015-09-29 17:21:16.738216 by cat

not cooling?help

On 2015-09-24 20:50:52.703363 by (mod)

Check for a FAN switch set to ON at the thermostat or at the air handler.

On 2015-09-24 17:59:37.396992 by Douglas

After turning off the ac
the indoor still runs

On 2015-08-14 16:38:24.052626 by (mod)

Clive:

If the control(s) is (are) working properly then when they try to turn the fan ON (say in MAN mode) a service tech would confirm that there is power (proper voltage) at the fan motor power leads. If so and if the fan doesn't run the problem is in the fan or its motor.

On 2015-08-14 02:38:01.666205 by Clive

My fan will not work in Manual or auto, AC and heating systems all work. Any easy diagnostics with a multimeter I can do?

On 2015-07-25 20:38:12.272463 by (mod)

Sharon

The decision to turn the air conditioning system on to cool the home interior can be based on how comfortable you feel. If the indoor temperature and humidity are comfortable you can leave the thermostat alone. If it is too warm then sure, turn on the system cooling.

Note that if your home does not have central air conditioning, turning the heating thermostat to "COOL" won't do much for you.

On 2015-07-25 19:41:18.471199 by Sharon Johnson

when it is hot outside should i have the thermsat a square box on auto to cool the inside of house.and the right of cool.?

On 2015-07-08 07:24:23.242900 by (mod)

Replace dead batteries in the thermostat.

If that does not work,

Check that electrical power is on at the air conditioner or heater controlled by the thermostat.

Check that the transformer powering the thermostat circuit is working.

Check that the wiring to the thermostat is intact.

On 2015-07-07 18:49:33.947990 by Anonymous

What do you do if the wall unit blanks out and you can't see the temperature etc and can't see the buttons to program anything?? Does this open and maybe need a battery?? Thank you




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 DIAGNOSTIC GUIDES A/C / 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

or see HEAT WON'T TURN ON

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 °F 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 the "More Reading" links at the bottom of this article 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?

Question:

(May 26, 2014) Anonymous said:
My A/C is not cooling properly. It is A holiday and can not call a tech until tomorrow. Temp says 79 in house and is set at 72. Unit keeps running, Should I turn it off so it is not continuely runningg?? Do not want to ruin something? Please advise!

Reply:

Turn the system off for 6 hours then try turning it on again.

If by luck the problem was that the cooling coil was ice-blocked, this will give you temporary relief.

Question:

(May 28, 2014) chuck said:
ac unitfan comes on but blower fan just clicks,tried anouther thermastat same result please help

Reply:

Chuck,

It sounds as if a control board or relay or fan motor has failed in the air handler.

Question:

(June 4, 2014) raffy said:
what does crc mean on the wall thermostat?

Reply:

x

Question:

(June 16, 2014) Frederick J Benning said:
how do u wire and program a honeywell T6169 fan coil thermostat during installation for auto cool

Reply:

Frederick,

Please see thermostat wiring details at THERMOSTAT WIRE CONNECTIONS

Question:

(Aug 16, 2014) Jay said:
In our building It's always a problem the air-condition. do we need to leave the setting of fan into auto mode or "On" mode

Reply:

Jay

In links at the top of this article by the photo see these two articles

CONTINUOUS BLOWER FAN OPERATION - separate article

FAN ON AUTO MAN Thermostat Switch - separate article

for how to set the fan switch. Briefly: "ON" will leave the fan running continuously. AUTO will cause the fan to turn on when there is a call for cooling.

Question:

(Aug 25, 2014) KarmaClad said:
My A/C works fine, haven't used the furnace to heat yet, but when the thermostat fan setting is switched from "Auto" to "On" the fan kicks on then sounds like it's cycling through 2 or three power settings and then at some point after usually a few minutes, it switches off. Then sometimes on and off again over and over, then randomly through the low, medium and high fan speeds and off again. When on "Auto" with the A/C on, it just kicks on when the temp gets too high, cools off the house, and then shuts off again without any issues.
Any thoughts? Bryant 80

(Dec 18, 2014) Teresa said:
What is the difference in 24 volt and 120/240 volt when putting in a thermostat?

Reply:

The thermostat purchased needs to match the voltage level supplied to it by it connecting wires - otherwise it will not work and the connections are unsafe.

Most residential thermostats use small gauge electrical wire and a 24V thermostat. Line-voltage 120V thermostats are used in some installations to operate electric heating baseboards and sometimes commercial equipment. A 120V/ 240V thermostat will be connecting to wiring rated to carry the higher amps and volts of such circuits, typically 14 or 12 gauge wiring.

See

inspectapedia.com/electric/Voltage_Measurement.php

VOLTS MEASUREMENT METHODS

Article Series Contents

...


Continue reading at THERMOSTAT SWITCHES, EXTERNAL or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see COOL OFF HEAT, THERMOSTAT SWITCH

Or see CONTINUOUS BLOWER FAN OPERATION

Or see FAN ON AUTO MAN Thermostat Switch

Or see THERMOSTAT SWITCHES, INTERNAL

Suggested citation for this web page

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

INDEX to RELATED ARTICLES: ARTICLE INDEX to ARTICLE INDEX to HVAC THERMOSTATS

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