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.
Green links show where you are. © Copyright 2017 InspectApedia.com, All Rights Reserved.
Recently-posted questions & answers about the settings of external switches on a room thermostat, posted originally at THERMOSTAT SWITCHES, EXTERNAL
On 2017 11 29 Mod said - what to check when the heat doesn't run at all
Melanie, and other readers, please check out one of the following "no heat" diagnostic procedures - choosing depending on the kind of heat you have in your home
On 2017-11-29 by Melanie
My thermostat has two bulbs of Mercury in it. Thanks
My heat does not come on at all. I have a Carrier heat pump thermostat. It is not working at all. It will not turn on at all. It was working yesterday morning when I left for work. When I came on yesterday evening it was not working.
On 2017-11-26 by (mod) - heat doesn't turn on
Are you sure if the heat is actually coming on or could it be that you're just feeling air movement from the fan being on?
Please check out the diagnostic and repair suggestions found at this page THERMOSTAT WON'T TURN ON
On 2017-11-26 by Calvin
When I leave my mode on heat, but fan on auto, the heat doesn't come on. When I put the fan on and heat on, then the heat comes on. For AC, it works on both on and auto.
On 2017-11-23 by (mod) - heat doesn't turn off
Using the InspectApedia search box just above to look for "heat won't stop" we find THERMOSTAT WON'T TURN OFF - https://inspectapedia.com/heat/Thermostat_Stuck_On.php
which contains diagnostic suggestions; please start there.
Or just go to the power switch for your furnace and turn it OFF.
On 2017-11-23 by Anonymous
My heat switch is off but heat is still coming out the tFitlrs as it's on 90 degrees what do I do to turn it off if switch is broke
On 2017-11-22 by (mod) -
Please take a look at the fan and limit control diagnosis and repair suggestions found at
And let me know if that's helpful or if you have follow-up questions.
On 2017-11-22 by Russell
That's the problem I'm having!
On 2017-11-22 by Russell
Blower set to auto comes on sometimes and then doesn't.
On 2017-11-04 by Janet
I live in an apt. Maintenance was out to change some kind of switch on the heater, then they installed a new thermostat. I've noticed that when the heat is set one degree above the room, shouldn't the heater reset to start the fan?
I keep hearing this "knocks", at least 10 of them, as if it is trying to come on (the blower). I hear the heater ignite but no blower. Don't they need to check it again? Thanks.
On 2017-08-29 by (mod) -
Set the fan switch to Auto. If you set the fan switch to on it will run continuously.
On 2017-08-29 by Dean
The a.c.fan is it to be on auto or on ?
On 2017-07-18 by (mod) -
I would ask the heating company service manager to send out an experienced repair person to make a repair that doesn't require continuous recalls.
On 2017-07-17 by Candace Clegg
I don't know if I worded that right I think he's doing it on purpose anybody know of this happening
Is there a way that the guy that fixes the things can somehow beat putting up a timer for stuff to turn off because first the heat wasn't working and he's the one put the whole thing in and then now the cooling ain't working and the fan was this is like the second time that happened every time we got to pay him like $150
On 2017-05-21 by (mod) -
I am, across the electronic distance between us, confused, uncertain just what equipment you have installed. It SOUNDS as if you're talking about a heat pump system whose outdoor compressor/condenser unit doesn't work so you want to run just your inside backup heat system.
If an outdoor unit is OFF then the whole refrigerant cycling system is OFF and you're relying on backup heat: I don't understand what defrosting in heating mode would be needed.
One could jury-rig a switch to interrupt power to an electric heater, but I can't recommend such an attempt, in part because I don't understand the problem and in greater part because fooling around with the wiring by someone not fully trained/informed could kill somebody.
On 2017-05-20 by zak
I had my outside unit (electricity only) disconnected before winter started by the furnace man so as to run just the inside furnace. (older Lennox) This is due to a fault on the circuit board causing the defrost cycle to keep running (defrosting) while in heating mode.
Otherwise the furnace works, but you just have to turn it off when it starts defrosting unlike the cooling which always works fine even still. But of course I can't use it while disconnected. It only took the furnace guy a few minutes to disconnect it. Right now I can't afford to pay him to come out and turn it back on.
I would like to turn it back on myself. How would I do this? Eventually of course I will have to replace the outside unit because they say it's to old (1975) and no parts.
On 2017-05-03 by (mod) - How do u switch from heating to cooling inside the unit itself
When you change a thermostat from HEAT to COOL mode, it will switch a second pair of wires that in turn operate the appropriate relays in your air conditioning air handler/compressor/condenser units. You should not have to change anything in the unit itself.
On 2017-05-03 by Dewey haney
How do u switch from heating to cooling inside the unit itself
On 2017-03-22 by (mod) -
That's because (perhaps) the FAN ON switch simply forces the fan on regardless of heating or cooling system temperature.
That is, your heater or air conditioner still isn't turning on.
If you've lost heat search InspectApedia for NO HEAT FURNACE using the search box just above to see helpful diagnostic and repair steps
On 2017-03-22 by D C Henderson
When turn unit to auto won't turn on but when turn fan on will come on
On 2017-02-26 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 by Anonymous
is my ac condenser blower supposed to run when my thermostat is in heat mobe
On 2017-02-26 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 by Anonymousis my ac condenser blower supposed to run when my thermostat is in heat mobe
On 2017-02-22 by (mod) Air conditioners do not "run out of coolant"Susann,
On 2017-02-22 by SusannHello, 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 by (mod)Manny
On 2017-02-20 by (mod)Jennie
On 2017-02-20 by JennieWhen 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 by MannyI 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 by (mod)Anon: check at the heater - is it running?
On 2017-02-11 by AnonymousI turned it up but no click as normal does heat comessage up few minutes after but now nothing
On 2017-02-07 by Sara WilliamsI was if it continuous running could it mean that I need to change my filter
On 2016-11-28 by LandonMoved 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 by RickThe unit is electric and the filter is changed regularly
On 2016-11-27 by AnonymousIt's an electric unit
On 2016-11-26 by (mod)Rick:
On 2016-11-26 by RickOur 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 by AnonymousI 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 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 by AnonymousJust put the switch on termostat to heat and nothing happened why ? And should it be on auto ?
On 2016-10-18 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 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 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 by (mod) Thermostat jumps from cool to off in the middle of the night.
How odd. You're not sleep-thermostat-adjusting, right?
Check for a programmable automatic setback on your thermostat.
If your thermostat doesn't have that feature then
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 by JasonMy Thermostat jumps from cool to off in the middle of the night. What's the problem?
On 2016-09-05 by AnonymousWe 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 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 by uniIf 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 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 by vicki robyturn 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 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 2016-07-31 by KathrineMy unit cools 3 floors. On floor 1 and 2 they each have their own thermostat. Floor 1 has this issue:
On 2016-06-14 by (mod)I am a bit confused by the question, Beth but it sounds to me as if either
On 2016-06-14 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 by shavonneHI. 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 by patair conditioning not coming on we had heater on and switched it
On 2015-11-14 by AnonymousSwitch in wrong position or thermostat wire damage or bad limit control, or heat is not coming on at all
On 2015-11-12 by RickyThe 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 by JulieJust 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 by BenoitWhen 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 by JohnLadies 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 by catnot cooling?help
On 2015-09-24 by (mod)Check for a FAN switch set to ON at the thermostat or at the air handler.
On 2015-09-24 by DouglasAfter turning off the ac
On 2015-08-14 by (mod)Clive:
On 2015-08-14 by CliveMy 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 by (mod)Sharon
On 2015-07-25 by Sharon Johnsonwhen 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 by (mod)Replace dead batteries in the thermostat.
On 2015-07-07 by AnonymousWhat 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
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?
Sydney, Australia - 6/10/11
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.
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.
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.
We have a new LG aircond 8month old it have runs only 5 minutes it becomes overload and compressor disconnected internally Jitu 8/11
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.
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
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.
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 email@example.com, 8/27/12
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.
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
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.
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
Depending on your type of heat, see the diagnostic "no heat" procedure starting at
or see HEAT WON'T TURN ON
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.
How do u change the Thermostat for it too blow cooler Air. - Christie 7/5/12
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)
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.
How do I test for a bad low voltage transformer - Den 7/2012
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.
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
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
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
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?
(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!
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.
(May 28, 2014) chuck said:
ac unitfan comes on but blower fan just clicks,tried anouther thermastat same result please help
It sounds as if a control board or relay or fan motor has failed in the air handler.
(June 4, 2014) raffy said:
what does crc mean on the wall thermostat?
(June 16, 2014) Frederick J Benning said:
how do u wire and program a honeywell T6169 fan coil thermostat during installation for auto cool
Please see thermostat wiring details at THERMOSTAT WIRE CONNECTIONS
(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
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.
(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?
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.
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