Thermostat Switch Setting FAQs

Room Thermostat Switches

Heat Won't Turn On FAQs

What to check when the heat doesn't run at all

My thermostat has two bulbs of Mercury in it.

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 2017-11-29 by Melanie -

Reply by Mod

Melanie, and other readers,

First check that your thermostat is set to HEAT mode and that the temperature setting is above the current room temperature shown on the thermostat's thermometer.

Then if your heat still doesn't come on, check out the following "no heat" diagnostic procedures - choosing depending on the kind of heat you have in your home

• NO HEAT - BOILER - hot water heat, baseboards, radiators, steam heat, hot water radiant heat
• NO HEAT - FURNACE - forced hot air heat
• HEAT PUMPS - heat pumps as primary heating system - or see LOST COOLING CAPACITY (the heat pump won't run)
• BACKUP HEAT for HEAT PUMPS - back up heat for a heat pump system

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

Start at

HEAT WON'T TURN ON

Also see

COOL OFF HEAT THERMOSTAT SWITCH

 

With Fan on AUTO & Mode set to HEAT, Heat doesn't turn on

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 2017-11-26 by Calvin -

Reply by (mod)

Calvin,

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 THERMOSTAT WON'T TURN ON

Heat Won't Turn Off FAQs

Heat doesn't turn off

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 broken - On 2017-11-23 by Anonymous-

Reply by (mod)

Anon

Using the InspectApedia search box to look for "heat won't stop" we f

ind THERMOSTAT WON'T TURN OFF -

which contains diagnostic suggestions; please start there.

Or just go to the power switch for your furnace and turn it OFF.

Also see COOL OFF HEAT THERMOSTAT SWITCH

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.

See the diagnostics at

• HEAT WON'T TURN OFF

Also see

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

Fault on the circuit board caused the defrost cycle to keep Heat ON

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 2017-05-20 by zak -

Reply by (mod) -

Zak:

I am 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.

Fan Settings at the Thermostat, FAQs

Should the A/C Fan be set to AUTO or to FAN-ON?

The a.c.fan is it to be on auto or on ? On 2017-08-29 by Dean

Reply by (mod) -

Dean,

Set the fan switch to Auto. If you set the fan switch to on it will run continuously.

See the instructions and diagnostics at

• BLOWER FAN WON'T RUN in AUTO - COOL
• BLOWER FAN WON'T RUN in AUTO - HEAT
• BLOWER FAN WONT START
• BLOWER FAN WONT STOP - LIMIT SWITCH
• BLOWER FAN WONT STOP - THERMOSTAT SWITCH
• FAN AUTO ON THERMOSTAT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH
• FURNACE FAN WONT START
• FURNACE FAN WONT STOP

• Fan Only Runs in the FAN ON position

  When turn unit to auto won't turn on but when turn fan on will come on On 2017-03-22 by D C Henderson -

  Reply by (mod) -

  DC
  
  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

  See the instructions and diagnostics at

  • BLOWER FAN WON'T RUN in AUTO - COOL
  • BLOWER FAN WON'T RUN in AUTO - HEAT
  • BLOWER FAN WONT START
  • FAN AUTO ON THERMOSTAT SWITCH

Fan will not turn on and air conditioner won't run

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.

Also see

• COOL OFF HEAT THERMOSTAT SWITCH
• FAN AUTO ON THERMOSTAT SWITCH
• FURNACE FAN WONT START

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.

See the explanation and advice

at FURNACE FAN CYCLES AFTER HEAT

Miscellaneous Thermostat Setting FAQs

How do you switch from heating to cooling inside the unit itself

How do u switch from heating to cooling inside the unit itself On 2017-05-03 by Dewey haney

Reply by (mod) -

Dewey

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.

See COOL OFF HEAT THERMOSTAT SWITCH

You should not have to change anything inside the furnace unit itself.

Should my outdoor condenser unit run if the thermostat is set to HEAT ?

is my ac condenser blower supposed to run when my thermostat is in heat mode On 2017-02-26 by Anonymous -

Reply 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.

If A/C is left switched to ON will that cause the unit to run out of refrigerant?

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 2017-02-22 by Susann -

Reply by (mod) Air conditioners do not "run out of coolant"

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.

Thermostat jumps from cool to off in the middle of the night.

My Thermostat jumps from cool to off in the middle of the night. What's the problem? On 2016-09-19 by Jason

Reply by (mod)

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?

What to do if the thermostat display is blank

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 On 2015-07-07 by Anonymous

Reply 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.

Old Carrier Nova air conditioner 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 GUIDE A/C or 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.

...

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: you can't do that at the thermostat

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.