Room Thermostat FAN AUTO ON or OFF Switch FAQs

Q&A on which thermostat fan settings to use
Thermostat switch safety
Blower Fan Won't STOP

Thermostat or Blower Fan Switch Safety FAQs

When I set my thermostat FAN to AUTO or to FAN ON smoke blows into my house

The Auto Fan is bringing smoke smells into my house from outside. How do I stop the fan! On 2017-11-24 by Joan

by (mod) - smoke coming in at return air inlet?

Watch out: if a heating furnace return air is taking air from near an oil or gas burner that can cause the movement of fuel or combustion odors (gases) into the occupied space and it is likely to create a combustion air defect, or even a fatal carbon monoxide hazard

Also be sure your building has properly-installed, properly-located, tested, working CO (carbon monoxide) and smoke detectors.

It's pretty unusual for the return air and a heating system to be bringing in outside air and smoke. I would start by looking at the return air and let's to see where the smoke source is and why you're drawing outside air. Just turning off the fan may stop the problem temporarily but it will also stop your heat.

Take care of that if there's a fire outside you need to turn everything off get out of the house and call the fire department

A/C or Heat or Fan Runs When it Shouldn't or it Won't Stop Running - FAQs

Blower fan never stops when the heat is on - is that normal?

My fan wont stop when heat is on is that normal? Or does the the fan have to be set to OFF when the heat is on ? On 2019-11-16 Rick

Reply by (mod) -

In HEAT mode you'd normally leave your furnace blower fan switch set either to AUTO (fan will come on and off depending on the call for heat and the temperature in the furnace plenum - it won't blow cold air)

Or you can set the fan to ON - in which the fan will run continuously, 24/7 - which is not usually what you want in the heating season as when the furnace is not running the blower may push chilly air onto occupants.

With the fan switch at the thermostat set on AUTO, turn the thermostat all the way up so you're sure it's calling for heat.

Then stand at your furnace and watch the fan limit control dial as the burner turns on. You should see the dial rotate to the point at which it should be turning on the fan.

If it doesn't rotate or doesn't turn on the fan the problem is probably there at that switch.

That's something your heating service technician can repair or replace.

Details of how to test your fan limit switch to be sure it's doing what it should are

at FAN LIMIT SWITCH TROUBLESHOOTING 

Why does Our Furnace Fan Never Turn Off?

When using the heat function and the house reaches the desired temperature the burners turn off, but the fan runs non stop even when switch is on AUTO

But then when I use the cooling portion and the house reaches the desired temperature the fan does turn off..is the thermastat ok ? Or is the heating relay to blame? On 2019-11-07 by Dean

Reply by (mod)

Common causes of the fan running continuously are in two places

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

You'll see that there are two possible switches that will keep the fan running continuously.

Why does my A/C fan come on if the thermostat is set to OFF?

With the thermostat OFF & I turn fan to ON then the A/C comes on too.

Why would it do that? - On 2020-10-13 by Anonymous

Reply by (mod) -

Anon

If I understand, your thermostat is set to OFF, and not to "heat" and not to "cooling"

But when you turn the fan control to "ON" which is designed to force the blower fan to run, your air conditioner compressor unit (outside) comes-on.

If that's so then there is either a wiring error or thermostat wires are shorted somewhere between thermostat and control board in the air handler or compressor unit.

My furnace runs even when my new thermostat is set to OFF

My furnace wouldn't kick on. I replaced the thermostat, but when I take the button off of AUTO on the heater itself, it runs On 2019-01-05 by Susan

Reply by (mod) -

Susan

If the heater runs when disconnected completely from the thermostat wires - at the furnace controller or limit switch - and if it won't run in response to the thermostat I suspect either

the thermostat is mis-wired

or

a thermostat wire (or wires) has been cut or damaged enroute between the thermostat and the control for the furnace

or

the thermostat itself is defective or is not receiving power

If you don't have the manual for your thermostat - to make sure you wired it correctly,

see THERMOSTAT INSTALLATION & WIRING MANUALS

My thermostat is set to AUTO but the fan won't ever shut off

My thermostat is set on "auto" however, my fan won't shut off at all, ever.

I have a hot water boiler for heating, if that is relevant.

Even if I shut my air off the fan continues to run even though is set to auto. Help! On 2016-07-20 by Jennifer

by (mod) - First things to try when the fan won't turn off

Susan, a hot water boiler system doesn't use the FAN ON OFF AUTO switch discussed in this article. At least not in most system designs.

If your A/C system is set to AUTO and never turns off

Set the thermostat SET temperature to above room temperature as shown on the thermostat.

If the system does not shut off after about a minute, then I suspect that thermostat wires are shorted, there is an A/C control board problem, a relay problem, OR someone has turned on a manual FAN ON switch in the air handler itself.

See these diagnostic articles

FAN WONT STOP - LIMIT SWITCH - that's a switch on the fan limit control at the air handler

FAN WONT STOP - THERMOSTAT SWITCH - controls on the room thermostat

My Lennox G1201-82 Furnace heat and blower both won't stop running

I have a Lennox G1201-82-1 where the heat and blower won't shut off.

I installed a new thermostat and is set to AUTO. There is no heat pump. Juset heat. On 2016-12-11 by Anonymous

Reply by (mod):

You could have an error in thermostat wiring or thermostat wires that are shorted together, or at the heateer a bad control board. Since you just installed a new thermostat I suspect it's a wiring error. But first

Set the room thermostat well below the room temperature, so the thermostat should not be valling for heat.

Here are the additional diagnostic steps I recommend:

• HEAT WON'T TURN OFF - diagnostic suggestions if your heating system runs even when you don't want heat
• FURNACE or A/C BLOWER FAN WONT STOP - step by step suggestions to figure out what's wrong when the blower fan never stops
• FAN WONT STOP - THERMOSTAT SWITCH - see if the switch is working properly
• FAN ON AUTO MAN THERMOSTAT SWITCH - instructions on using the thermostat AUTO setting

Heat stops but the blower fan keeps running, 24/7 in Heat Mode but in A/C mode it's OK

Rheem blower doesn't shut off in heat mode. Had 2 new units installed with heat pumps. Heat strips were also added. Everything was installed in march.

So I used the A/C thought the summer with no problems. Unit would turn on and cool and shut off normally. Now I switched to heat and the blower does not shut off.

According to my thermostat once the selected temp is reached you hear a click and the light/lights turn off but the fan continues to blow non stop. As the temp goes down in the room you hear a click and the light turns on and starts to heat the house again.

This continues 24/7. I'm setting the heat anywhere between 65 to 68 degrees. Only way to turn off the fan is to turn system to off.

I have checked to be sure it is set to auto. I did look at the thermostat wiring guide and noticed the B wire (blue) reversing valve (heating mode) is wired and in the book says activates reversing valve in heat mode-ALWAYS ON in HEAT MODE

I pulled the thermostat off and disconnected the blue wire....didn't get any heat out of the vents but when I lowered the selected temp to at or below the current temp the you hear a click and the lights go off and the fan stopped.

I'm not an expert but I don't think the fan should be blowing 24/7 which would cause for a high electric bill. On 2016-12-04 by Mark -

Reply by (mod)

Check the step by step diagnosis and "fix" I suggest at

• FURNACE or A/C BLOWER FAN WONT STOP - step by step suggestions to figure out what's wrong when the blower fan never stops
• FAN WONT STOP - THERMOSTAT SWITCH - see if the switch is working properly

Air conditioning blower fan won't stop running

My ac blower fan wont shut off. I tried to set the fan on auto, turn(AC) it to off on thermostat; the AC shuts off but fan keeps running. I changed the fan relay, same issue.

I disconnected thermostat , fan still runs.

The cooling process is runniong properly with temperature settings. Any help appreciated. Thanks On 2016-09-13 by Antoine Sayegh

Reply by (mod) -

Antoine

You say that you've disconnected the thermostat. But if you did that at the thermostat itself, he trouble could remain as shorted TT wires - so try disconnecting those wires right at the blower unit. If the fan keeps on, then

see FAN WON'T STOP - LIMIT SWITCH for step by step help in diagnosing and fixing the trouble.

My old Vulcan furnace turns itself on in hot weather

I have a gas wall furnace located outdoors which pumps heated air via ducting to all rooms. It is a Vulcan unit, 40 years old, and do not want to replace yet.

On extreme summer heat, the fan on the outdoor unit comes on continuously and blows "room temp" air.

Although the indoor control is Off. Is the cause due to severe heat "tricking" the thermostat or the fan switch in the outdoor unit ? Please advise as annoying to switch off fan at outdoor power point. On 2018-01-09 by peter osborne

Reply by (mod) -

Peter,

When an interesting question. Indeed a typical Honeywell fan limit switch uses a rotating dial with three temperature set points to turn the fan on and off; in turn, that dial is caused to rotate by a coiled bimetallic spring within a probe placed in the warm air plenum of the furnace.

It is possible that the supply plenum is getting so hot that the furnace fan limit switch thinks it needs to do a heat purge cycle to avoid heat exchanger damage.

At FAN LIMIT CONTROL SETTINGS we note that

From the factory the FAN ON is usually set to about 130 °F.

So your outdoor furnace plenum air temperature would have to be reaching that temperature for the fan to turn "ON"

Two other articles give reasons that a furnace blower fan may run when you don't expect it to do so, though they're not exactly your situation.

Take a look nevertheless for additional operating details found at

FURNACE FAN CYCLES DURING HEAT
and at

FURNACE FAN WONT STOP

You can answer the question for us by watching the position of the rotating temperature dial on the limit switch.

Another odd possibility is that at high ambient temperatures around the furnace a loos connection or switch is malfunctioning and in essence turning on the blower unit.

How can I turn off heat without turning off the gas?

Heat on and off switch. What happens inside single stand stove when set to OFF?

I want to know how to stop heat without turning off gas fully.

My Floor unit is not used for anything but winter heat . I have a separate stand alone stove heater On 2019-04-22 by Kim

Reply by (mod) -

Kim

If you have access to the thermostat that controls your heat, simply set the thermostat dial (or digital display) to a temperature 5-10 degrees BELOW the actual room temperature (as read on the thermostat) and the heat should turn off and stay off. You don't need to do anything else.

You can, optionally, also set the thermostat switch to HEAT OFF if that switch is present - not all thermostats will provide that feature.

My thermostat doesn't turn of the A/C unit when the room reaches the set temperature

Hi, I have a question about my a/c unit. I have a two ton unit, my pressures are reading correct levels, I have new filter in my return, new thermostat wired correctly with fresh batteries

My unit will get to the satisfied temp and will not cut off within the time frame. I have been mainly raising temp to make it understand it is satisfied and it immediately shuts off like it should.

Temp raises back on the thermostat quicker then it actually does in my house; therefore turning the unit back on.

After about 7p.m., I can set it to 70 degrees and it will run for about an hour and satisfy. I do have vaulted ceilings in my living room, however; my house is very small under1200 sq.ft.. What else do I need to check or do? My electric bill of course is high. HELP! On 2016-06-18 by Dee Cooper

by (mod) - Temperature raises back on the thermostat quicker then it actually does in my house

I'd ask my heating service tech to take a look at the thermostat location and the cooling requirements of the space; if the thermostat is in a location where it improperly senses room temperature (exposed to sun, drafts, supply air) that could be the problem.

 

Why does our A/C blower Never Shut Off and after a while it won't produce cool air.

I have my air conditioning set on auto and set a on schedule for Day at 26 and Sleep 25 and cool is on.

The problem is cool air comes through the vents but the blower fan does not shut off when it reaches 26, it runs continuously and after a few hours it will no longer produce cool air.

What is the problem? - (Aug 19, 2014) Brian

Reply: check for cooling coil icing

Interesting, Brian, I'm not sure.

If the room temp satisfies the thermostat it should shut off the equipment.

But thermostats are not necessarily absolutely precise instruments.

If room temp is actually just below the trip-off point for the thermostat and the AC system is not able to drop the temperature further (for example because the cooling coil is frosting over, impeding airflow) then the system might keep running.

Your second point about no cool air makes me think that the coil is iced OR the compressor is staying off.

Check for FROST BUILD-UP on AIR CONDITIONER COILS 

Also check our diagnostics at AIR CONDITIONER / HEAT PUMP WON'T START

HVAC blower runs continuously and loud thumps occur when cooling cycle ends

I have an open floor plan/vaulted ceiling/two story home which is very difficult to keep evenly cooled in the summer.

It seems to help to run the blower fan continuously but I have noticed a loud thump inside the home when the cooling cycle completes.

To clarify the FAN is always on, when the condenser starts its cycle, I get a loud thump inside the house.

This thump does not occur when the FAN is set to auto and it turns on at the same time as the condenser.

It seems/sounds like the thermostat or something else in the system is telling the blower to turn off and then back on again.

I can imagine this is good for the blower. What can I do to stop this? (Aug 19, 2014) Nate

Reply:

Nate

Have you checked for

- loose motor mounts at a blower assembly or fan

- sudden pressurization of metal ductwork causing a section of duct to flex?

See diagnosis and repair details at BANGING NOISE at AIR HANDLERS or in DUCT SYSTEMS