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Fan ON AUTO switch settings explained Fan Works Only on FAN-ON not on AUTO
Heating Furnace Fan Won't Run if Thermostat set on AUTO

Why does the air blower fan or furnace fan run only if the thermostat is set to FAN-ON and your system is in HEAT mode? Why won't the fan work if the thermostat is set to FAN AUTO?

We describe at least 17 different possible causes of "Fan Wont' Run on AUTO", some more-likely and most quite easy to test.

This article series explains the use, setting, and adjustment of the FAN ON- AUTO button or switch or the FAN ON-MAN switch on room thermostats.

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Heating System Blower Fan ONLY Runs if on FAN-ON or MAN

Fan limit switch settings (C) Carson Dunlop Associates

Question: fan works only if thermostat set to FAN ON

2020/01/21 Anonymous asked:

Fan will work only when thermostat set to fan on.

Fan won’t work when thermostat set to fan auto

This Q&A were posted originally at FAN ON AUTO MAN Thermostat Switch FAQs

Reply: Why the blower fan runs only in the MAN-ON switch but won't run on AUTO

Thank you for the question, Anon, as it's a common A/C, or heating fan operating problem that needs to be understood:

I put it as "The blower fan runs only in the MAN-ON switch but won't run on AUTO".

We will begin with a few very basic and simple things to check that can explain why the indoor air handler blower fan won't run when the thermostat fan control is set to AUTO - these pertain to both heating mode and cooling or air conditioning mode.

Then on this page we'll continue with diagnosis and repair steps for the heating-mode problem, while sending you to a separate page for cooling-mode problems with the FAN AUTO function.

If your HVAC blower fan won't run in AUTO for either cooling or heating modes, step through the diagnostics on this page.

Diagnosis and Repair Steps

Below we continue with diagnostics for why the indoor air handler blower fan won't run in AUTO for two different cases: Air Conditioning or Heating mode (set at the thermostat).

  1. Rule out bad fan motor or no power to fan.

    First, by noting that the fan will run when you set the thermostat to FAN ON we know that the fan has power and the fan motor starts and runs just fine. So the problem is elsewhere.
  2. Rule out thermostat wiring to fan

    Given the simplicity of most thermostat wires and thermostats themselves, while we may come back to it, most-likely we also know that the 24VAC wires between thermostat and the blower or air handler as well as the thermostat itself are probably OK.

    So for now we'll rule out the thermostat and thermostat wiring.

combination furnace control Honeywell L4064B with cover on... furnace fan limit switch

In our photos above that white switch found on some Honeywell fan limit controls can be used to turn the fan on continuously - another way to confirm that the fan can be forced to run.

  1. Test MANUAL ON or FAN-ON at the Thermostat and/or at the Fan Limit Control

    Manual-On furnace fan:

    If you push the white fan override button "IN" to set the fan to its MANUAL position that will force the furnace blower to run continuously. If your fan was set to run continuously at either the thermostat or at the limit switch you'll need to turn that feature off.

Below we continue with diagnostics for why the indoor air handler blower fan won't run in AUTO for two different cases: Air Conditioning or Heating

Diagnose a Fan that won't run in AUTO mode on a call for HEAT

  1. Be sure the fan is set to AUTO:

    We'll continue with the fan NOT set to run continuously. Be sure that at the thermostat the FAN is set to AUTO and if your limit control has a white switch as shown in our photos, pull it OUT to prevent running the fan manually.
  2. Prepare to inspect the limit control:

    To do this you'll have to open the furnace front access door giving view of the burner and the interior controls - (not the blower door as that will prevent blower from running).

    Find the FAN LIMIT SWITCH (See that article and return to this page if you don't know what the fan limit control switch looks like)

    and if your control sports a metal cover, remove that too.

    Watch out: Take care not to touch wires or any switches as you could get shocked.

    Watch out: Also do not adjust, touch, twist, or press on the round dial that you will see inside most fan limit controls;

    doing so can bend its bimetallic spring, put the switch out of calibration, and can seriously damage your heating system or even make it fatally dangerous. N/
  3. Turn up the heat

    to cause the furnace to run.

    While you watch conditions at the furnace, have a helper turn the thermostat all the way up, as high as it'll go - thus well above room temperature - while at the furnace you WATCH the BURNER and the fan limit control.

    Depending on fuel type you may hear a buzz or click of a gas igniter or the whir of the oil burner, or the click of a relay on an all-electric heater.

  4. The burner should turn on.

    But then the heat source - let's say burner - will turn ON.

    Oil burner: you'll hear the burner run.

    Gas burner: you'll see the burners ignite and may hear the flame.

    If the burner never turns     on then the problem isn't the with the fan control and you'll need to take other steps we'll discuss below.

furnace fan limit switch

  1. NOW with the burner ON, Watch the round dial

    on the fan limit control (most limit controls have one but depending on model yours may not).

    As the air plenum in the air handler is heated by the burner the sensor on the fan limit control will respond. IF your limit control has a round dial you will see the dial rotate as temperature increases.
  2. If the limit control is working,

    you'll see the dial start to rotate towards the FAN ON temperature;

    when the plenum temperature reaches the FAN ON temperature (read on the face of the dial)
  3. The blower should start.
  4. If the blower does not start,

    the most likely the limit control temperature sensor is not working and the control needs to be replaced. More details are below.

If the limit control is not working to turn the fan on when the plenum is warm, the plenum will continue to get hotter; as the burner remains on the plenum temperature will reach the HI LIMIT and a safety circuit in the limit control will turn the burner OFF.

Watch out: If this happens don't keep trying to run your heater as you could seriously damage the heat exchanger and even cause it to crack, risking fatal carbon monoxide poisoning of building occupants. Leave the heater off and call for repair.

Possible Explanations for Why the Fan Runs only in MANUAL or FAN ON Mode:

  1. No heat (if the fan problem occurs in HEAT mode):

    The furnace heat source is not working, at all - no fuel, no power, off on safety reset - the burner never comes on
  2. Bad temperature sensor / bad limit control switch:

    The furnace heat source is working but the fan limit control or temperature sensor that turns ON the fan is broken

    or has a loose wire or bad electrical connection
  3. Bad control board:

    The furnace control board that operates the blower assembly is damaged or defective

...

Uncommon Problem at the Thermostat Prevents Fan from Running on AUTO

  1. Bad thermostat -

    There is an unlikely possibility that there is a problem at the thermostat itself, in the thermostat.

You can eliminate that chance by the following step to remove the thermostat and its wiring from the equation and run the furnace directly.

Watch out: if you are not trained in safe and proper electrical and control wiring, don't touch wiring or control boards inside your HVAC system: you could be shocked or killed or you could damage the equipment. Ask your trained HVAC service technician for help.

  • At the heater or air handler, disconnect the thermostat wires entirely - from the two T T terminals and use a small jumper to connect the two T T terminals together.

    This is in effect exactly the same as what happens when the thermostat calls for heat - it's acting as a simple on-off switch (in response to room temperature).
  • If the burner starts, the plenum heats, and the blower starts as it should, then you have isolated the problem back to the thermostat or its wiring.

...

Other Blower Fan Won't Run Problems

  1. The fan will run and blow warm air but only in FAN or FAN ON mode -

    delivering heat - but NOT in AUTO mode.

    This is a bad temperature sensor in the unit; replace the fan limit controller.
  2. Fan never runs:

    In AUTO mode the furnace will heat up but the fan won't run, the heater then reaches the HI LIMIT and the system shuts down - the fan never runs.

    For this case or if your blower fan or furnace fan won't start at all, see

    FAN WON'T START where we give 12 reasons why the fan won't run.
  3. Wrong Fan Control Wiring

    Watch out: All of the blower fan diagnostic steps we gave earlier PRESUME that no one has recently changed any wiring or controls on the furnace or air handler.

    It is possible that a wiring error or a PRIMARY control change may cause the blower to run only in MAN or FAN ON mode

    Example: If the furnace was originally controlled by and wired for a Honeywell R7284 limit control and if that control was replaced by a Honeywell R7184 limit switch, the latter has no "LIMIT" wiring terminal. An installer may have spliced the original red limit wire in to the new R7184. If so, that wire may need to be removed.

    Special thanks to @EdTheHeaterMan who explained and illustrated this over at HeatingHelp.com - retrieved 2020/01/21 original source: forum.heatinghelp.com/discussion/176516/furnace-only-runs-with-fan-on-manual

What the Fan-on-Man-Only problem is NOT likely to be

  • It's not likely to be the thermostat wires themselves between thermostat and air handler/blower control board as you're able to turn on the fan from the thermostat
  • It's not likely to be fan wiring, fan motor, fan motor start/run capacitor as the fan runs in MAN or FAN ON mode
  • It's not likely to be the fan relay itself since if it were a bad relay the fan wouldn't turn on in MAN or FAN-ON mode either.

...

Diagnose Blower Fan that Won't Run in AUTO in COOL or Air Conditioning Mode

This topic has moved to

BLOWER FAN RUNS ONLY ON FAN-ON / MAN - COOL

...

Fan Limit Control & Thermostat Wiring References

White Rodgers Fan/Limit control switch No. 5D51-78 with an 11-inch probe (C) InspectApedia.com

  • FAN LIMIT SWITCH INSTALLATION & WIRING links to manuals and wiring diagrams for all brands of fan limit controls switches
  • HONEYWELL CONTROLS On HVAC Equipment, thermostats, primary controls, zone valves - complete list of references, manuals etc.
  • HONEYWELL TRADELINE PARTS CROSS REFERENCE GUIDE [PDF]
  • HONEYWELL L4046B / L4064 FAN LIMIT SWITCH INSTALLATION & WIRING at InspectApedia.com
  • HONEYWELL L4064 FAN LIMIT CONTROL INSTALLATION INSTRUCTIONS [PDF]
  • HONEYWELL L6064 UNIVERAL TWO SPEED FAN & LIMIT CONTROL MANUAL [PDF] (2006) L6064A Universal Two Speed Fan and Limit Controllers
  • WHITE RODGERS 5D51-35, -78 & -90 FAN & LIMIT CONTROLINSTALLATION INSTRUCTIONS [PDF] PART NO. 37-1558B White-rodgers Division Emerson Electric Co. 9797 Reavis Rd., St. Louis, Mo. 63123-5398 (314) 577-1300, Fax (314) 577-1517 9999 Hwy. 48, Markham, Ont. L3p 3j3 (905) 475-4653, Fax (905) 475-4625
  • WHITE-RODGERS 5D51-78 LIMIT CONTROL SPECIFICATIONS [PDF] (also lists the Honeywell L4064 Limit Control)
  • [4] Azel Technologies Inc., P.O. Box 53138 10 Royal Orchard Blvd. Thornhill, Ontario, Canada L3T 7R9 Ph: 905-223-5567 Fax: 905-223-3778 Email: info@azeltec.com, Website: www.azeltec.com.
  • [5] Honeywell Controls, the company wants you to use their contact form at this web page: http://www51.honeywell.com/honeywell/contact-support/contact-us.html
    Honeywell Consumer Products, 39 Old Ridgebury Road Danbury, CT 06810-5110 - (203) 830-7800

    World Headquarters, Honeywell International Inc., 101 Columbia Road, Morristown, NJ 07962, Phone: (973) 455-2000, Fax: (973) 455-4807 1-800-328-5111
    • Honeywell product model numbers & instruction Manuals:

      see yourhome.honeywell.com/home/Applications/FindYourModelNumber.aspx
    • HONEYWELL R7284 OIL PRIMARY CONTROL MANUAL [PDF] (2016) R7284B,P,U,G Electronic Oil Primary, EnviraCOMTM Enabled 69-2467EFS-07 Honeywell International Inc. 1985 Douglas Drive North Golden Valley, MN 55422 customer.honeywell.com
    • HONEYWELL R7284U OIL PRIMARY CONTROL OVERVIEW [PDF] Digital, Universal replacement for Honeywell R7184s Honeywell R8184s and also primary controls on Beckett, Carlin, and White Rodgers oil primary controls - note that this is a primary control not a limit switch
  • [6] White Rodgers Thermostats and HVAC controls,

    Homeowner information: http://www.emersonclimate.com/en-US/brands/white_rodgers/Pages/wr-homeowner-info.aspx

    Contractor information: http://www.emersonclimate.com/en-US/brands/white_rodgers/wr_contractor_info/Pages/white-rodgers-contractor-info.aspx

    White Rodgers Product Catalog (don't misspell the company's name as White Rogers Thermostats) -

    http://www.emersonclimate.com/Documents/thermostats.pdf - Thermostat Catalog
  • [7] White Rodgers 1F90 Low Voltage Digital Comfort-Set thermostat Installation Instructions, PN 37-3654, White-Rodgers Division, Emerson Electric Co., 9797 Reavis Rd., St. Louis MO 63123
  • [8] "Automatic Oil Burner Controls - Thermostats", Domestic and Commercial Oil Burners, 3rd Ed., Charles H. Burkhardt, McGraw Hill, 1969 (and later editions), ASIN B0000EG4Y8
  • [9] Thermostat wiring color codes & conventions, Thanks to reader " Helpful Pointers" Regarding 24V T, 10/7/2012
  • [11] Proliphix Corporate Headquarters [Website: proliphix.com] , 3 LAN Drive Suite #100, Westford, MA 01886 Phone: +1.978.692.3375 Toll Free (U.S.): 866-IP-LIVING (866.475.4846) Fax: +1.978.692.3378 - Sales: sales@proliphix.com Marketing: marketing@proliphix.com Customer support: support@proliphix.com http://www.proliphix.com/

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2023-07-15 by Rafael - diagnostic checks for fan AUTO stops working after outdoor compressor/condenser failure

I have the old AC outdoor 567G and furnace 394G system.. It started working when thermostat was on "cool" and stopped immediately when its fan blades separated from the hub , that is the fan malfunctioned..

The above expert didn't consider this case. What made the blower to stop ( it worked on AUTO).

But when new blades were installed and the fan started working properly the cool air come in house only under "fan on." The above expert didn't consider this case.

I think that the above 2 cases are interconnected. It is possible to assume a short circuit when the blades separated from the hub which was eliminated later.

Please share information raftc1@yahoo.com

On 2023-07-15 by InspectApedia Publisher (mod) - after repairing outdoor condenser fan indoor fan won't come on in AUTO

@Rafael,

Thank you so much for adding this HVAC fan diagnostic tip - we'll be sure to keep it with the article.

I'd summarize for other readers as follows:

Fan ON vs Fan AUTO: control failure following mechanical damage

A mechanical failure such as the blower fan itself coming apart might not only stop the blower from working, it may cause damage to the blower unit control board, perhaps due to an over-current when the broken fan components jam the blower fan motor.

Let me know if you think I've got this right - or not.

DF

On 2023-07-16 by Rafael Yanushevsky - fan works on FAN-ON but not on AUTO MODE

To have a cool air in my house I use "cool" on my thermostat. But nothing happens in the house although the condensing unit fan (outdoors) works well. Since the outdoor AC works properly, I decided to use FAN ON instead of AUTO on the thermostat.

Then a cool air comes to the house but this is not a normal mode for the cooling system. That is why I asked your help to understand what happened with the system.

Today I checked the control board fuses, made some measurement but didn't find anything explaining the situation. After that I tried the system with " "cool" and "auto" on the thermostat.

The system started sending a cool air bur only 5-10 sec (maybe this was also yesterday but I didn't notice that). After that I was forced to change AUTO" to "FAN ON".

Maybe this new information is useful. As to the Fan Limit Control, I don't know where it is to use. First I want to understand what happened to the system and how to fix the problem. Thanks in advance.

Thank you for your fast response. You classified my problem as "Fan ON vs Fan AUTO: control failure following mechanical damage." It looks like I haven't explained all properly.

Fan On and Fan Auto relates to the furnace blower. But damage relates to the outdoor fan of the condensing unit. As I indicated the colling system started when its thermostat was on "cool" and the blower on AUTO.

But when damage of outdoor fan happened, the blower, which was under AUTO stopped sending air. When the outdoor fan system had been fixed, the indoor blower still doesn't turn on when the thermostat is on "cool" and the blower on AUTO.

But when I change AUTO to "Fan On" cool air comes . It is not clear what happened and why the blower doesn't work in its normal mode AUTO.

On 2023-07-17 by InspectApedia Publisher (mod) - Blower Fan in air handler fan runs in "FAN-ON" but not in "AUTO"

@Rafael Yanushevsky,

Thank you for clarifying, your question.

Because of the way I read your question and also because we're posting on a page that has to do with the indoor blower unit or air handler fan, that's the fan damage that I thought you were talking about.

Let's first be sure we're discussing the same equipment: outdoor compressor/condenser fan or indoor air handler blower fan.

The fact that the indoor air handler fan will run when you turn the fan ON manually tells us that the fan and fan motor themselves are not damaged.

So we are probably left with either a control board, relay failure, or wiring error.

On 2023-07-18 by Rafael Yanushevsky - why does an outdoor compressor/condenser unit problem turn the indoor air handler fan OFF?

I have the old AC outdoor 567G and furnace 394G system. Several days ago it started working when thermostat was on "cool" and “AUTO.” But after several minutes the blower inside the furnace stopped sending any air in the house.

I approached the AC outdoor unit and found out that its fan blades became separated from its hub so that the fan doesn’t cool the condenser, The unit was damaged.

However, it is not clear what made the furnace blower stop working. The furnace and AC systems are connected only by the thermostat terminal. So that the blower should have continue working (of course, it will not be a cooled air).

But the blower is not damaged; it works on the thermostat “Fan ON.”
I would be thankful if someone explains what made the blower stop when the AC outdoor unit became damaged.

On 2023-07-18 by InspectApedia Publisher (mod) - diagnostic checks for fan AUTO stops working after outdoor compressor/condenser failure

@Rafael Yanushevsky,

Thanks for a helpful question that I'd put (for others) as

I've split out and added some diagnostic steps for this question where we'll also include all of our discussion, now

at BLOWER FAN RUNS ONLY ON FAN-ON / MAN - COOL

...

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