Why, when the room thermostat is set to COOL or "air conditioning" mode, does the indoor air handler blower fan run only if the thermostat is set to FAN-ON.

Why won't the blower fan work if the thermostat is set to FAN AUTO?

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

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

Problem: the indoor air handler bloer fan works only if thermostat set to FAN ON, and won't run if the thermostat fan control is set to AUTO.

Cooling Fan Won't Run in AUTO: Diagnosis and Repair Steps 1-5

Steps 1, 2, and 3 below are used when the indoor blower fan won't run in AUTO for either COOL mode or HEAT mode because these snafus can happen in both heating and cooling modes.

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.
   
   If you need to rule out the thermostat itself and its wiring between thermostat and the air handler, first heck the thermostat switches and wires where they connect to your air handler's control board to find those locations that can be connected by a local jumper wire.
   
   Watch out: if you are not trained in safe and proper electrical and control wiring, don't fool with the controls and wiring in your equipment: you could be shocked or killed or you could damage the equipment. Ask your trained HVAC service technician for help instead.
   
   For this teste example, simply disconnect the thermostat wires from the COOL terminals on the air handler control board and temporarily connect those two terminals together.
   
   That should be functionally identical to turning the thermostat down to a low setting and placing it in COOL mode.
   
   

In our photo 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 and thus telling us that the blower fan motor itself is getting electrical power and is not damaged.

3. 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 note that we have diagnostic procedures for two different cases: Air Conditioning Mode or or Heating Mode.

• Cooling Mode: INDOOR BLOWER FAN WON'T RUN in AUTO - COOL or Cooling Mode - the thermostat is set to COOL - just below on this page
• Heating Mode: INDOOR BLOWER FAN WON'T RUN in AUTO - HEAT or Heating Mode - the thermostat is set to HEAT separate article
  
  - If your HVAC blower fan won't run in AUTO for either cooling or heating modes, step through the diagnostics on the HEAT page linked here.

 

Continuing with Steps 4 & 5 to Diagnose a Fan that won't run in AUTO mode on a call for COOLING (Air Conditioning)

4. 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.
5. Check the Thermostat Temperature Setting
   
   The set temperature on your room thermostat must be set to well below the present room temperature in order for the thermostat to call for cooling or air conditioning.
   
   Check the thermostat temperature reading and adjust its setting to below that temperature.
   
   

Diagnose Blower Fan that Won't Run in AUTO in COOL or Air Conditioning Mode - Steps 6

In addition to the first things to check: indoor air handler blower fan diagnostic steps 1 - 5, here more things to check when the "fan wont' run in AUTO" problem is occurring when the system is in air conditioning or COOL mode - set at the room thermostat.

We also address the question: what would keep the indoor AC air handler blower fan from running (thermostat set to COOL & FAN-AUTO) if the outdoor condenser unit is off due to problems of its own?

The blower fan troubleshooting steps below assume that you have already shown that the fan will indeed run when set to FAN ON or MAN mode at the thermostat or at the fan limit control at the air handler/blower unit.

If the indoor A/C blower unit / air handler fan won't start, at all, see our diagnostics are at

FAN WON'T START

What Check Next If the Indoor A/C Blower Fan Won't Run in AUTO mode when the Thermostat is Set to COOL

6. Check the thermostat temperature setting
   
   Thermostat temperature setting: For the indoor blower fan to run when using air conditioning, the thermostat (TT) switches must be properly-set for cooling (COOL mode with FAN set to AUTO)
   
   AND
   
   the thermostat temperature setting must be set to below the current indoor temperature.
7. Check that power is ON
   
   to the indoor air handler unit - its power switches are on and its circuit breaker or fuse in the electrical panel are "on" and "good".
8. Bad Fan Relay
   
   One of the most-common causes of the indoor blower fan not turning on when it should is a bad fan relay.
   
   Now why might a blower fan relay fail after the outdoor compressor/condenser unit has failed? Perhaps a continued, unsatisfied call for cooling (by the thermostat) or constant switching on and off at the relay + age + environmental conditions has caused the indoor fan relay contacts to burn or seize.
   
   Your service tech will first check that the blower will run normally with the thermostat set to FAN ON.
   
   If the fan runs then the problem isn't a fan motor, belt, bearing, relay, etc. so we're back pointing to the indoor blower fan control board.
9. Fan Actually On but Weak or No Air Flow
   
   Are you sure that the fan is actually not running or are you just not feeling air coming out of air supply registers?
   
   Stop by the air handler itself: is the blower fan running?
   
   If the fan is running but air flow is poor or weak,
   
   see  AIR FLOW TOO WEAK
10. Power Surge Damage
    
    When an indoor blower fan failure appears to be independent of an outdoor compressor/condenser unit failure, but both are being reported at the same time, I'd look for a possible power surge or lightning strike or other electrical event that may have damaged
    
    - the indoor fan / blower unit control board
    
    - the fan relay
    
    - the fan start/run capacitor
    
    - the fan motor itself
    
    
11. Bad Fan Belt or Pulley
    
    And of course on belt-driven blower fans we could have a broken fan belt or loose fan pulley.

...

Less Likely Problem: the Thermostat Itself Prevents Fan from Running on AUTO

12. 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

• 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 in COOL Mode Problems

13. The fan will run and blow warm air but only in FAN or FAN ON mode -
    
    delivering heat - but NOT in AUTO mode.
    
    This may be a bad temperature sensor in the unit; replace the fan limit controller.
14. 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.
15. 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: https://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 if you were able to turn on the fan from the thermostat
• It's not likely to be fan wiring, fan motor, fan motor start/run capacitor if 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.

...

Fan Limit Control & Thermostat Wiring References

Found at

BLOWER FAN CONTROL REFERENCES

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

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

What would keep the indoor AC air handler blower fan from running (thermostat set to COOL & FAN-AUTO) if the outdoor condenser unit is off?

You'll want to check the "fan wont' run on AUTO" diagnostic steps above on this page where we'll add a couple of tips to cover not just heating mode but cooling or air-conditioning mode.

Let me know if these diagnostics work for you or if you have suggestions or questions.

Thanks for a helpful question.

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.

This discussion was posted originally

at BLOWER FAN RUNS ONLY ON FAN-ON / MAN - HEAT - that page has been edited to pertain only to heating mode.

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 2023-07-16 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 2023-07-15 y 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 2023-07-15 by Rafael

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 2022-09-28 1 by InspectApedia (Editor) - Blower motor won’t run in auto mode for COOL nor for HEAT

@Rick s,

Best bet: try stepping through the diagnostic suggestions above on this page. Let me know what you find.

On 2022-09-28 by Rick s

Blower motor won’t run in auto mode in heating and ac mode. No batteries used in thermostat

On 2022-08-14 by InspectApedia-911 (mod) - check that the batteries are good

@Dawn,

Check that your batteries are inserted in the right direction and then check that the batteries themselves are good.

On 2022-08-14 by Dawn - BLOWER ONLY runs on FAN ON

I put new batteries in my thermostat but it will only run on fan and you can’t see the temperature numbers on it, I put it on auto and it won’t work what can I do

...

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