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.
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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.
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.
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.
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
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You can eliminate that chance by the following step to remove the thermostat and its wiring from the equation and run the furnace directly
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Found at
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Below you will find questions and answers previously posted on this page at its page bottom reader comment box.
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
@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
@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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