FAQs on why the forced air heating or cooling air handler troubleshooting set #3.

This article series describes what to check if the furnace or forced air blower fan does not stop when you expect it to do so

. These same diagnostics also aid in air conditioning blower assembly diagnosis when an A/C blower continues running. We explain what switches or controls may be set improperly or what else, such as a shorted wire, can cause continuous HVAC fan operation.

FAQs on why the blower fan won't stop

These questions and answers were posted originally

at FURNACE FAN WONT STOP - be sure that you check that article for help in diagnosing a furnace fan or blower that doesn't stop running. Questions & answers about why a furnace blower fan does not stop when you think it should are discussed here.

Remember to check that the FAN ON switch is not set to ON at your thermostat and that the FAN ON switch is not set to ON at the fan limit control in your heating furnace or air conditioner (if your control there includes that feature).

On 2019-07-17 by (mod) - service tech says can't get a replacement control board for my air handler / blower

Paul,

Let's start by identifying your furnace blower unit brand and model; a photo of the unit data tag would be helpful. From that we also want the unit's age, and the country and city where the unit is installed.

Then you can contact the manufacturer to ask their advice. It ought to be possible, at worst case, to replace the blower controller: circuit board and relays; it would be a surprise for the manufacturer to say you have to replace the entire unit. Also there may be third party generic parts that will work.

On 2019-07-17 by Paul

Blower fan keeps running and contractors tell me they cannot find the manufacturers original part to replace the failed one and they want us to buy an entire new unit, can anyone help?

On 2019-07-17 by (mod) - replacing control board fixes problem with blower fan that would not stop: signs of overhreating on control board

Doug,

Thank you so much for the photo, the follow-up, and the detailed look at the browning that suggests overheating on the Honeywell fan control board in your air handler - that information will certainly help other readers.

Working together makes us smarter.

As we're testing an update to Comments Box code and so as not to lose your question and our reply we have moved recently-posted questions & comments into

FURNACE FAN WONT STOP FAQs-3 at https://inspectapedia.com/heat/Furnace-Fan-Wont-Stop-FAQs3.php

Please take a look.

On 2019-07-17 0 by Doug Ziebro - overheat at resistors on Fan Controller: Carrier 361AAN036075/ Control board Honeywell ST9103A1002

Again, thank you. I was able to replace the board, and luckily, the swap solved the problem. It is amazing to hear the AC shut off, and the fan stop as well.

We have gone for almost a year, having to manually hit the power kill switch, on the out side of the unit. to get the fan to shut off.

The beauty of the internet and the sharing of information, is a truly wonderful thing. Timing is great too, because we are supposed to be getting a 100 F+ heat wave here in NJ this week.

I have attached a picture, with an area circled, where I think the failure point potentially was. It seems slightly burned. Rear side of board looked fine.

One the new board, where the those two resistors are, is a large square block style ceramic heat sink. Makes me ponder of this was a recurring issue, spurring Honeywell to update this control board. Thanks again.

Above: Carrier 361AAN036075/ Control board Honeywell ST9103A1002

[Click to enlarge any image]

On 2019-07-09 by (mod) -

Tony let's start by checking out the diagnostics on the page above. Let me know what happens.

On 2019-07-08 by Tony

My rheem hvac ac blower has started to run loud and hard

On 2019-07-07 by (mod) -

Keep us posted on what happens - that'll help other readers.

D

On 2019-07-07 by Anonymous

OK, thanks again for the advice. I'll move forward with ordering a new board. Good karma to you bud.

On 2019-07-06 by (mod) - Goodman Amana Furnace Fan Relay B1370751 B13707-51

And as another example here is a Goodman Amana Furnace Fan Relay B1370751 B13707-51

On 2019-07-06 1 by (mod) - Supco relay used as an A/C fan relay switch.

An replaceable electromechanical relay is usually a physically separate device with a visible, moving relay part that you could even push down with a finger (don't unless power is off). There's none in your photo.

Or it's labelled "relay" - here's a general purpose Supco relay used as an A/C fan relay switch.

On 2019-07-06 by Doug Ziebro - Bad Fan Controller: Carrier 361AAN036075/ Control board Honeywell ST9103A1002

Thanks for the input. I appreciate it. Silly question...how do I know if I have a replaceable fan relay switch? I know I have a " continuous low speed fan switch", but that is always set to "off".
For a point of reference, I have worked on motorcycles and cars, including rebuilding engines.

I have also fully serviced the beckett oil heater(burner) side of this unit. So Hopefully, I will able to troubleshoot this fan switch. Thanks for the help.

On 2019-07-03 by (mod) - suspect bad circuit board or on-board relay

Could be a failure on the control board or on a replaceable fan relay switch. See if your unit has a separate fan relay.

When the system fan should be OFF your tech will check to see if the relay is dropping power to the fan. I'd like to actually do some tests before replacing the board.

On 2019-07-03 by Doug Ziebro

So, hope you can help. I have a blower fan, that does not shut off. This is the same situation on the heat side AND the cooling side. NOTE: When heat or AC itself shuts off, fan slows down, but does not fully stop running. When heat or AC kicks on, it blows at full normal speed.

New issue, did not used to be there, for years. Oil fired furnace/AC unit. Checked all thermostat wiring, no issues. High temp switch checked and replaced( was probably fine). No change. Unit: Carrier 361AAN036075/ Control board Honeywell ST9103A1002

It has been suggested to me that there may be a "cold weld issue" on the circuit board and to replace it. I want a second opinion before I spend the $100+ on a new board and spend the time replacing it. Any help appreciated.

On 2019-04-30 18:23:25.000875 by (mod) -

At the furnace or air conditioner air handler or blower unit: the fan limit switch may include a switch that manually keeps the fan running.

On 2019-04-29 17:05:30.616941 by James

hvac blower will not stop running. Disc. thermostat, change ckt board and transformer, ck wiring, still runs.

On 2019-04-28 06:20:29.661378 by (mod) -

If in fact its not dry in your home, and you dont want it running,
Then
Lets start by turning off power to the humidifier


Then youd check flr shorted control wire or a bad cotrol board or relay.

On 2019-04-28 03:40:10.328894 by Alicia

My a/c has been off all winter. While still off all of a sudden the handler started running with air coming through vents.
How did this happen? What do I do?

On 2019-04-30 by (mod) - hvac blower will not stop running

At the furnace or air conditioner air handler or blower unit: the fan limit switch may include a switch that manually keeps the fan running.

FURNACE FAN WONT STOP contains additional easy diagnostic checks you should try.

On 2019-04-29 by James

hvac blower will not stop running. Disc. thermostat, change ckt board and transformer, ck wiring, still runs.

On 2019-04-28 by (mod) -

If in fact its not dry in your home, and you dont want it running,
Then
Lets start by turning off power to the humidifier


Then youd check flr shorted control wire or a bad cotrol board or relay.

On 2019-04-28 by Alicia

My a/c has been off all winter. While still off all of a sudden the handler started running with air coming through vents.
How did this happen? What do I do?

On 2019-03-19 by Sharon G

Thanks!

On 2019-02-09 by (mod) -

Patty

Please step through the "fan won't stop" diagnostic steps in FURNACE FAN WONT STOP, then let me know what you found.

On 2019-02-09 by Patty

I have an oil furnace. I’ve replaced the thermostat but the fan will not turn off??

On 2019-01-29 by (mod) - new air filters fixed a constant-on furnace

Mary

Thank you for the nice note, I'm glad our information was helpful. Your experience will probably help other readers as well.

On 2019-01-29 by MARY

Thank you so much for your article. I noticed my furnace was running constantly but the house was staying at the temperature on the thermostat.

After reading your article I put clean filters in and that did not work (they were not too dirty.) Then read about making sure the fan is on auto I checked the thermostat and it said fan on. Then I remembered that when I turned the heat down before I went to bed that I missed the button and hit the button next to it. You saved me a service call!

On 2019-01-13 by (mod) -

If you are sure that there is no FAN ON switch set on the thermostat or on the limit control switch, And IF you can rule out a bad control board or contactor relay then I'd be looking for wires shorted together.

On 2019-01-11 by Phillip

I have a Goodman air handler that fan motor won't shut off, I've replaced the board and motor and wiring harness ass two of them were damaged and everything worked the way it's supposed to two out of five times and now it still won't shut off

On 2019-01-09 by (mod) - fan switches tiself from AUTO to MANual

Paul

Thanks for a stump-the-chump question: I am puzzled too. I suspect a bad electrical connection or bad control.

Try going through the fan limit control switch diagnostic procedures at

FAN LIMIT SWITCH TROUBLESHOOTING https://inspectapedia.com/heat/Fan_Limit_Switch_Test_Procedure.php

and let me know how that works out.

ALSO be sure to confirm that none of your thermostat wires are shorting or touching one another nor loose at their connectors.

On 2019-01-09 by paul

I have an old coleman furnace. The fan wont shut off after cycling. I have replaced the limit switch and fan switch with a universal. I have it set to auto and it starts and cycles fine. During the cycle, for some reason, it will automatically switch from auto to manual thus keeping the fan from shutting off. Any ideas? Thanks, Paul

On 2019-01-13 by (mod) - Goodman air handler that fan motor won't shut off

If you are sure that there is no FAN ON switch set on the thermostat or on the limit control switch,

And IF you can rule out a bad control board or contactor relay then I'd be looking for wires shorted together.

On 2019-01-11 by Phillip

I have a Goodman air handler that fan motor won't shut off, I've replaced the board and motor and wiring harness ass two of them were damaged and everything worked the way it's supposed to two out of five times and now it still won't shut off

On 2019-01-09 by (mod) - The fan wont shut off after cycling.

Paul

Thanks for a stump-the-chump question: I am puzzled too. I suspect a bad electrical connection or bad control.

Try going through the fan limit control switch diagnostic procedures at

FAN LIMIT SWITCH TROUBLESHOOTING
and let me know how that works out.

ALSO be sure to confirm that none of your thermostat wires are shorting or touching one another nor loose at their connectors.

On 2019-01-09 by paul

I have an old coleman furnace.

The fan wont shut off after cycling. I have replaced the limit switch and fan switch with a universal. I have it set to auto and it starts and cycles fine.

During the cycle, for some reason, it will automatically switch from auto to manual thus keeping the fan from shutting off. Any ideas? Thanks, Paul

On 2018-12-06 by Anonymous

The article above on this page lists the reasons I've been able to find for a fan not shutting off.

If you go through that list and you can rule out all of them what's left is probably a bad control board or incorrect dip switch setting on the control board in the air handler

. I tend not to suspect the fan limit switch in this case but that's also something I would watch.

On 2018-12-06 by Ben

In cooling mode, it works fine, the temperature reaches the setting on the thermostat, the 'cooling on' system switches off, and about 2 minutes later the furnace blower fan also shuts off.

However, heating mode, the temperature reaches the setting on the thermostat, the 'heat on' system switches off, BUT the furnace unit blower fan stays on and never shuts off. A this point, even if I set the AC system to 'Off' (at the thermostat), the blower fan keeps on running.

I can even remove the whole thermostat control unit from the wall, and the blower fan keeps on running, never shuts off.

Any thoughts about why it works correctly in cooling mode, but not in heating mode?

Thanks

On 2018-11-30 by (mod) -

Richard

CHeck that you haven't bumped a FAN ON switch either on the thermostat or on the fan limit control in the furnace.

Then check for wiring that was changed inadvertently in the air handler.

On 2018-11-29 by Richard

I have an old furnace that I had to shut off to replace the belt and clean it. Now it will not stop running. Even if you change the temperature down.

On 2018-11-29 by Anonymous

Kenneth

If you haven't done so I would start by disconnecting the thermostat wires at the heater to see if that shuts off the blower. If it does then the problem is the thermostat wiring.

On 2018-11-28 by Kenneth

Changed blower switch changed sequencer when heat satisfy itself blower keeps running ?

On 2018-11-28 by (mod) -

Without knowing anything about your system my best guess would be one of the reasons given in the article above.

On 2018-11-27 by Christine Brooks

Why does my fan on when the system is turned off

On 2018-11-22 by (mod) -

Locked out usually means that the system has shut off on safety because I burner would not ignite. That doesn't sound like your problem. Check through the Diagnostics giving in the article above.

On 2018-11-21 1 by Kristine

Furnace fan won't stop running. Owners Manuel says it's locked out. What does that mean was running fine until we changed filter

On 2018-10-12 by (mod) -

Disconnect the thermostat wires from the control - to rule out shorted TT wires.

On 2018-10-12 by Brian

My blower fan continues to run at low speed when t-stat isn't calling for it. In cooling mode the high speed fan setting works when calling for cooling, but drops down to low once satisfied. My fan setting is in auto

I have 24 vac between the R and C terminals.

I verified that both limit switches are closed because I have 24 vac at Lim 2 at the board. I disconnected the green wire from the Tatar and the fan continues to run.

There is no fuse in the sec-1 circuit. I don't know what else to check before replacing the board. I also tried switching over to heat. The t-stat called for heat, the fan continued in low but the burner wouldn't light. Help!

On 2018-10-02 by (mod) -

it could be, but first try disconnecting the thermostat wires right at the limit switch

On 2018-10-02 by scott

the fan in my furnace will not stop running, i replaced the thermostat and no change. could it be the control board??

On 2018-09-08 by (mod) - Diagnosing a Keeprite gas furnace fan that won't stop

Ian

Thanks for an important question about what we can almost call a ghost fan controller.

You did the first thing I'd suggest - disconnecting the thermostat wires at the thermostat.

The second step is to disconnect the thermostat wires at the air handler. If this shuts off the fan then the wires are shorted together somewhere.

The third step is to look at the fan controller card or relay in the air handler- more suggestions are in the article above.

AND as a last resort, let's find the model of your Keeprite gas furnace, then we can find the technical or service manual for it.

See KEEPRITE HVAC MANUALS for help finding the manual for your Keeprite furnace if you don't already have it.

Below is an excerpt from the Keeprite/ICP condensing gas furnace manual for the F/G 9MVE model

Adjust Continuous Fan Airflow/Low Speed Cooling Airflow

NOTE: When the furnace is used with a two−speed cooling or heat pump unit, the airflow selected for Continuous Fan via setup switch SW2−3, SW2−4 and SW2−5 will also be the airflow used for low−speed cooling, and vice versa.

NOTE: When the furnace is used with a two−speed cooling or heat pump unit, adjust the Continuous Fan CFM setup switches SW2−3, SW2−4 and SW2−5 to match the airflow required for low−speed cooling.

Select the required Continuous Fan airflow using setup switches SW2−3, SW2−4 and SW2−5 as shown in Figure 5 and Table 5

On 2018-09-06 by Ian

Hi there,
I have a Keeprite variable speed furnace that seems to have developed a mind of its own.

The fan runs at high speed off and on all times of day and night without the call for cooling or heat. I tried disconnecting the thermostat, but the fan still cycles without it. There are no error codes read from the flashing light on the circuit board. Do you have any suggestions? Any help is appreciated , THANKS!

On 2018-08-15 by Mike

My a/c thermostat continues to run even after reaching the set temperature. Ex. when I set it at 72 and it reaches that temp. it continues to run and the only way to get it to stop aside from just shutting it down is setting it to 73.

...

Continue reading at FURNACE FAN WONT STOP or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
• BLOWER FAN CONTINUOUS OPERATION
• BLOWER FAN OPERATION & TESTING - home
• BLOWER FAN WONT STOP - LIMIT SWITCH
• BLOWER FAN WONT STOP - THERMOSTAT SWITCH

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FURNACE FAN WONT STOP FAQs-3 at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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