Air Handler Blower Fan Won't Stop in Heating Mode - FAQs

Indoor AHU or Furnace Blower Fan that Will Not Stop in Heating Mode

Air Handler Blower Won't Stop - Heating Mode, FAQs

Can blower be deliberately set to run constantly

Can blower running constantly 2019/10/18 Henri Nseke

Reply by (mod) - yes by setting a FAN-ON switch

See these articles for complete detail covering both deliberate running of the blower fan continuously OR accidental or malfunction (the fan doesn't turn off when it should)

• FAN ON AUTO MAN THERMOSTAT SWITCH - settings
• FAN ON SWITCH ADDITION

In Heat mode our Furnace will not turn off and keeps blowing out cold air

When the house reaches the desired temperature sometimes the fan keeps blowing even though the burners have stopped and the fan then blows cool air and does not stop.

I have to then turn the heat off and in a few minutes back on and then it runs properly by turning off. The fan is always set to auto. - On 2018-01-15 by Anne

Even took the batteries out of the wall unit and it is still on and blowing out cold air - Christina 2018/01/01

Reply by (mod) -

Anne, and Christina:

On most forced warm air heating systems the fan will run until the call for heat is satisfied at the thermostat, and for a brief period thereafter.

But if the fan never turns off the:

SOME (not all) fan limit switches include a white push-pull knob that forces the fan to run continuously - or leaves it on AUTO.

That's what I suggest you look for first.

Then check the room thermostat FAN settings. Set the fan to AUTO, not to ON.

The indoor air handler blower can be forced to keep running, as we explain at

• BLOWER FAN WONT STOP - LIMIT SWITCH
• BLOWER FAN WONT STOP - THERMOSTAT SWITCH
  
  That switch and its operation as well as other causes for a fan that won't stop are listed in the diagnostic steps given in the article
• FURNACE FAN WONT STOP

Christina, Just taking the batteries out of a thermostat might not even disable the thermostat.

Also see

• FURNACE BLOWS COLD AIR

Watch out for air delivery problems

Also remember to check obvious defects that would keep your air handler blower running in heating or cooling mode, such as a dirty or blocked air filter or crimped or disconnected air duct - both of which mean that the heating or cooling system runs but the thermostat is never satisfied so the system keeps running.

In Heating Mode my ADP Air Handler (w/ hot water heating coil & a cooling coil) pump won't stop running

I have an ADP air handler with a hot water heating coil and a refrigerant coil. My thermostat controls the HW recirc pump.

In heating mode after the recirc pump shuts off the fan keeps running, forever.

I do not see an aquastat or fan limit switch in the housing anywhere.

My question is what controls the fan on/off?

Is it programmed right into the circuit board or am i missing something? - On 2020-11-13 by ovsa

Reply by (mod) - ADP Air Handler with heating/cooling coils: fan control

Ovsa

There is usually a fan limit control in the air handler that turns the fan on or off depending on air temperature sensed in the plenum just above the heater's heat exchanger.

If the fan won't stop on your unit look for one of the controls described at FURNACE or A/C BLOWER FAN WONT STOP and that can be used to manually turn the fan on for continuous operation.

One of those is on the fan limit control, another is often on your room thermostat, and on occasion there may be a fan control right on the printed circuit board in the air handler.

Look furhter for a control board failure: if your air handler does not use a separate timer-containing fan limit control the time delay in fan shut-off may be controlled by dip switches on a control board or by a hard-wired timer.

If you don't have the manual for your unit see

ADP ADVANCED DISTRIBUTOR PROUCTS heating & cooling systems

Or this may be the IO Manual for your unit:

ADP B-Series Multi-Position & Hydronic Air Handlers IO Manual [PDF] (2017) Electric or Hot Water Heat, with available Variable-Speed High Efficiency ECM Motor

Follow-up by ovsa

Thanks.

I looked all through the duct work and there is defintely no controller in there. The stat is set at auto and it is a fairly new stat.
The manufactures data says,

"Time delay for blower activation: 60 seconds (std.)

130 deg. F Aquastat (w/optional aquastat).

So i think that 60 sec delay is just built into the board

I don't see any options or markings for time delay nor any DIP switches.

Anyway I can test it?

thanks for your help

Found a bit more info looking through the data sheets,

Units without Factory Installed Aquastats or Deactivated Aquastats

A time delay of 60 seconds follows before the circuit between R and G are complete activating the indoor blower motor.

The Air Handler fan will turn off 30 seconds after the Thermostat stops calling for heating.

Reply by (mod) - important for the blower fan to continue running for a brief time after the thermostat stops calling for heat

thank you for that it, too. Your paragraph yes I agree and we discussed in this article series The observation that it's absolutely important for the blower fan to continue running for at least a brief time after the thermostat stops calling for heat.

That's to extract the remaining heat out of a heat exchanger and thereby to avoid cracking the heat exchanger from high temperatures which in turn could make the furnace Unsafe and risk carbon monoxide poisoning.

So to be clear we should distinguish between the fan continuing to run for a brief interval typically 30 seconds or a minute after the call for heat of satisfied as distinct from the fan that just runs continuously.

If the fan runs continuously with never shutting off that's the situation discussed at

FURNACE or A/C BLOWER FAN WONT STOP

and is not that purging cycle that you found in the instructions and that we also discussed in this article series

Brand New HVAC blower fan motor keeps running, never stops

Hi looking for some technical help with our HVAC system. I just replaced the blower motor in our furnace and it is running constantly since the change.

I've disconnected it from the thermostat to isolate the problem and it still fires up as soon as the power comes on and stays that way with or without the furnace engaged.

The furnace itself still turns on and off as normal so I don't think it's the furnace limit controller reading overheat or going bad. I suspect it could be a bad relay.

The thing that really confuses me is that every morning when I wake up it tends to be off and stays that way until triggered either by the thermostat, the fresh air exchange or even me cutting the power and turning it back on gets it going again.

Then it stays running until the next morning again. - On 2020-12-05 by Nathang

Reply by (mod) -

Nathan

Let's start by asking you to review the causes of blower fan not stopping that are given in FURNACE or A/C BLOWER FAN WONT STOP because that's more complete than if I try to make that up again in an off-the-cuff reply.

Please take a look and let me know what you think and let me know what questions remain.

Followup by Nathan

Thank you. I have looked through your article extensively and tested at each place that is applicable to my system. Though I very well could have missed things as I get lost in the different types of systems.
It is an electric heating system with a heat pump and furnace.

1. Thermostat fan set to auto. Still runs all the time. Smart thermostat indicates that fan is off. Thermostat still calls for the fan when the furnace kicks on but when the furnace turns off and the fan is no longer called for at the thermostat it still runs.

2. It doesn't seem as though there is a shorted wire as when I disconnect the thermostat from the furnace by disconnecting it's wires at the Air Handler, the fan still runs.

3. There is no fan limit switch that I am aware of. It is a Trane and the furnace is BAY96X141502.

4. This system has a snap disk type limit switch. But I have checked it with a multimeter and it appears to be working correctly. Also the furnace is running as it should, just with constant blowing so if i understand correctly the limit switch would also be keeping the furnace from running normally.

Let me know if i'm missing checking something and I'll check it out.

Reply by (mod) - Trane air handler using snap-disc & control board for fan limit controls

Nathan

Good going on those diagnostic steps.

You're right that a furnace that uses "snap disc" controllers to manage the fan won't have a Honewyell dial-type fan limit control; but it's of course still performing those functions: fan ON OFF and HIGH limit settings are controlled by those snap-disc controls (whose wiring you may want to follow & check out)

With the thermostat wires completely disconnected at the air handler end, and when the fan won't stop, we rule out an issue with shorted TT wires as well as a bad thermostat or thermostat setting.

Of course "won't stop" bears some clarification: it is proper for the fan to continue running for a minute or so after a call for heat, to extract the remaining heat from the heat exchanger and thus avoid damaging it.

If you are absolutely certain that there is no FAN ON control anywhere on the furnace, and it's disconnected from the thermostat wires at the furnace end then there's not much left: I'd suspect a bad snap disc, (I hear that you tested it), a shorted wire in the air handler, or a bad relay in the air handler.

Furnace Blower doesn't stop when it should until I tap on the limit switch cover

@Roy,
I have the exact same issue. My 1985 York furnace. The thermostat calls for heat, the heat comes on but when the thermostat is satisfied the blower continues to run. Tapping (lightly) on the cover of the switch turns is off. Or, turning off the breaker will do the same thing. So, is it the limit switch or a board? I think the switch is sticking. - On 2017-01-27 by Michael

Reply by (mod) -

Sounds like a bad fan limit switch but first check for a loose or corroded electrical connection anywhere in the controls.

See these "fan won't stop" control diagnosis and repair details:

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

 

Carrier furnace locks out every 3 minutes but the blower keeps blowing cold air

My carrier furnace locks out every 3 minutes but the blower keeps blowing alternating warm and cold air blowing into the room.

It's a carrier weathermaker. Is there a way to adjust this to turn off the blower during lockout, so I don't get cold air blowing between heat cycles. (Nov 6, 2014) mike

Reply: check for a manual FAN ON switch

Mike

The air handler should stop after purging heat from the heat exchanger.

It should NOT be stopped early as doing so leaves a hot heat exchanger that can crack the unit, making it unsafe - risking flue gas or CO leaks.

Check that your fan control switch is not in MAN or ON - that would cause the fan to run continuously. There may also be a similar FAN ON switch on your wall thermostat.

My furnace fan won't shut off

My fan will not shut off.

What do I do ? - On 2011-10-04 by Shawn -

Reply by (mod) -

Shawn
First take a look at our page top photo -at FAN LIMIT CONTROL SETTINGS - if your switch looks like the one we show, check that white pull/push knob - it may have been switched to keep the fan on continuously.

Second check your thermostat - it may have a "FAN ON" switch - be sure your switch is set to AUTO or try OFF to see what happens.

And see the diagnostic and repair steps

at

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

Finally, those steps don't work I'd call a service tech as you may have a bad thermostat or fan limit control switch.

No white fan button and the fan never shuts off.

there is no white fan button and the fan never shuts off. The limit setting is really low, can I change it? is this something I can do myself or should I call a furnace tech?

The off is at 50, on at 100 and OFF at 150. I see above it should be 90, 130 and 200 from factory. I also noted the the disc it self is not level (text doesn't read straight across it is on an angle) any advice? - On 2011-10-19 by Shanna -

Reply by (mod) -

Shanna

Watch out: if the dial on your fan limit switch is bent askew I am afraid it may be unsafe and need replacement. Mishandling the dial during setting can bend internal parts.

The modified temperature settings could also point to inexpert hands on the switch.

Because the dvice depends on an intact bimetallic spring to work, it may not safely respond to a high furnace temperature. I'd ask your heating service co to take a look and to replace the switch if appropriate.

Followup by Shanna

Thanks Dan. the dial isn't bent, it just appears as though it has been turned about 72 degrees counter-clockwise.

Will a heating company charge to look at it? Or will they come and give a free estimate?

Reply by (mod) -

Shanna

The heating company may, depending on where you live, charge for travel and have a minimum service call fee, or may charge for an estimate and then credit you back for that charge if you buy a service or repair from them.