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Furnace blower motor

Hi all,

I've got a gas fired, forced air furnace with a programable thermostat. My problem is the blower motor runs continuously. There is a switch on the front of the thermostat to change from auto or manual. This makes no difference the blower just runs continuously.

Anyone have any ideas where to start looking for a problem?

Thanks in advance for any help!


Re: Furnace blower motor


Could be any one of 5 different things; we can try a few exploratory things that may pinpoint the problem.

I assume you're using the main on/off furnace switch to activate the furnace when you need heat.

Is this an old furnace, or new; could you post back with the brand & model number.

Has this been a problem for a while, intermittently, or just happened.

Is there also AC attached, or just heating.

Any photos you can post of the limit/fan control, wall T-stat, Circuit Board would be a big help (these controls may be located on the back or front of the furnace).

Consult the site below, & scroll down to the diagram of a furnace that illustrates all its components.

You will notice a small rectangular box at the upper back part of the furnace labeled as the "Fan and Limit Control"

You may have to remove a furnace back cover to gain access to the control.

See if there is a furnace schematic pasted on any of the furnace back covers (there are usually 2 back covers).

If this looks a lot like the fan/limit control on YOUR furnace, try giving it a gentle tap with the meaty part of your clenched fist.

On some units this box contains relays that electrically "pull in" and "release" the fan contacts.

These fan contacts often get stuck as they start to wear out, & a light rap will often release them & the fan will go off; try rapping it 2 or 3 times.

On other furnaces, there are on this same box 2 screwdriver slots, or temperature adjustment wheels that control the furnace high temperature limit and the fan (blower) on/off temperature settings.

You might be able to see temp settings on the wheels; the high limit should be about 160 degrees and the fan should be set at about 90 degrees.

If there is a little slot that a screwdriver can be inserted in the one marked "fan", insert the screwdriver & turn it slightly clockwise to see if this shuts off the fan.

Make a note of the name and part number of the limit/fan control.

On newer furnaces these controls are bunched together on a circuit board, if you can see the circuit board, also give this several light taps with the back end of a screwdriver to see if it has any effect.

While you're at it, tighten any wire connectors that you might see in the limit/fan box.

Sometimes the vibration of the furnace over time can loosen wire connectors causing intermittent conditions.

Please post back to advise if these procedures have had any effect.


Re: Furnace blower motor

If nothing as been added or changed and this is a problem that just showed itself three things come to mind that may be the issue.

1- The switch on the thermostat could be malfunctioning and stuck at the "manual" setting. Or the wiring from the thermostat to the furnace may be faulty with the "fan" wire shorted.

2- Inside the front cover of the furnace where all the control wiring and the burners are located there is typically a device that is an over temperature safety interlock. It's usually a black housing with four wires attached to it along with a round silver face having numbers stamped into the face . You will also notice typically three little tabs located in the slot on the round silver face. The round face on this device is attached to a bimetallic spring which expands and contracts with temperature causing it to rotate. The tabs are for setting certain heat values to turn on and off the blower , similar to a mechanical type of electric timer.

This device actually does three things :

A- it turns on the blower when the furnace reaches a certain preset internal temperature. In other words when the thermostat calls for heat it signals the furnace to fire up the burners then once the internal temperature reaches the preset value contacts inside this device will close and the blower turns on.

B- when the thermostat signals the furnace to turn off the burners after reaching temperature this device allows the blower to continue running evacuating heat that is still inside the furnace. This will continue till the internal low temperature reaches the preset value that's set on this device.

C- this third tab is the important one it's the safety over temperature shut down. When the furnace burners are on and the blower malfunctions the furnace will continue to heat up until it reaches this preset value then this device will signal the burners to shut down preventing damage.

It's possible the tab for the low set value is set too low which will keep the blower running or the face is not rotating freely.
You could try adjusting the low set value up to see if it shuts off the blower. If you do this hold the face from rotating and move the tab to the right or for a higher temperature.

3- the motor start relay contacts may be stuck in which case should be replaced.

Hopefully this helps.:)

Re: Furnace blower motor

Darn ... while writing my novel it seems JacktheShack was quicker at typing ... sorry for duplicating information ...Oh well.:D

Re: Furnace blower motor


Try canuk's procedures first, they look more informative than mine!


I think we're gonna need all the help we can get on this one before it's solved.

Re: Furnace blower motor

Jack ... I hear you.:)

I think with the two posts combined there's a good start for troubleshooting. Probably we can help.;)

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