# Low speed blower won't work in auto mode



## Eric F (Mar 11, 2008)

When my furnace turns on, it heats but the blower does not run. After a short time, the furnace shuts down so it does not overheat. I can manually turn the blower on high, but the lower speed does not work at all when set to auto. I have a Lennox furnace. Any suggestions? I do know it is not the thermostat.


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## biggles (Jan 1, 2008)

the low speed winding on your supply motor might be shot..feel ambitious:thumbsup: shut the furnace down and check the wires off the motor you will see one taped off with 2 connected the 2 connections are 1 for hi/cool and the other for low/heat...the taped off one is medium speed not used but can be in this situation.the 2/brown wires are for your capacitor usually mounted on the fan shroud or motor itself.the dial that starts the fan then knocks the heat off might be bad on the contacts for the ON fan call ...hooking up the medium speed into where the low was will prove the switch is working on a call for heat. te motor tag label will tell you what color is what speed..original manual might have a schematic on the motor speeds.


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## Eric F (Mar 11, 2008)

*Blower motor wires*

My blower motor has three wires: RED-heating speed tap terminal, BLACK-cooling speed tap terminal, and WHITE-neutral terminals. Does that man the white wire can be moved to the heating speed tap terminal? I believe somebody said the three wires were for different speeds. If I changed the location (change the taps) of the red wire at the harness connection at the blower motor, would that allow me to determine if the problem was with the blower motor itself?

Before I start looking for a blower motor, I want to make sure that is the problem. Any suggestions would be appreciated. 

Does my Lennox G26 series unit have a fan limit control? Is it possible that is the problem?

Thanks


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## HvacWiz (Nov 24, 2006)

*Sounds to me like maybe you have a bad control board, reley is not switching on the blower after burners come on. causing furnace to overheat.*


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## biggles (Jan 1, 2008)

OK so you only have a 2 speed motor by flipping the hi speed over to the heating side and trying a cycle of heat this will proof out that 1)the low speed is shot ....result/hi motor runs1 OR 2) the relay board that controls the FAN in the heating mode is shot/result hi motor doesn't run!then to verify the low speed hook it to the cooling when you flip the hi to heat go up and run the fan in ON off the stat to make sure the low motor is OK


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## Eric F (Mar 11, 2008)

I did as you suggested and switched the leads from the blower motor. I attached the high speed to the heating terminal and the low speed to the cooling terminal. Now when I turn the fan on manually, it runs on the lower speed. Therefore, I guess the problem is not with the low speed windings on the blower motor. The problem is still the same, however. When the thermostat is set to auto and the furnace turns on automatically, the burner ignites and the combustion air blower turns on. However, after it heats up, the supply air blower (the main blower) never comes on automatically. The furnace ends up shutting down because it overheats. I can set the thermostat to "on" so the main blower is always on and the furnace will cycle on and off to keep the house at the desired temperature. Is there a sensor connected to the control box that tells the main blower to turn on once the heat exchanger reaches a certain temperature? Is this all done on the control board? Any additional test I can conduct to determine the problem?


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## biggles (Jan 1, 2008)

OK...that proofs out the low speed is good thru the flipping of the speeds,now it is the question what controls that delay after the heat-X heats up on start.the older units have a metal disc mounted on the side of the heat-X and expands with the heat and closes the fan circuit and cools down and opens on the heatbeing satisfied,ooor a silver covered control with 2 slide pins on a black dial....fan ON and hi limit OFF as your seeing.the newer units run a time delay thru a circuit board with no temperature involved just call for heat times out...main gas on and fan comes on.you need to trace the WHITE/LOW wire and see where it ends up whatever it is tied into is what controls it...if it lands on that solid state board the board needs to be replaced.if you need to work the fan from the stat till the repair is made might want to keep that low speed were it was in the TEST with the FAN/ON from the stat to get a more normal heat run


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## HvacWiz (Nov 24, 2006)

*Change the circuit board!!*


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## Eric F (Mar 11, 2008)

I really appreciate everybody's help. My furnace is not too old, it is now 11 years old so it has a circuit board. Sounds like that is the problem. I will replace and let you know. Thanks again!!


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## biggles (Jan 1, 2008)

check out his site to get an idea on prices good luck http://bestbuyheatingandairconditioning.com/index.htm


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## Eric F (Mar 11, 2008)

*Thanks*

I would just like to thank those of you that assisted me in diagnosing the problem with my furnace. Last week I ordered the new circuit board and finally had an opportunity to replace it this morning. My furnace now works perfectly. Once I removed the old circuit board, it was very clear by looking at the back of the board that it had been burnt.

Thanks again for your help!!!!!


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## Vlad1 (Oct 22, 2008)

Eric F said:


> I would just like to thank those of you that assisted me in diagnosing the problem with my furnace. Last week I ordered the new circuit board and finally had an opportunity to replace it this morning. My furnace now works perfectly. Once I removed the old circuit board, it was very clear by looking at the back of the board that it had been burnt.
> 
> Thanks again for your help!!!!!


Hi,

Could you tell me where this circuit board located? My blower has 2 speed controls for LOW & HIGH speed fan. I'm a newby in this field, please explain me if these controls should be turned off or turned on for heating. My thermostat is in AUTO position, but looks like the blower fan doesn't turn on automatically and when I tryed manually to turn on speed controls, air coming from outlets is cool ( furnace works fine & water is hot).

Please help,

Vlad


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## dac122 (Sep 5, 2008)

Just to clarify, you say the furnace fan comes on when tstat is set to AUTO, but when you change the tstat to MANUAL the fan _does not_ come on? Is that correct?


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## Vlad1 (Oct 22, 2008)

Yes, that's correct. And I don't know how to keep "Low speed" fan switch in "OFF" position or turn it on. I see 2 tubes from my furnace, and one is hot, other cold, so furnace is fine.


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## Vlad1 (Oct 22, 2008)

And when I set thermostat in "ON" position, fan turns on, but air coming from outlets just a little bit warm


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## hvaclover (Oct 2, 2008)

Eric F said:


> My blower motor has three wires: RED-heating speed tap terminal, BLACK-cooling speed tap terminal, and WHITE-neutral terminals. Does that man the white wire can be moved to the heating speed tap terminal? I believe somebody said the three wires were for different speeds. If I changed the location (change the taps) of the red wire at the harness connection at the blower motor, would that allow me to determine if the problem was with the blower motor itself?
> 
> Before I start looking for a blower motor, I want to make sure that is the problem. Any suggestions would be appreciated.
> 
> ...


 
Nope. It's a 90% furnace. Uses a control board.


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## simpleman1962 (Oct 29, 2008)

If you manually had to turn the fan on by cutting the switch to on and not auto then you need to replace the fan switch which controls fan by tempature.


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