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Old 12-22-2011, 08:59 PM   #1
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Help with an old furnace


An older furnace in an apartment rental had the blower motor not working due to a blown fuse. Replacing the fuse and sliding a switch for cooling fan only caused the new fuse to blow again. It tuned out the motor was bad. Replaced the motor and tested it before re-installing it back into the bottom of the furnace. Now, the fuse is ok, but when a call for heat is made, the furnace shuts off after running for few minutes, probably due to the fan not coming on.

With the new motor, sliding the switch, the fuse is ok; however, the motor isn't kicking in.

Can someone help as to what might be the cause or how I go about troubleshooting the issue.

Thanks

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Old 12-22-2011, 09:14 PM   #2
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Help with an old furnace


Brand, model, age and a possible picture would help.
Is this a direct drive motor or does it have pulleys and a fan belt?
Is there a capaciter attached to the fan housing?
If it has been wired up correctly I'd be looking for burn wires or connections within the furnace.
Do you have a fan/limit control with a temp reading dial inside that would give us an operating temp?

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Last edited by how; 12-22-2011 at 09:19 PM.
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Old 12-22-2011, 09:35 PM   #3
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Help with an old furnace


The brand is Comfort something as the sticker is somewhat torn. The age, it's more likely about 20-30 yrs of age; however, it has a new gas valve, new thermocouple, and pilot pipe. As for the motor, it is direct drive and I don't have a picture handy. The furnace is real basic, it does have a slider switch located well above the heat exchanger with the top position indicating heat and lower position labeled Fan only cooling. The original motor didn't have a capacitor and the new one does. All wires have been checked and motor was tested once placed into the housing with the fan wheel on for proper operation and direction. All wires are new, I simply used the blue wire for mid speed connected to black and white to white to power the motor.

In terms of the fan/limit control, not sure where that is; however; due to the age of the unit, it seems to be fairly basic with the power coming from a screw-in type of fuse on a switched j-box down to another j-box where there's a transformer is mounted. One leg of the transformer is going to the sliding switch above while the other going into either the gas valve or t-stat (sorry, it's been a long day), then wire from gas valve and another going to the t-stat on the wall. Hope that provide some answers to the questions.

It seems that once the heat starts, the blower is never getting the request to go on and whatever part responsible for that, is more than likely the cause. I'm just not sure which part that is.

Thanks
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Old 12-22-2011, 09:42 PM   #4
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Help with an old furnace


You may need check that motor wiring again. If it a universal motor white is neutral black is high blue med hi yellow med and low is red. What I'm get that motor black to high speed
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Old 12-22-2011, 10:10 PM   #5
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Help with an old furnace


It's a 3-speed motor with blk > hi, Blu > med, red > med and white as you stated is neutral. The it is wired line white to motor's white and line black to motor's blue for med speed.
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Old 12-22-2011, 10:27 PM   #6
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Help with an old furnace


Does it work fan on or cool?
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Old 12-22-2011, 10:41 PM   #7
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Help with an old furnace


You didn't mention the two brown lines from the motor that should be going to the capaciter? Do you have and know how to use an electrical meter?
Anyway...

Some where on that furnace is a control that turns on the furnace blower when it gets up to a certain temp. It is usually connected to your slider switch (Summer fan switch) unless your slider switch is exoutside of the furnace. Usually its a box thats attached to the front wall of the furnace and will have three or four wires connected to it. If you see this box,the cover can be removed to expose the innards by pinching in the sides of the cover.
Another way of tracking it down is to follow the power lead from the motor into the junction box where it will be marreted to the line that then goes to the fan control.
Let us know if this makes sense.
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Old 12-22-2011, 11:30 PM   #8
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Help with an old furnace


Yes, the brown and brown/white are connected to the capacitor. As for using an ohm/volt-meter, absolutely no problem there. I'm just trying to narrow it down to the part where the motor is signaled to run.

As I previously stated, there's a slider switch toward the center top of the unit where it's marked fan-heating at the top and fan-cooling towards the bottom. Previously when the furnace's standing pilot is lit and then the t-stat call for heat, the burners will fire up but no fan and when started to push the slider toward the bottom position, the fuse will blow hence replacing the motor as it was shorted.

With the new motor in place, moving the slider down no longer blows the fuse; however, the blower never runs and after few minutes of heat, the burners will shut off, more than likely due to safety and reaching some limit.

I hoping that someone can point me in the right direction in terms of finding the part responsible of signaling the blower to turn on.

The line powering the blower motor is indeed as you stated, goes into a j-box on which a transformer is mounted. I presume that's where you think I should look for the fan/motor control. The question I have, what kind of part is it normally used to control the motor, is it a relay or something else and what usually goes wrong with it, can it be cleaned or doesn't need to be replaced.

Any help you can provide is greatly appreciated.
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Old 12-22-2011, 11:38 PM   #9
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Help with an old furnace


the part you are looking for is the combination fan and limit switch should be located above on plemum....pictures would help...
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Old 12-22-2011, 11:46 PM   #10
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Help with an old furnace


OK, I am wondering if the slider control is part of the temp control that turns on your motor. The slider contol determines the temp that the furnace motor turns on at.
How many wires are conected to the slider control?
Is there a cover that comes off the slider control box? I am thinking that you have at least one 120V live line to that slider control and another one with 0 Volts. If so, that line with 0 volts is supposed to measure 120V when the slider is pushed to the summer cooling position or when the furnace reaches the temp set by the slider control.
Remember we are talking about 120 V lines that can be deadly so take the appropriate precautions like turning of your furnace breaker when exploring or setting up your meter.
What I am trying to find out is if the previous motor short damaged your fan control and if the slider control is part of the fan control. This slider control/fan control will have to be in a situated where it can measure the temp within the furnace.

Last edited by how; 12-23-2011 at 12:01 AM.
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Old 12-22-2011, 11:46 PM   #11
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Help with an old furnace


Thanks for replay, currently I don't have access to the unit; however, can you tell me if that is the same as the slider I mentioned in my previous message or is it different and if so what would it look like given the furnace is over 20 yrs old (I'm guessing, possibly even older) and very basic unit compared to today's furnaces where there's no controller board or many relays for that matter.

Can the fan/limit switch be located in the j-box where the ac line powering the motor originates. The same j-box has a transformer mounted on the face of it. if that helps??

Thanks for your help!!
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Old 12-22-2011, 11:49 PM   #12
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Help with an old furnace


Quote:
Originally Posted by arky View Post
Thanks for replay, currently I don't have access to the unit; however, can you tell me if that is the same as the slider I mentioned in my previous message or is it different and if so what would it look like given the furnace is over 20 yrs old (I'm guessing, possibly even older) and very basic unit compared to today's furnaces where there's no controller board or many relays for that matter.

Can the fan/limit switch be located in the j-box where the ac line powering the motor originates. The same j-box has a transformer mounted on the face of it. if that helps??

Thanks for your help!!
it could be but i don't so we really need some pictures we just guessing....seem problem is between that slide switch and fan control?????
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Old 12-22-2011, 11:51 PM   #13
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Help with an old furnace


I will take a closer look tomorrow and I think I have some good info to go with here.

Thanks for all the responses. I will report back soon.
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Old 12-23-2011, 12:18 AM   #14
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Help with an old furnace


What you call a slider control should have temp increments engraved beside it so you can set the temp that the fan will come on at. If not it is probably just a summer fan switch. The location of that fan control can be found by following the motor power lead back to it as the fan control is the last power leg on route to the motor.
All temp fan controls need to be situated where a bimetal probe or disc plate can access the furnace. The can be concealed in electrical boxes but only if that box is directly attached to the wall of the furnace. I doubt that your J box is.
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Old 12-24-2011, 12:55 PM   #15
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Help with an old furnace


Attached you'll find 2 files, one is an image of the furnace that has the problem with the firing then shutting off because the blower motor isn't kicking in. The second image is a block diagram detailing the wiring of the component numbers in the first image.

Just to recap...

- Furnace started with the above problem and as it turned out, the screw in fuse was blown. Further checking, it turned out the blower motor was shorted

- Replaced the motor and checked it once mounted into the bottom furnace housing before installing back into the furnace and it works fine

- Once installed into the furnace, standing pilot on, t-stat calls for heat, burners fire up, after few minutes burners shut off. Based on the smell thru the register, more than likely because the motor never starts

- Measured AC voltage across the transformer (5) = 27.9 VAC

- Measured resistance across the AM - Stat - Fixed limit 170F Model number: L59 - 3A - 170 - 50 (4) = 1.6 Ohms

Any help with shedding some light on what might the cause for above issue is greatly appreciated. Also, please let me know if you have any questions or need to more info.

Thanks
Attached Thumbnails
Help with an old furnace-fur.jpg   Help with an old furnace-fur_blck.jpg  

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