exhaust fan keeps running

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  #1  
Old 10-17-04, 08:50 PM
dmeskis
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exhaust fan keeps running

I have a gas furnace with a exhaust fan that has recently taken upon itself to work overtime and not kick off. It runs continuously and i'm sure that is not good for it or my electric bill. HELP! It has was looks like a vacuum solenoid connected to it with a couple of wires and a clear plastic hose, other than that I'm not sure what would control it.

Donm
 
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Old 10-18-04, 10:34 PM
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I would suspect the control board by the symptoms described. Does the furnace work as normal aprt from the continuously running fan.

A malfunction in the microcontroller/PIC on the circuit board or a short could cause the fan to run continuosly.

The fan does have a pressure switch to prove to the micro that the fan is running for ignition to take place. It does not control the fan directly.

Have you tried to turn the power off and then back on to the furnace. The control board may also have a reset switch you can press (if it exists). It is common practice to have one on any digital circuit but I'm not sure if the manufacturers who make the modules for furnaces have this on all the designs.
 
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Old 10-21-04, 12:42 PM
dmeskis
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Ok, it works well sort of, well only intermittantly

Ok, the day after my original post, I called a service tech, they were busy until the next day, so in the meantime, the thing starts working again. Ok, that normally means it works for a while,then... But they can't fix what isn't broken, so cancel the service call. Anyway, just now it has started running constantly again. The furnace is a luxaire, it does have a vacuum solenoid with the clear hose running to the motor. I tried to tap the switch on the solenoid (tip from another thread), is this switch possibly malfunctioning? and staying in a closed position? if so I can replace the solenoid myself and save the service call.

thanks for the help

don
 
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Old 10-21-04, 01:39 PM
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Still not too clear about your original posting. Normally the micro will check the pressue switch before starting the motor. When stopping the motor it may or may not check the switch depnding on the person who wrote the code for the micro (I cannot see any logical reason why they would want to check this when stopping the motor.)

You can try disconnecting the wires to the pressure switch and see if this helps although I have my doubts. To prove the point you can disconnect the wire form the control board to the motor.

I would suggest not replacing anything unless you are sure it is the fualty part or you may end up wasting money.
 
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Old 10-21-04, 08:12 PM
dmeskis
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pressure switch

i tried disconnecting the wires one at a time, none of them caused the motor to stop. So i guess that solves that one. The microprocessor is that big box in the front with all the wires running into right? Man I know nothing about these things, I wish it were a car, then I'd have it fixed. Cars I know, furnaces, not. It seems that I'm in for a service call, no?

don
 
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Old 10-21-04, 08:23 PM
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The box is the control board. The micro is the main chip on the board but there are a bunch of other things so that the micro can talk to the outside world - things like relays, DACs etc.

Unfortunately I don't think the manufacturers provide circuit diagrams to help troubleshoot the board at the board level although the micro is usually not made by the furnace manufacturer - usually it is a chip/semiconductor maker like Microchip. You normally have to replace the whole board.

You should be able to buy it online and replace it yourself if you feel upto it.
 
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Old 10-22-04, 04:05 AM
dmeskis
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replacing the control box

do you have any good suppliers in mind? I wouldn't know where to start.

don
 
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Old 10-22-04, 05:20 AM
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Start Locally

Hello: Don

Furnace parts can be purchased locally, in many areas. Look in the phone book under any one or more sections. Furnaces, heating, appliances, etc. There will be or should be several, if not many retail dealers/suppliers depending, upon your area.

Important points to consider:
Have the brand name, model and serial number with you.
Take the part(s) wanted or needed with you.
Be aware that electrical and electronic parts are often not returnable and or not refundable nor exchangeable. Once purchased and you leave the store, the part(s) are sold and most likely will not be taken back.
May not and most likely electronic/electric parts will not be warrantied when purchased & installed by a do it yourselfer.
(IMO) Buying such parts online can be risky too. Usually best to buy & install only OEM parts. Opinions and experiences vary when buying online. Some turn out okay while others do not have good experiences and or get quality parts and services. (Personal Opinions Only)
 
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Old 10-22-04, 08:13 AM
dmeskis
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parts

thanks for the tips, I have called a few local places, they all seem really stunned at to what the problem may be, and gee they would have to send a guy out. i realize that its a business and they have to eat too, and for liability reasons they don't want to just throw out random information about what is admittedly a cold call about an obscure issue, but couldn't that be addressed a little better... Anyway, enough of my complaints, I have recorded all the model numbers, serial numbers etc, do you have an idea of what the likely culprit would be? Replacement of any of the items seems relatively straight forward, I'm no expert but I've worked on race cars for almost ten years so I know my way around wires, and tools. Is there anything that I should know before I start pulling on wires (besides the obvious of turning off the power).

thanks so much again.

don
 
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Old 10-22-04, 09:36 AM
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Have you tried disconnecting the fan wires and then measuring voltage across the terminals. If this is energized all the time, (which it should be) then there may be a short.

Woking on the furnace electrics is really no different to the electrics on other thing including cars. The connectors either pull off or you may have screw type connectors. There is a large run capacitor that you should avoid touching or you may get a shock but this is not usually part of the board.

Here are a couple of links to furnace parts. I've never used them so I cannot say how good they are but you can talk to them. I think they may halp you with indentifying the problem and the part required to fix it.

www.heatcoolparts.com
http://arnoldservice.com/

good luck
 
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