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Old 09-14-2012, 10:12 PM   #1
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Furnace problems....


I have a 20 year old Lennox furnace that is acting strangely and I could use some help.
First issue is that the a/c unit does not turn off? Furnace fan shuts off but a.c. unit keeps running. Only way to shut it off is the breaker!

Second issue is that when I turn to call for heat the blower only turns very slowly and i start to smell burning... it's only a single speed fan and the lack of air flow causes an overheating situation.

I've replace the thermostat and the problems remain....

Any assistance would be appreciated.

Thanks

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Old 09-14-2012, 10:28 PM   #2
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Furnace problems....


Contactor for the a/c unit outside may be stuck. That would be internal to the outside unit.

Have your evaporator coil cleaned.

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Old 09-14-2012, 11:23 PM   #3
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Furnace problems....


That explains the ac portion any idea what could cause a slow fan when I turn the thermostat to auto and call for heat? I tried to install a new fan centre and I may have mixed a wire.... funny thing is that the fan works fine if I set the thermostat to fan on not auto.
What contactor would I order? Are they standard?

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Old 09-15-2012, 06:39 AM   #4
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Furnace problems....


Slow blower motor could be a bad/weak capacitor.
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Old 09-15-2012, 07:45 AM   #5
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Furnace problems....


Quote:
Originally Posted by strosek View Post
That explains the ac portion any idea what could cause a slow fan when I turn the thermostat to auto and call for heat? I tried to install a new fan centre and I may have mixed a wire.... funny thing is that the fan works fine if I set the thermostat to fan on not auto.
What contactor would I order? Are they standard?

Thanks
You'd need to check the contactor itself, it'll have the numbers on it. It'll be something like 240/50VA. Make sure to see if it's a single pole or a double and get the same one. Might want to take a clear picture of it so you have it with you when you look for another one. Grainger will sell to the public and should have it.

I say take a pic versus removing and taking it with you so that when you have the new one you can simply remove one wire from the old and put on new contactor, one at a time. Less confusing that way although pics of the wiring help as well.

Make sure the breaker AND the thermostat are off when handling or working the contactor, even when simply taking a pic.

If the fan being slow is a new issue since you replaced the fan center than perhaps you do have a wire misplaced. Was it doing this before?
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Old 09-15-2012, 08:06 PM   #6
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Furnace problems....


No not doing that before but the strange thing is that when I put the thermostat to fan on as in always on the speed is fine.... when I put it to auto that's when it turns too slow. Could it be the new thermostat? I've done too many things at the same time. I'm going to put back the old thermostat and double check the wiring of the fan center replace the contactor.
Thank you very much for the help!!

Last edited by strosek; 09-15-2012 at 08:09 PM.
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Old 09-15-2012, 08:27 PM   #7
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Furnace problems....


shut the disconnect off out at the condenser cotrol power is from the furnace airhandler section...probably stuck contactor as mentioned...on the fan speed situation.the FAN ON from the stat is the HI sped for he cooling ...the fan for the heating side is lower as in Lo Med/LO and is thru a temp.disc or if you have a control module in that furnace a time delay.remove the stat off the subbase and jump r to W for a straight heat call.. pilot,burner then supply fan.low speed winding has been off sice last winter might just be a dust brn off even dust on the heat echanger burning off...is the fan section clean and squirrel rounds are free a major dust...motor has to be 2-speed.if you have a control module the black relay on that board are for the fan relay
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Old 09-15-2012, 08:43 PM   #8
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Furnace problems....


Old motor. Trry oil.
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Old 09-17-2012, 09:47 PM   #9
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Furnace problems....


Update: ok so I checked the contactor and you guys were correct! It was seized!

Now for my second issue... I ran the furnace on auto/heat and I still think the fan is running too slow because the plenum got very hot and I could smell a burning. I replaced the old thermostat and the problem remains. I checked the wiring of the fan center and it looks correct as per the picture of the wires I took. From what I remember this furnace has only ever ran at one speed...high!

This is my mother's house and I really want to be sure it's working properly.

The furnace is a Lennox G8Q3-105-2

Greatly appreciate the help!!

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