furnace blower sticks on

I have a York Stellar furnace that is 16 years old. After the heat cycles on and off, the blower will occassionally stay running. If I tap the fan/limit control very lightly the metal disk unsticks, jiggles a bit, and then the blower shuts off. Then it may work fine for a few cycles, but sooner or later it will stick on again and need to be tapped. I assume that this is the part that needs to be repaired, but is it pretty easy? The fan/limit control is a Honeywell, so I assume I can find the model/part# and get a replacement or just take the old one in. I've attached a pic at
http://img199.imageshack.us/img199/2626/img1509fo.jpg
Thanks!
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wrote:

You should be able to find a tapper at a reasonable charge. The Home Depot parking lot is a good place to look, or Craigs list whereever au peres are listed. When the burner is on and the blower should be running, since he's in the basement, he can do laundry or something.

Should the lower limit switch really be set at 50? Or is that for AC too? (I don't have a control like that, for some reason.)
I don't see the metal disk, but it seems if it sticks, one needs to find where it sticks and either bend whatever it touches away from it, or file down the disk a little at that point. But I have no idea how your gizmo work or what I'm talking about. Except that filing seems preferable to buying another one.

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The contacts will spark from breaking the curent when the contact opens as the temperature in the plenum gets cooler. The sparking roughs up the surface causing little pits and valleys. Eventually the pits stick in the valleys and the contact does not open. If you can get near them, a women's emery stick for filing nails can be used to smooth the surface and keep the switch working for several months. I have to use an emery board on our 50 year-old gas furnace about every 2 years when the blower does not shut off after the air has cooled (when on the Heat setting).
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On Thu, 5 Nov 2009 22:15:12 -0800 (PST), "hr(bob) snipped-for-privacy@att.net"

Very interesting.

I thought emery sticks had sand on them that embedded in the contacts and made them wear again faster than they would if filed with an all metal file. At least that's what I recall from instructions on filing contact points on auto distributor points and/or voltage regulators.
Wouldn't a metal file, like a point file (which is thin and fits between contact points, be better, last longer.

How come no one's nagging you to get a new furnace when they used to give me a hard time here for having a now-30-year-old oil furnace?
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wrote:

Thanks for the advice Bob. Where are the contacts? Do I likely have to take the fan/limit control off the furnace or take the whole thing apart? No problem doing that, I just want to know what I'm looking for.
Thanks
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They are not made to be dissembled. Have to take the whole fan limit switch out, and replace.
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Hard to tell from the pics, but is your low limit set on 50? That should be bumped up a little bit. Otherwise you are blowing 50 degree air in your house. Maybe that is your problem, because the fan will not shut off until the plenum cools off to 50 degrees. Try 75 degrees.
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You may not be able to get the exact part but a universal replacement part can be purchased.
First I would move the low limit up to about 80 or 90 degrees like another poster suggested.
Colbyt
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I'm with some of the other posters here... I think the control is misadjusted. That little metal tab sticking out near the "50" marking controls when the fan turns on and off, and can be moved. 50 is way too low a setting, it should be closer to 100. The "Off On" markings under the word "Fan" indicate the range where that is supposed to be.
Eric Law
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Okay, sorry I should have clarified before. I took the pic at a point where I was experimenting with the adjustments. The metal tabs are usually set around 130 (high) and 80 (low). I had pushed the lower one down to 50 so that I could slowly move it up and see at which point the blower shut off. The problem is that when it does stick, it requires only the slightest touch to un-stick - so it's impossible to hold the dial and move the tab without manually "unsticking" it. The issue is definitely not the setting, since when I have it set on 80, the blower continues to run until I manually unstick it - at one point i was out of town and my wife said it ran for 2 days straight, and upon coming home and measuring the temp of the air in the plenum, it was much lower than 80.
So really I either need to replace it or file the contacts. I'd like to try the filing, but can anyone tell me where the contacts are? If I do need to replace it, can anyone tell me if it's a straighforward replacement job?
Thanks
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It is straightfoward. All it is is just a probe sticking into the plenum, and the unit is held by 2 screws. Plus you have to remove the wiring, so make sure the power is off. The only thing you have to check is the length of the probe, they make some with 5", 8" or 11.5". Get the model # off the inside of the cover case. If your not in a rush and if the weather cooperates, you can get one online for cheaper in a few days.
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Do you have a humidifier attached to the furnace?
My old furnace had a humidifer and the probe-type fan limit switch described in this thread.
When I had a problem with my blower staying on/not coming on, I pulled the fan-limit probe and found it coated in mineral deposits - that white flakey stuff.
I replaced the limit switch and yanked the humidifier and all was well.
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When I described filing the contacts, I should have said replacing the limit switch was a good option if you can get a replacement with similar mounting holes and probe size. The humidifier dust collecting is logical, altho I have never heard of it being a problem before.
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wrote:

re: dust collecting
Trust me, it was not dust.
The probe was caked with minerals deposits from our hard water.
I did replace the entire fan-limit switch (White-Westinghouse) although the replacement was not an exact match so I had to adapt the mounting location a bit.
The biggest problem was that the exterior housing was much smaller and it was really tough to get the wiring inside the housing and clip the cover back on.
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Okay, sorry I should have clarified before. I took the pic at a point where I was experimenting with the adjustments. The metal tabs are usually set around 130 (high) and 80 (low). I had pushed the lower one down to 50 so that I could slowly move it up and see at which point the blower shut off. The problem is that when it does stick, it requires only the slightest touch to un-stick - so it's impossible to hold the dial and move the tab without manually "unsticking" it. The issue is definitely not the setting, since when I have it set on 80, the blower continues to run until I manually unstick it - at one point i was out of town and my wife said it ran for 2 days straight, and upon coming home and measuring the temp of the air in the plenum, it was much lower than 80.
So really I either need to replace it or file the contacts. I'd like to try the filing, but can anyone tell me where the contacts are? If I do need to replace it, can anyone tell me if it's a straighforward replacement job?
Thank
Just change it out Matt. Those exact models sticking with age is very, very common. It's super easy to replace. You just need a nutdriver to remove it, and a small regular screwdriver to release the wires. Two things to remember 1.) Put those wires in the exact same place!!! Your furnace may have both 24 volts and 120volts going through that switch at the same time since it is both a fan and limit switch, so you don't want to fry anything. 2.) On the new unit, there will likely be a jumper between the two bottom terminals. This was for three wire set-ups, and would make the switch all 120volt. You have a four wire set-up, so you will want to remove it. There are three different lengths available, and also some have a internal heater to bring the fan on sooner. It is'nt really going to make a lot of difference which one you use, any of the Honeywell L4064 fan/limits will work essentially the same in your older furnace, and you probably could'nt tell the difference which one you had used unless you're the kind of guy who times the blower with a stopwatch. There are also other fan limits available, but it would be way simpler for you to find an L4064. Don't forget to set the dial pins at the temps that the furnace manufacutrer recommended.
HTH, Lefty
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On Thu, 5 Nov 2009 17:40:13 -0800 (PST), Matt

Just replace it.
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That's a great picture of a fan limit switch.
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