Olsen Mid-Eff. keeps on cycling before fan limit switch returns home

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Old 11-18-14, 10:07 AM
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Olsen Mid-Eff. keeps on cycling before fan limit switch returns home

Hi, furnace cycles like this, induced blower comes on, 40 secs. later ignitor glows and gas valves open lighting burners, fan comes on 2 mins. later and blows heat for 2mins., gas valve shuts off and induced blower shuts off 15 secs. later. But furnace cycles again before fan shuts off. The fan limit switch never rotates counter-clockwise to the left of the fan off pointer before it starts to cycle again. Just wondered if the fan limit switch is not working properly?
 
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Old 11-18-14, 10:58 AM
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What's the model number of the furnace?
 
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Old 11-18-14, 11:13 AM
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HMS2 75TML highboy mid-efficency
 
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Old 11-19-14, 07:01 AM
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This furnace is in someone else's house but it is the exact furnace that we have and the only difference that I can see is the operation of the fan limit switch. I have already cleaned the flame sensor. They had a technician come a month before to do a fall tune-up and install a new thermostat. This was a used furnace that was installed by the previous owner. I've tried adjusting the fan off and on pointers but no luck. Not sure if previous owners or service techs damaged the limit switch by not gently holding rotary dial part in place while making pointer adjustments. Any ideas would be appreciated. TY
 
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Old 11-19-14, 07:30 AM
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Could be the fan off is set to high.
 
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Old 11-19-14, 08:36 AM
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I have adjusted the off and the on pointers in several different combinations. The off pointer nevers rotates past the stationary mark(reference point found on plastic housing). I assume the switch is a heat sensitive coil spring which expands and contracts. So it could be at the lack of proper movement.
 
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Old 11-19-14, 09:11 AM
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The fan and limit has a bi-metal probe that sticks into the furnace. As the bi-metal is heated or cools it moves up and down to either close or open the contacts. Over time debris can clog it up and prevent it from spiraling. The attached photo shows an example. You could turn off power to the furnace, remove the fan and limit and clean it to see if it will restore proper operation.
 
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Old 11-19-14, 09:48 AM
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Thanks Fire

What is the best way of cleaning, shop vac or compressor, etc.?
 
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Old 11-19-14, 10:50 AM
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It depends on what is in the probe. Sometimes insulation gets jammed in there and might require a small "picK" tool to dig it out. Low pressure compressed air will usually work pretty good if you have it. I carry a can of compressed air used to clean computer keyboards and etc. A vacuum will usually work also.
 
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Old 11-19-14, 12:01 PM
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Check my threads in this forum
I am having exactly the same problem
The gas valve switches on and off because of the fan and limit switch
It reaches the 200F limit too quick
That is because the furnace overheats because not enough air is getting through to cool the heat exchanger OR the heat exchanger is cracked and the heat can make it out of it and it warms up the switch sensor too quick.

I think that this is happening to me, I have an older model but it is pretty much the same story.

Solutions? Switch the blower to high speed or replace the heat exchanger. I believe that it is pretty difficult to find a new heat exchanger for these older furnaces.
The blower motor seems to be hard wired for medium speed in my case. See the diagrams in my older threads
 
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Old 11-19-14, 02:19 PM
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Thank all of you for the great info. Will try these ideas on our own furnace if this situation arises. My friend will get a service person to check out the fan limit switch.

p.s. PF try cleaning the squirrel cage(veins) on the fan motor. I've cleaned ours twice. It increased the air flowing out of the floor registers and may cool your heat exchanger quicker.
 
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