Lenex Furnace Short-Cycling problem


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Old 01-22-13, 08:12 AM
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Lenex Furnace Short-Cycling problem

My Lenex 3-jet gas burner furnace stopped working (was found to be short-cycling) yesterday but by (1) shutting off-and-on the power to the electronics of the furnace -OR- (2) by turning the thermostat temperature to real low and back up to real high with heater on -OR- (3) by leaving the thermostat at 80degF and turning the HEATER switch off-and-on, a new short-cycle was initiated and furnace eventually started to work ok.

During the several forced short-cycling event, I was able to verify that glow igniter comes on and when all three of the jet becomes lit, even though the flame sensor is exposed to proper looking flame, the gas would shut off and another short-cycle event would occur within a minute. After two more short-cycle event that result in flame sensor mal-functioning, the furnace would wait probably for one hour before next three short-cycle event is initiated. I didn't wait an hour but force-initiated the three short-cycle event by using one of three methods stated above.

While watching the short-cycling event, I blew on the flame that was supposedly heating the flame sensor and this supplied more oxygen(?) and changed the color of the flame and furnace worked ok. I also tapped on the flame sensor later (probably cleaned it or delivered a needed mechanical shock to make the flame sensor operate properly for another 24 hours?) with a plastic pen.

The furnace did not go through any more short-cycling for about 24-hours now and operates faultlessly (the temperature outside is -3degF today!) and my question is: Did I inadvertantly clean the flame sensor and everything is ok for now and for another 6-7 years or is the flame sensor a trouble waiting to resurface in a couple of days regardless of how much cleaning is done to it? In another words, is the flame sensor's failure mode depend heavily on the cleanlyness of the sensor rod or an intermittent flame sensor is a definite precursor (within a day or two) to a permanantly bad flame sensor?
 
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Old 01-22-13, 11:08 AM
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Flame sensor will go out again. Get some steel wool. Home Depot sells it. And clean the flame sensor. You will also need a 1/4 inch nut driver to remove the screw holding the flame sensor.
 
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Old 01-22-13, 11:43 AM
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Usually if the flame sensor isn't working properly, the burners will shut off within 3 seconds or so after lighting. But marginal operation of the flame sensor function might cause a longer delay.

Cleaning the flame sensor is routine maintenance that shouldn be done anyway, and might solve the problem.

If it doesn't, you need to take the cover off the furnace compartment of nthe furnace and carefully observe the sequence of events that occurs when the thermostat is turned up. Post that sequence in order and in detail here.
 
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Old 01-22-13, 12:25 PM
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You guys are GREAT!

About ten years ago, in trying to troubleshoot a similar problem on a different furnace, I found the 120VAC powered glow igniter was dirty/intermittent. I used an old tooth brush and cleaned and the fragile filament simply broke and I had to buy a new one for $60.

I am wondering if the flame sensor is made of same/similar material that I may accidentally break it while cleaning.

Is it ok to vigorously rub steel wool on the flame sensor without removing it at the same time vacuuming the powder as I clean?

Is the flame sensor made of fragile material or is it a non-brittle metal stick?

And, yes, when the flame sensor was mal-functioning, the gas shuts off exactly 3 seconds after good flame is torching the flame sensor. By blowing on the flame during the 3 seconds, short cycling ended and everything worked ok. I am wondering if blowing on the flame sensor cleaned 10% of dirt but tapping on it with a plastic pen cleaned ~50% of dirt.

So my ultimate second question is: Is the 4 inch long (1/16" diameter) flame sensor made of brittle material or is it a very strong/maliuable metal that I can scrape?
 
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Old 01-22-13, 04:34 PM
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The flame sensor is a rod of metal and you need not fear breaking it while cleaning it.


The odds are good that cleaning it will solve the problem.


My suggestion is to consider cleaning the flame sensor annual maintenance that you do in the future to provide reliable operation of your furnace.
 
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Old 01-22-13, 04:37 PM
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The flame sensor is a metal rod, so cleaning it won't cause any problems.

Igniters on the other hand, should never be cleaned.
 
 

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