Flame sensor questions?

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Old 10-08-16, 12:05 PM
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Flame sensor questions?

Hello DIY,

Gas unit: 1988 Janitrol Goodman GUI 125-4 rev s input 125,000

I had an issue with my gas heat unit not turning on. I proceeded to clean the flame sensor, and the unit responded somewhat but it was still having problems detecting the flame from the pilot. The flame from the pilot has always turned on and that does not seem to be the problem. I initially thought that the flame sensor was dirty so I cleaned it, nothing. Thought it was defective so I bought a new one, nothing. The new one did not worked as expected either. I started investigating more and inspecting the unit and realized that if I moved the flame sensor closer to the pilot flame (actually getting it off the intended place and manually putting it close to the flame), the unit would go thru the motions normally (turn on main gas, etc). It seems that the flame from the flame sensor is not "spreading" enough to activate the flame sensor at the distance it was designed for. Now this unit is old (30 years) and rusty. It worked for two winters without a glitch since I moved to this old house. This is the third year and it is when it is giving me trouble.

So what I did was to bend the flame sensor with pliers to have it closer to the flame (see pictures). I bent the old sensor and voila! the heater unit is now sensing the pilot flame and turning on the main gas immediately after. blower turns on, etc.

Pictures:
the one one the right is the stock photo of my pilot and flame sensor (circle red notice the gap between sensor and flame).
the one on the left is my system before I bent the sensor (was working for 2 years, stopped working this year)
the one on the bottom is bent flame sensor (working now)


http://imageshack.com/a/img921/6711/O0ggcf.png



Questions:

(1) is the flame sensor capable of withstanding being closer to the pilot flame, heatwise or otherwise? I would suppose so since the flame sensor is next to the main flames once the unit goes on. it does get hot in there.

(2) is the flame sensor too close to the flame that it can "overheat" the electronics on the other side of the flame sensor? It is literally touching the pilot flame once the thermostat turn it on.

(3) i thought flame sensors were delicate. I bent and mistreated the old one and is working. I had also cleaned it with a fine wire cloth (didn't work) and to an extreme scrapped it with a blade a lot beforehand trying to clean it (back when I thought the sensor was dirty). The old sensor is really beat up. How come is still working?

(4) how come the flame heat from the pilot is not "reaching" the flame sensor any more to turn on main gas? Is the pilot system somehow "plugged"? It definitely worked without a hitch for the last two years. How come it got "plugged" this year?

(5) any other comments?

Thank you guys,

TL;DR: bent my flame sensor so it is able to detect pilot flame. is my unit going to blow up? Look at picture
 
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  #2  
Old 10-08-16, 12:30 PM
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It would be better to correct the pilot flame problem. Have you taken out the pilot assembly and cleaned the pilot orifice? Likely it is partially plugged.

What you did won't hurt the flame sensor and there are no electronics in the actual sensor itself. But if the pilot flame is too small, it is possible to get a greater build up of gas in the combustion area before it ignites and that can be exciting, and not in a good way.
 
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Old 10-08-16, 12:46 PM
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1. The "flame sensor" is a thermocouple; a junction of two dissimilar metals that generates a tiny electrical voltage when heated. It is quite rugged but what you have done is NOT acceptable as I will detail later.

2. The thermocouple SHOULD be engulfed by the pilot flame when in the normal position. That it wasn't means you have an insufficient pilot, most often caused by a dirty pilot orifice but sometimes by a maladjusted pilot regulator.

3. Thermocouple itself is located inside the tubing that you have manhandled. Bending and scraping the tube has accomplished nothing.

4. Nothing works forever without maintenance. The pilot orifice can become partially plugged from dirt particles in the air or in the gas itself.

You need to disconnect the gas tubing right at the pilot burner and the orifice is located in the fitting that is part of the pilot burner itself. Sometimes compressed air will clear any foreign material and sometimes you need to use a wire from a wire brush. Totally removing the pilot burner from the furnace is usually necessary to be able to do the work.
 
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Old 10-08-16, 02:08 PM
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Based solely on your pictures.... this furnace is a newer version and does not use a thermocouple although a thermocouple IS listed for your exact model.

Your system uses a flame sensor and spark ignition.
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It appears from the picture that the flame spreader is cracked and broken with a piece missing.

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You cannot run your furnace on an improper pilot. Bending the flame sensor rod didn't hurt it but now it's registering a pilot that is too small.
 
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Old 10-08-16, 03:39 PM
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<<1. The "flame sensor" is a thermocouple; a junction of two dissimilar metals that generates a tiny electrical voltage when heated. It is quite rugged but what you have done is NOT acceptable as I will detail later.
>>


A flame sensor is NOT a thermocouple.


A thermocouple generates 20-30 millivolts of DC voltage which energizes a magnet in the gas valve, keeping the gas valve open. If the pilot goes out, the voltage goes away, the magnet fails and snaps the gas valve closed, shutting off the gas.

A flame sensor has an AC voltage applied to it via a wire from the circuit board. If a flame engulfs the flame sensor, a small DC voltage is rectified from the AC voltage as it passes through the flame. That DC voltage is detected by the circuit board, which verifies that the pilot or burners are lit.

So they are quite different systems.
 
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Old 10-08-16, 03:39 PM
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Thanks for looking that up, Pete. It is the same pilot burner that uses either a flame rod or a thermocouple, the latter often having an electric ignitor. The flame spreader IS important as it directs the (proper size) pilot to the main burner. By bending the flame rod over so it works with the smaller pilot flame it could cause delayed ignition of the main burner and that could under adverse conditions cause a major explosion.
 
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Old 10-08-16, 03:45 PM
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<<It would be better to correct the pilot flame problem. Have you taken out the pilot assembly and cleaned the pilot orifice? Likely it is partially plugged.
>>


That is very likely the actual problem.

<<(1) is the flame sensor capable of withstanding being closer to the pilot flame, heatwise or otherwise? I would suppose so since the flame sensor is next to the main flames once the unit goes on. it does get hot in there.
>>


The flame sensor MUST actually be touched by the flames for the circuit board to be able to detect that the burners are lit. Close doesn't count!


Yes, the flame sensor needs to be engulfed (touched) by the burner flames. However, too hot and you might burn away the metal on the flame sensor. Any metal (such as steel) can be used as a flame sensor, but metals such as steel are easily burned away by flames.

(If you connect the wire to a flame sensor to a screwdriver, you can use the screw driver as a flame sensor) this can be a useful way to test things occasionally.

<<(3) i thought flame sensors were delicate. I bent and mistreated the old one and is working. I had also cleaned it with a fine wire cloth (didn't work) and to an extreme scrapped it with a blade a lot beforehand trying to clean it (back when I thought the sensor was dirty). The old sensor is really beat up. How come is still working?
>>


The main problem with bending a flame sensor is that they can break off and then need to be replaced.
 
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