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Old 10-02-2011, 11:42 PM   #1
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Roll out switch problem


So I've been dealing with this problem for about 3 weeks now and I'm at a loss. I turned my furnace on and noticed it was blowing cold air. Went downstairs, opened the furnace, no flame. Shut it off, started it up again, flame comes on, then when the igniter turns off the unit shuts off the gas and the flame goes out. After doing some testing and surfing the web, I came to the conclusion that it was either my flame sensor or the circuit board.

Ordered a new circuit board, swapped it out, fired up the furnace and the flame stayed on, everything back to normal.

About a week later the same thing started to happen again. Frustrated, I went back down into the basement with my multimeter and started doing continuity tests again on all the wires, made sure my ground was good, everything checked out. Then something interesting happened.

Just for grins I was checking for voltage across my rollout switches to make sure they weren't bad/tripped. I discovered that with one lead of my multimeter touching the rollout circuit at any point in that circuit and the other lead touching the frame of the furnace, the flame stayed lit and the yellow LED that would indicate a flame was sensed stayed on (I should have mentioned before that the LED was either off or flashing previously which is what made me suspect the flame sensor or circuit board initially). To make sure it wasn't my meter "tricking" the furnace by sending some small voltage or signal, I replaced the meter with a piece of #12 THHN and the system is still holding.

I have replaced my circuit board and flame sensor so those shouldn't be the problem.

At this point I don't even care that I probably replaced my circuit board for no reason. I just want to know why this is happening. I know what they're there for, but how do the rollout switches physically work that they would behave in this manner?

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Old 10-03-2011, 12:03 AM   #2
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Roll out switch problem


They only normally are closed and open (break the circuit) at a certain point, at a certain temperature, effectivley cutting the furnace off.

I'd replace that rollout switch and be done with it.

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Old 10-03-2011, 07:09 AM   #3
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Roll out switch problem


Take the burners out and clean them. Just a little rust on the face of them will make your flame sensing weak.
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Old 10-03-2011, 06:46 PM   #4
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Roll out switch problem


I'll try that when I get home later tonight. Thanks for the suggestion.
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Old 10-03-2011, 10:53 PM   #5
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Roll out switch problem


Well I took out the burners, there's three of them (brass), and they all appeared to be clean. No rust or obstructions. tried to fire it up but it didn't make a difference. Thanks for the suggestion though.
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Old 10-03-2011, 11:08 PM   #6
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Roll out switch problem


Sounds like a bad ground connection.

Do we have a good ground wire connection to the furnace?

Are any mounting holes on the new board labeled as a ground?
Is the board connected directly to the furnace body or a metal panel that mounts to the furnace body? Is this area clean?

As Marty has posted it would be a good idea to wire brush some of these surfaces.

The furnace is looking for microamps dc. This is a very small amount of current.
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Old 10-03-2011, 11:56 PM   #7
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Roll out switch problem


Houston204, whereabouts you at? I'm in Houston as well. Southwest.
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Old 10-04-2011, 12:12 AM   #8
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Roll out switch problem


I'll have to look into it more deeply tomorrow but the furnace does have a good ground both at the panel and where it comes into the furnace. I was thinking something along the same lines though. Bad ground=charge build up on board=interference with small signals that it needs to operate. Which would also explain why replacing the board temporarily cured the problem if it would take time to build such a charge. Or I could have no idea what I'm talking about.

The only ground going to the board is through a nine pin connector which I'm sort of leary about. To make sure that I was getting a good ground there, I took a piece of #12 THHN from the base of my flame sensor and then shoved some strands into the backside of the nine pin where the ground wire goes into. So I don't think that's the issue. Besides, I did continuity checks on all the grounds and they all ring out. Although that dosen't speak to the quality of the ground, just tells me that there's enough of one there for my meter to see. Sigh.
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Old 10-04-2011, 12:22 AM   #9
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Roll out switch problem


Lightly sand paper the sensor itself, without touching the sensing end if possible.

Those things are touchy feely and they themselves will not recognize the current through the flame if "dirty.".
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Old 10-04-2011, 12:47 AM   #10
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Roll out switch problem


Can you measure less than 3vac between 24 volt common and 120vac neutral?

I have seen dirt and oxidation between a mounting plate and furnace cause this. I have also seen a missing ground screw on the board before.

I'm also from the sw side of town. I work in the River Oaks and Memorial area.
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Old 10-04-2011, 12:48 AM   #11
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Roll out switch problem


Another piece of information. I was just testing the voltage across the rollout switches and noticed that I only have .1v supplying the rollout switch circuit. Isn't that normally a full 24v?
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Old 10-04-2011, 12:57 AM   #12
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Roll out switch problem


Now measure to 24 volt common...
24vac each side to common now indicates that a limit is not tripped.
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Old 10-04-2011, 01:03 PM   #13
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Roll out switch problem


If you have not done so already, clean the flame sensor probe with fine (400 grit) sandpaper or steel wool. The milivolt current that it needs to see is easily interupted by corrosion or even dust. Also check the other end of the probe where it plugs into the board to ensure it is corrosion free. I recently had one of these that had me stumped for a bit. Sensor was fine but the pilot light that it was looking for had a weak enough flame that it barely made contact with the probe. When the burner kicked on it would pull the pilot down far enough that it lost conection with the sensor and shut the system down. A simple cleaning of the pilot orfice cured the problem
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Old 10-13-2011, 06:10 PM   #14
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Roll out switch problem


Figured it out! First I'd like to thank you guys for all the worthwhile posts, and thank you for not just telling me to "get a professional to look at it" like some forums do. Turns out though that it wasn't specifically any of the things that we'd talked about. But in theory it kinda was. It was a connection quality issue but in a different place. The nine pin wire harness connector. It must have had some residue built up on the pins. I could get continuity with my meter, but as we had talked about, when the circuit board is looking for microamps, you need to have a really good connection. Cleaned it on a hunch because it was the one place I hadn't been able to bypass with new wires to eliminate as the problem. I could eliminate the nine pin coupling between the upper and lower compartments of the furnace by running continuous pieces of wire temporarily in between.

Disconnected it, sprayed some electrical cleaner in it, and got a sustained flame right away. However, after it had run for a while the yellow flame LED started to blink again, but I knew I was on the right track. Didn't have any vinager so I loaded the end up with ketchup. The acidic properties of the vinager in the ketchup is known to eat the layer of oxidization off of metals. Let it sit for a half hour or so, shook it back and forth in a bowl of hot water to get the ketchup out, sprayed it good with the electrical cleaner, let it dry, problem solved!

Last edited by zalexander1; 10-13-2011 at 06:19 PM.
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Old 01-05-2012, 03:52 PM   #15
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Roll out switch problem


Success - I had the same roll out error and could not sleuth it out with my volt meter. Suspecting that Z was onto something, I took a small piece of very fine (0000) steel wool, wrapped it around a toothpick and lightly "scrubbed" around each of the 12 pins of the socket and then the mating half likewise. Vinegar might have worked as well, but that would have been tried next.

NOTE: Disconnect furnace power before doing this ! Also clean the flame sensor probe while you are in there, I did, but that was not my problem.

After reconnecting everything the furnace came right on line without any more blinking lights. This seemingly trivial fix may have something to do with the flame sensor that generates a tiny ~ 3 micro-amps of current. A little oxidation goes a long way when the sense levels are so low.

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Last edited by halleydog; 01-05-2012 at 03:56 PM.
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