trader_4 wrote, on Tue, 23 Dec 2014 08:00:29 -0800:
The flame sensor is $20, and it's on order.
At the moment, it could be any one of three things:
I know you suggested I check the ground on the circuit board, but, try as I might, I can't find a single connection "labeled" ground!
Tony Hwang wrote, on Tue, 23 Dec 2014 13:57:56 -0700:
I don't understand that test.
It "sounds" like a good idea, and it's *easy* to run:
But, whatever it does, doesn't tell me anything. It just runs 'stuff', and that's about it.
I must be missing the magic of this so-called "component test". I *wish* it gave me useful information.
But, I get nothing out of it, and, I'm not even sure that the "component test" tests the flame sensor anyway, since the flames never start up during that so-called "component test".
So, at the moment, I think it's the most over-rated test that has been suggested, to date! :)
Tony Hwang wrote, on Tue, 23 Dec 2014 13:57:56 -0700:
I taped over the door switch, so that's not a problem.
Thanks for the warning on the brittle igniter.
Here, where I've removed the three burner tubes, you can see that the speckly igniter is only at the top burner, while the rod-shaped flame sensor is only on the bottom burner, with the middle burner being left alone:
Tony Hwang wrote, on Tue, 23 Dec 2014 14:04:50 -0700:
I was only able to get two quotes for the board, the first being $450 and the second being $250, neither of which was in stock.
I spent at least an hour (or more) on my back, looking the board over, and I don't see a single burn mark or obviously bad component:
I did figure out the terminal sequence though, which you can see I drew a simplified diagram of on the chassis in the picture above. COM === the unswitched neutral out of the 24VAC (nominal) transformer RED === the power to the thermostat (which is switched in the thermostat) WHI === the power coming out of the thermostat to call for heating YEL === the power coming out of the thermostat to call for cooling GRE === the power coming out of the thermostat to call for the fan
The transformer, nominally 24VAC, tested at 28VAC, by the way.
Scott Lurndal wrote, on Tue, 23 Dec 2014 21:28:09 +0000:
Thanks Scott,
I have a call to a scientist friend, who might have the capability to measure 1 to 5 microamperes of current.
A funny thing is that one of the service personnel told me to measure "5 ohms" (I swear I heard him right) of the flame sensor.
When I questioned him, he said he always measured if it was 5 ohms. When I asked him *how* he measured it, he *clearly* explained that he hooked one lead to the disconnected wire and the other lead to the spade on the flame sensor while the flames were heating it up.
He kept telling me that was ohms, and I told him two or three times that he can't possibly be measuring ohms that way, but he insisted he tested the ohms that way all the time, and determined whether a flame sensor was bad that way.
Sometimes these technicians scare me.
Pat wrote, on Tue, 23 Dec 2014 11:01:12 -0500:
I will fully agree with you! I do *not* know how the flame sensor works.
It appears to have two *external* electrical connections.
What is not clear are the *internal* connections (if any).
When I tested it cold, there was no connection between the spade end and the body ground:
Oren wrote, on Tue, 23 Dec 2014 14:46:23 -0800:
I remember that story, as you've said that before, I forget why, but, probably in reference to an AC part, when I had to fix my sister's AC unit.
If it matters, I took all three burners out, but, there was nothing I could see to do with them once they were out, except snap a photo:
That did enable me to confirm that the flame sensor, which is on the lower burner in the photo below, was mounted squarely in the flame:
Googling for how the flame sensor works, it seems very few people actually know, or, if they do, they are all explaining something different.
How does a flame sensor work and how do you test it:
How does a furnace flame sensor work:
How does the gas furnace flame rod work:
etc. (I'm still reading up on this...)
makolber wrote, on Tue, 23 Dec 2014 16:08:42 -0800:
I'm trying to make sense out of these four descriptions of how the typical gas furnace flame sensor works (because if I knew how it works, I could devise a test that might not require measuring from 1 to
5 microamperes, which I can't adequately measure with my Fluke 75).How a flame sensor works:
How does a flame sensor work and how do you test it:
How does a furnace flame sensor work:
How does the gas furnace flame rod work:
Danny D. wrote, on Wed, 24 Dec 2014 04:45:28 +0000:
It's not clear, to me, whether that's because of RMS versus average measurements, but the measured 28 VAC is close enough for our purposes to assume the switched power to the RED terminal isn't a problem.
What's harder to understand is the unswitched COM terminal.
It seems to be an unswitched neutral, coming out of a center-tapped transformer, especially since it doesn't seem to go to the thermostat, but, it does go to the AC unit.
Anyway, for some purposes, I assume it's not a center-tapped transformer, and that the COM is simply the unswitched other side of the transformer, where the RED is the switched side of that transformer.
But, for other purposes, I assume the COM is a "neutral" of sorts, coming out of the middle of a 12VAC + 12VAC transformer, which makes it a ground, of sorts.
All this probably just tells you that I don't really understand what exactly the switched RED and unswitched COM terminals are, but, in reality, that's not the problem with this furnace, so, that's just a diversion away from the real problem, which is determining whether the failed part is, either: a) A bad flame sensor ($20 part is on order), or, b) A bad control board ($250 to $450 part can be ordered), or, c) A bad circuit board ground.
Scott Lurndal wrote, on Tue, 23 Dec 2014 21:28:09 +0000:
I got an idea for a test of the "flame rod" from this video.
The guy shorts out a good flame rod, and the flames stop.
If I short out mine, during the flames, it should also stop the flames quicker, and, if it doens't stop the flames right away, that would be an indication that it's not doing anything.
Of course, either way, I still wouldn't know if the control board is bad, but, it's worth a test to see what happens.
Also, I noticed his autopsy of a flame rod shows that there is apparently
*no connection* from the flame rod to the steel body of the flame rod.So, it *is* a single-wire sensor!
Danny D. wrote, on Wed, 24 Dec 2014 05:11:13 +0000:
Now that I've seen the insides of a flame-sense rod, I can't imagine, other than getting dirty, how they can *not* work.
It's just a steel rod inside a ceramic insulator.
Nothing more than that. It's about as simple a construction as can be imagined.
In fact, this guy, who says he's an instructor, says he can't get a flame rod to fail!
He says you can clean it with just about anything (even a file):
Tomorrow I'm going to three more aggressive ways to clean it:
But, it's looking less and less like the flame rod went bad. :(
Scott Lurndal wrote, on Tue, 23 Dec 2014 21:28:09 +0000:
Here, they use a < $10 cheap (#69096 Harbor Freight) meter!
Danny D. wrote, on Wed, 24 Dec 2014 05:44:44 +0000:
As much as I hate cheap tools, I might pick one of these up:
trader_4 wrote, on Tue, 23 Dec 2014 08:00:29 -0800:
This is probably one of the better explanations of the flame sensor:
Pat wrote, on Tue, 23 Dec 2014 11:01:12 -0500:
Here's a good explanation of how the flame rod works.
Now that I know how it works, I should be able to devise a test that doesn't require me to measure 5 ua of current.
trader_4 wrote, on Tue, 23 Dec 2014 06:56:45 -0800:
You're definitely on to something here!
This video says reversed polarity will cause teh flame sense to not work!
Look at the test here for ground using AC voltage (< 2VAC):
From this video, the FS terminal (Flame Sensor) on the board is key:
That control board FS terminal sends out a signal of 90VAC to the flame rod. The flame itself, acts like a diode and a resistor, connecting the flame rod to ground, which is the burner metal itself. Completing the circuit causes the control board to hold open a relay, which keeps the gas flowing.
I noticed my burners were rusty on their ends, so, I will try to clean them up.
Any ideas how to clean up this burner end?
Well, you learn something new all the time. I thought the flame sensor was a thermocouple. But from some googling, I see you're right. I never even knew this phenomenon existed. It apparently works by using the flame itself to conduct a small current. And somehow the AC gets half-waved rectified in the process.
One big difference with testing, would appear to be that the rod type sensor won't generate any voltage from just heating it. It has to have an AC current applied, so the current test would have to be done with it installed, connected to the controller.
Now the advice to clean them makes sense. If it was a thermocouple, whether it had some dirt on it or not should not affect the operation.
Presumably the AC line is connected to the controller. Follow the green wire/s. The incoming ground will be connected to the cabinet frame, typically right where the connection is made. And typically, you'd have a green wire at the controller, together with the hot and neutral, that probably goes back to that incoming connection point too.
It's not over-rated if the inducer blower, air handler blower, etc has failed.....
Maybe he's not as dumb as he sounds. If you put a VOM on the ohms setting, it's providing a voltage source. You now have that connected to the flame sensor. The flame sensor apparently only conducts in one direction, but if you had the polarity right, you would think it would pass some current through the flame, just like under normal operation. That in turn would result in the VOM reading some kind of resistance. But you'd think it would be a lot higher than 5 ohms. And that also assumes he meant you should test it installed, with flame.
Now that we all know how the flame sensor works, I'm left wondering what there is to fail? It's just an electrode. About the only thing that I can see that could fail would be either the attachement wire coming loose or the insulator failing. Both of those should be testable with a VOM, without the need for flame, etc. But maybe something can change in the surface of the metal that reduces it's ability to conduct too.
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