I didn't watch the video, but I'm guessing that he did that with a furnace that started up and was operating, ie it was past the first few seconds. From what you've said, yours isn't doing that. The sequence is the furnace turns on the gas, ignites. At that point I would think it doesn't care what's going on with the flame sensor. It's only some seconds later, when there should be flame, that I would think the flame sensor would matter. So, past that point, if you shorted it out, then I'd expect it would force a shutdown. But you aren't there, you're in that initial fire up window.
Of course there's no connection from the rod to the metal body. It relies on the flame to make the connection. That's how it works.
I wouldn't bother. It's just going to get rusted again. The existing rust probably serves as a protective barrier. Remove it and you just expose more fresh metal to oxidize.
AC voltage and the flame lets the current flow in one direction only in ef fect creating a small DC current. The battery analogy is not exactly right .
Hi trader. yes I just learned this too, and it is interesting. Apparently the flame sensor circuits have evolved to this. I read that th e original flame sensors were just measuring the DC resistance through the flame, no flame is an open circuit, and a flame completes the circuit but w ith a high resistnace. The problem is that this is not fail safe. If the insulator that hold the flame sensor gets soot, that leakage resistance to ground can look like a flame. So if the insulator is sooty and the flame s hould go out, the controler would see the path through the soot and think t he flame is present. This of course is very dangerous. So they improved t he circuit, instead of just looking for DC resistnace, they found that a fl ame (unlike soot) conducts current in one direction better than the other. So the newer circuits apply an AC voltage, looks like a high impeandace an d pretty high voltage like 90V AC and the circuit senses that there is more current in one direction than the other. This indicates that there is an actual flame and not just leakage from soot.
So I'm guessing now that the 90 or so volts comes from the AC line and the controller circuit can't work right if the AC hot and neutral and ground ar e not connected correctly.
OP, what kind of VOM do you have. Not all can read MICRO amps. You would have to put the meter in series between the flame sensor and the controlle r. Unless you are good at this, I would not try it, you can cause damage.
Instead if you used an ohmeter to test the flame sensor by itself, connecti ng one lead to the flame sensor and the other to chassis ground, ___ IM GUE SSING__ you might see a resistance of 5 MEG Ohms (that is 5,000,000 Ohms not 5 Ohms) when the flame is present and if you reverse the ohm meter lead s the resistance should be a little lower in one direction than the other. And open circuit (infinity ohms) when no flame is present.
So the practical things to check are that all the ground connections are in place, that the AC supply polatity hot vs neutral is correct, that the fla me sensor insulator is not dirty, that the flame sensor probe itself is not very dirty and that the flame sensor probe is properly engulphed in the fl ame whatever that might be.
I agree, if there is loose rust. From the pic I saw, it just looked like one end was rusty. If it;s just a light rust coating that isn't loose, I'd just leave it be. If you take it to bare metal, it's just going to repeat the rusting process.
Hi, Don't worry about the voltage out of transformer. It does not have tight specs. Can be as high as 32V AC. IIRC, the test runs the logic sequence up to the point of running normally. If some thing is not right it'll tell via error code. Did you see the HSI glowing to ignite? If so definitely next step is sensing flame by sensor, if not it'll quit and go into lock down mode but blower is still running to cool off. Then you have to power down and up to get out of lock down mode. If it has ground problem then clip one meter probe to furnace frame and probe all the [point on the board Ohmming it out to see any unusual high resistance at any point.
Hi, If you are stuck at this point then new sensor or board, only two choices, right? Being retired from Honeywell I can lean on guys still working there, electricians, process control technicians, etc. When I need parts I get help from them. On the control board there ought to be a chip(maybe op amp) which will convert that small amount of current to normal logic level(5V DC) to feed to ASIC(the brain of board) telling flame is on. One time there was a TSB out regarding one high Wattage resistor over heating causing solder joint to fail intermittent. I used tyo reinforce the solder joints and drill holes on the board cover near the resister for more cooling. If this resistor has poor solder joint, it will produce all kinda funnies.
They may have told you that, but I've never had a problem buying from them, and I'm not a contractor. It's close, just drive over. Then stop at the falafel drive-in across the street for a gyro & baklava.
The COM terminal is used in conjunction with the RED terminal to provide power to 'smart' thermostats like the Nest or the Honeywell units that have Wifi radios and other circuitry that needs constant power.
trader_4 wrote, on Wed, 24 Dec 2014 04:13:39 -0800:
I have to agree with you.
One of the videos I watched was from a teacher who sabotages furnaces as a test for the students, & he said he can't make a flame sensor fail.
It's simply a stainless steel rod, mounted in an insulator, which is just about the simplest and most rugged of all possible sensors.
a. The rod can get carbonized (hence less conductive) b. The insulator could lose its insulating abilities (not likely) c. The burner can lose ground (or develop high resistance).
Even though I have a new flame sensor on order, I can't imagine that there is anything wrong with my current flame sensor, since, as you note, the thing is just a steel rod.
trader_4 wrote, on Wed, 24 Dec 2014 04:04:51 -0800:
Fair enough. In my case, everything is working, so I guess that the "component test" didn't tell me anything.
The only problem is that the flame is incorrectly *not* being sensed, but the "component test" doesn't seem to turn on the flame, so, it wouldn't know that.
trader_4 wrote, on Wed, 24 Dec 2014 04:20:58 -0800:
Fair enough.
Since the flame lasts about 7 seconds, there was time just now to test the shutoff by shorting the flame sensor to ground within that seven second window. But, as you guessed, nothing different happened.
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