trader_4 wrote, on Tue, 23 Dec 2014 06:56:45 -0800:
That's a good point in that the *control board* needs to be grounded, as well, especially, since the *only* DC I can find is in the flame sensor circuit.
Right now I'm looking up how to test the handful of microamps that the flame sensor circuit needs.
I'm not sure how it works yet. I suspect the heat lowers the resistance which then increases the current past a trip point, so, if that's the case, I will try to measure that increased current when I figure out how to measure the current.
makolber wrote, on Tue, 23 Dec 2014 07:05:32 -0800:
You're the first person to describe *how* the flame sensor works, so, I appreciate that input, as the first step of measuring something is knowing how it works.
I had suspected that the DC current through the flame sensor changes with heat, such that the controller senses that change.
In the same vein, that's the same as saying the resistance changes, such that the voltage drop across the flame sensor and ground changes (which, I think, is your battery analogy).
I guess I could remove the flame sensor and put a bunsen burner on it and measure the resistance change, but, right now I'm looking up for how to test the current at such low microamperes, when my Fluke 75 only has a 300ma scale.
I do have a cheap Radio-Shack emergency open-jaw ammeter in my car trunk, so, I'll dig that up, but, I doubt it's sensitive enough to go down to a microampere.
The flame is grounded? (I don't understand that statement.)
makolber wrote, on Tue, 23 Dec 2014 07:05:32 -0800:
This guy tests the flame sensor in situ with an ammeter:
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The current for a good flame sensor starts at 0ua with no flame, and then, with flame the ammeter reads 4ua.
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He mentions that a dirty flame sensor will read lower current, so, now I know at least which direction the thing goes.
a. There is no current when the sensor is cold. b. There is low current when the sensor is dirty. c. There is a handful of microamps when it's hot.
This guy removes the flame sensor and checks continuity all along the stainless steel rod, using a technique I have never seen before for a metal (conductive) rod:
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Here they hook up a Fieldpiece HS36 ammeter into the flame sensor in series,
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They had a reading of 0ua when cold, 2ua when dirty, and 3ua when cleaned. The difference between not working and working was only 1ua, so, a bad ground would certainly affect microamps!
Danny, I don't think you fully understood his battery analogy. It really generates voltage. Google thermocouple. If you put it in a bunsen burner, use your Fluke on its DC voltage range and measure the voltage to see if it is good. (It is also possible you are correct and the device is a variable resistor that changes with heat, but thermocouples are very common in gas heating appliances).
Hi, I use O'scope to check flame sensor. If that is the problem, I'd just replace it. If furnace fired and soon after it quits probably sensor did not sense the flame. Isn't there a test point on the board when temporarily grounded it will go thru logic steps of the operation. I believe all Carrier board has it and Bryant is Carrier 2nd tier product. Control board is quite common amongst brands. My cabin Trane furnace has Carrier equivalent board in there. Furnace operation is just a simple one line yes/no logic flow.
APED Appliance Parts on W San Carlos street in San Jose has igniters, flame sensors and control boards for most furnaces. I've been happy with my purchases from them.
Scott Lurndal wrote, on Tue, 23 Dec 2014 16:49:18 +0000:
Hi Scott,
Thanks for that tip, as they're very close. They're at 2280 Stevens Creek Boulevard, San Jose, CA 95128.
I called them at 408-977-0404, but, maybe they're on vacation as they didn't pick up the phone (9-6M-F, 9-3 Sat) so I will call again later.
I think, no matter what, I'll get a price on the flame sensor and the control board, since they're the two implicated parts at the moment (other than ground itself).
Tony Hwang wrote, on Tue, 23 Dec 2014 09:09:33 -0700:
I always wanted an oscilloscope; but I don't have one. :(
Wouldn't you know it, but none of the service manuals have the part numbers. I called Bryant and they said that both Bryant and Payne use the same part numbers, but they didn't have a parts list at their 800-428-4326x3 phone number.
They gave me four dealers to call, and all had messages, so, I left a message with all four, hoping they'll get back to me on the part number for the flame sensor and control board.
I agree. The current conditions are classic for when the flame sensor does not sense the flame (based on all that I've learned yesterday).
Yes. But ...
The test is confusing (to me) as to what the correct procedure is. Here's the test, on the bottom of page 10, of this manual:
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Here's what that test "says": "To initiate the component test sequence,shut OFF the room thermostat or disconnect the "R" thermostat lead. Briefly short the TEST terminal to the ?Com 24V? terminal. Status LED will flash code and then turn ON the inducer motor. The inducer motor will run for the entire component test. The hot surface ignitor, blower motor-heat speed, and blower motor-cool speed will be turned ON for 10-15 seconds each."
It's not clear which "R" needs to be disconnected, as there is are two "R" wires at the thermostat, and one "R" wire at the control board. I disconnected both.
Then I attached an orange wire to the "COM" terminal and briefly touched the "TEST" spade on the control board.
Nothing happened.
I then turned on the power, and nothing happened when I did that. I then turned on the power and held down the door switch, and, this time "something" happened - but the results were confusing.
First the inducer motor went on (as expected), and then the igniter lit brightly (as expected) and then nothing was visibly happening until the inducer went off, about a minute or so thereafter.
OK. I "think" I ran the test. What did it do? I don't know.
What did it tell me? I don't even know what to look for.
I called Bryant who said that Carrier and Payne and Bryant are all owned by the same company, and that Payne & Bryant are parts compatible.
Hi, If you want to run test just set the 'stat to lowest setting so it won't call for heat or cool interfering with the test. Just with a short piece of wire jumper the TP and ground momentarily the test will start. Regarding sensor only thing could be different would be connector. If you get one with wrong connector cut the leads and use connector from old one. I know same is the case with HSI(hot surface ignitor) Never touch HSI with bare hand, finger oil will shorten it's life, also it is quite brittle. And cover plate removed, don't forget to cheat the interlock switch. If you want to reset every thing just turn the power on/off otherwise it'll take hours B4 it comes out of lock down mode. All my houses I built I used Carrier product so I am quite familiar with it's design. I always keep a spare HSI. I once got caught in one cold winter night with burnt out HSI. I have to light the furnace with BBQ lighter whenever furnace came on all night. Two fire places in the house were not enough to keep warm, LOL! Good luck. NO white Christmas this year. Day time it is above freezing.
Hi, One more thing. If you indeed need board, it could be universal type with wiring adapter harnesses and instruction sheet. Board is quite pricy, I hope you don't need one.
Ok,the flame sensor is just a wire. It does not change resistance with hea t. Also the flame sensor has only 1 connection and as you shoild know, a c omplete circuit needs 2. The circuit is completed THROUGH THE FLAME to the metal burner. And the current does not come from the controller. The flame actually generates the current. The controller senses therescense of thos small current. So the flame is a small battery where the neg terminal is t he metal burner and the positive terminal is the probe in the flame. The c urrent is very small and i would not try to measure it unless you have the right kind of meter and know what to do.
Also the flame sensor in NOT like the thermocoouples which have two wires a nd generate current inthe two wires when heated. The flame sensor is diffe rent.
The flowing hot gas from the burner tothe probe creates a small current.
I donT know how important it is for the probe itself to be clean, but the i nsulator that holds the probe needs to be clean or else the small current w illleak directly back to the grounded metal ans not make it to the controll er.
Ok, im a little wrong. The flame senor works by rectification. The controller applies a small AC voltage and the flame lets the current flow in one direction only in effect creating a small DC current. The battery analogy is not exactly right. Goggle flame rectification sensor.
Stormin Mormon wrote, on Tue, 23 Dec 2014 14:45:50 -0500:
No problem. I had guessed that you might have been alluding to a situation where a conclusion would be reached without the scientific logic behind that conclusion.
While a *lot* of people just throw parts at a problem, and even more make inferences that have no basis in fact, I "try" to make logical inferences, and I try to test the fact.
I'm not always right, and, in fact, I'm often wrong; but in the end, the item gets fixed, usually by fixing the right part.
In this case, I "may" have to throw parts at the problem, because every test I "can" do on the flame sensor circuit shows a good sensor, yet, clearly, the flame sensing circuit is not working.
I contacted at least 5 distributors today, and only two would sell a consumer parts. One had the flame sensor for $20, the other for $18, but the $20 lady called first, so I ordered over the phone. Since she didn't require an up-front payment, I presume she has ordered it by now, so, I'll buy it from her anyway.
She had the control board for $450, while the other guy had it for $250, so, that's a bigger difference. I spent a few hours (literally) on my side, in the crawlspace, staring at that darn furnace, reading the three PDFs, staring at that darn thing some more, and, like an idiot, turning it on over and over and over again (even disassembling and reassembling the burners).
In the end, I gave up on debugging further. a) I can't definitively test the flame sensor (I can't even figure out yet how it works) b) It's either the flame sensor or the motherboard, and, c) If it's a ground, I can't find it.
Stormin Mormon wrote, on Tue, 23 Dec 2014 14:48:27 -0500:
I called a few today.
My hastily written notes are below, but I may have mixed a bit up as almost all had to call me back so I invariably didn't have my paperwork with me when they called back, so, it was hard keeping them straight since they all sounded the same.
Slakey Brothers 1480 Nicora Avenue, San Jose (sells only to contractors).
Appliance Parts, Bob 408-265-5030, San Jose (sells to humans, $18 flame sensor, $250 control board)
APED Appliance, San Jose 408-977-0404 (sells only to contractors)
Residential Heating and AC 888-818-6374, Campbell (sells to humans, $20 flame sensor, $450 control board)
Air Care Heating & Cooling, San Jose 408-513-3089 (doesn't sell parts, $80 for a service call)
Coldcraft 888-918-8662 (doesn't sell parts, $139 for a service call)
makolber wrote, on Tue, 23 Dec 2014 16:19:30 -0800:
There are only two ways to test a component: a) (Perhaps blindly) following a test procedure, or, b) Understanding how it works and improvising an appropriate test.
I can't blindly follow the stated test procedure because I can't easily measure from 1 to 6 microamperes.
I can't logically improvise a reasonable test procedure because I can't figure out how the thing works.
I did try measuring resistance from spade to tip, which varied: 0.2 ohms
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0.6 ohms
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And I verified that the flame sensor "body" is attached to ground: 0.2 ohms
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I doubled up on the ground from the flame sensor body to the chassis ground, (it's hard to see the black alligator clip wire in the photo below, nor the tape on the door switch, but this is that test):
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With the flame sensor body ground doubled, I cycled the furnace, just to make sure that a bad flame sensor ground wasn't the problem:
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In the hopes of doubling the control board ground, I looked (in vain) everywhere I could on the controller board for a named GROUND, but never did fine a single connection called "ground":
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All these tests were in vain.
I still don't know if it's a bad flame sensor or a bad circuit board.
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