see Fig 12.
Note #5 in the lower right corner.
This wire must be connected to furnace sheetmetal for the flamesensor to work.
THe note is for the return wire of the gas valve.
I'm not sure what they mean exaclty, but something there needs to get to furnace sheetmetal ground for the flame senor to work.
Looking up videos on troubleshooting a Bryant 373LAV flame sensor,
these guys clean it with a brand new five-dollar bill:
This looks like a similar furnace to mine, with a similar issue,
and this guy does not recommend cleaning the flame sensor:
After replacing the flame sensor, he shows the whole cycle:
These guys use #0000 fine steel wool to clean the flame sensor:
All say not to touch the sensor with fingers, which I didn't
know when I first had cleaned it.
But, interestingly, *all* (of those) simply clean or replace the flame sensor
without *any* of them testing it for the 5 or 6 uamps of control current.
(I'll google separately for videos *testing* the flame sensor circuit
but this is already long.)
Normally, I test things before replacing them, but, none of the
technicians in the youtube videos I've found bothered to test it
before replacing it.
Even so, I might buy a spare flame sensor and igniter, and keep
it next to the furnace. Any suggestions as to good suppliers?
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
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
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.
On Tuesday, December 23, 2014 7:19:34 PM UTC-5, firstname.lastname@example.org wrote:
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.
On Wednesday, December 24, 2014 6:58:04 AM UTC-5, trader_4 wrote:
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
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
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.
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
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.
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.
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:
The current for a good flame sensor starts at 0ua with no flame,
and then, with flame the ammeter reads 4ua.
He mentions that a dirty flame sensor will read lower
current, so, now I know at least which direction the
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:
Here they hook up a Fieldpiece HS36 ammeter into the flame
sensor in series,
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!
On Tue, 23 Dec 2014 15:23:20 +0000 (UTC), "Danny D."
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).
Scott Lurndal wrote, on Tue, 23 Dec 2014 16:49:18 +0000:
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.
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:
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 âCom 24Vâ 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.
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
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
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.
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.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.