Today’s units react to unsafe conditions in less than a second. This is accomplished through the use of “computerized control modules” and a flame sensing control system known as flame rectification.
The safety used in today’s furnaces that accomplishes flame rectification is the flame sensor. A flame sensor is nothing more than a stainless steel rod partially encased in porcelain. By itself, it does nothing. There are no moving parts, no switches.
When attached to a control module, the module sends out an A/C voltage to the sensor. This voltage is always present at the sensor. The reason voltage is always present is because the sensor is always looking for flame, even if there is no call for heat.
If it sensed flame without a call for heat (ie. – gas valve stuck open), the control would see this and make sure the blower and inducer turned on or stayed on to prevent the furnace from becoming a little Chernobyl (complete melt down) in the home.
When there is a call for heat and the burners ignite, flame hits this sensor creating a “path to ground through the flame” for the A/C voltage. When this happens, a pulsating D/C signal, in the form of micro amps goes back to the control and the module decides if the signal is strong enough to allow the unit to continue operating. Because this system is “electronic”, it reacts to unsafe ignition problems in fractions of seconds instead of minutes.
As stated above, the flame sensor does not contain any moving or functional parts. The flame sensor is energized with AC voltage, which is rectified to DC in the presence of flame. The DC current flow is measurable in micro amps (uA). A minimum micro amp measurement is required to continue the heating cycle. On gas furnaces, 3.7 uA is a typical measurement and measurements less than .5 uA result in interruption of the heating cycle.
On many models, the flame current may be read at the flame current pad built into the furnace control board. The control board converts the uA measurement to DC voltage (1 uA = 1 volt DC,).
Alternately, on furnaces that do not have a flame current pad, the micro amp measurement must be read by placing a meter set to DC Amps in series with the flame sensor and the control board.
One of the most common causes of flame failure is the sensor. Keep in mind that the control is sending out an A/C voltage signal. If the sensor becomes dirty or corroded, it is like covering that stainless steel probe with insulation and the current can not pass to the flame (ground) and create the D/C signal back to the board. Or, if the wire from the board to the sensor is cracked or the porcelain is cracked, the voltage “bleeds” off to ground at the point of the “leak” changing the micro amp signal to the board and not allowing it to lock in.
If the flame signal is not adequate for flame proof, check the following:
- A dedicated ground wire must be connected to the furnace
- All wiring connections must be tight
- Proper line voltage polarity.
- Manifold pressure must be correct per the furnace rating plate
- The flame sensor must be in the flame.
- The flame sensor must be clean and free of oxidation. If necessary to clean, use extra fine steel wool. Do not use sand paper or anything course as this can create “grooves” on the sensor which will cause it to become contaminated quicker. If corrosion or contamination is severe enough that cleaning with steel wool will not improve the sensors performance, the flame sensor must be replaced.
- Porcelain on the flame sensor must be intact. If the ceramic insulator is cracked, the flame sensor must be replaced.
- Check to see if there is voltage at the sensor. Without a call for heat, there should be voltage (approximately 90 to 110 volts) between the sensor and ground. The sensor is always looking for flame and is always energized. If no voltage output is found, check the wiring to the board or replace the board if no voltage output is present.
- Flame sensors replaced due to contaminants in the combustion air or cracked ceramic are not covered under most manufacturer’s warranties.
Since the flame sensor is the primary safety for ignition, the signal should be check every time a unit is cleaned and serviced. If the signal is getting weak, the sensor should be cleaned. Cleaning it is a fix if the signal does not reach the nominal micro amps required by the control. Keep in mind that this is the primary safety in the furnace. It is important to make sure that it functions properly.
Will technology stop here? Not likely! Service technicians need to keep up with the latest diagnostics and have the proper tools to work on the furnaces of today as well as those of tomorrow.
York Central Tech Talk 
Hi Mike,
This article was excellent, but I have two questions. First, I never knew flame was conductive…are you telling me current actually flows through the flame when you say ‘A path to ground through the flame’? Or, is it similar to a thermocouple which creates a (very small) voltage based on temperature? Second, it seems that the flame sensor can fail when coated with soot and prevent it from generating enough current, but can the opposite be true, will a short or cracked insulator always provide a small current leak, enough to make the fan kick on all the time even with the thermostat off? I am experiencing this now and, without even measuring this current, two contractors have simply told me to replace the control board.
Thanks again for all of these great articles!
Rob:
Thermocouples rely on “heat” to cause the dissimilar metals inside the thermocouple to come in contat with each other causing a small millivolt to be created to hold in the magnet of the baso
With flame rectification, heat has nothing to do with it. The sensor could be glowing “cherry red” and you could still not have a signal. It is a combination of A/C voltage present on the sensor and a good ground to the furnace and the presence of flame that creates the micro-amp signal.
Yes, the flame is conductive.
If there is a “slight crack” in the porceline, that voltage can bleed off to ground creating a “false flame signal” which is why most control have a fault code “flame present when it should not be there”. Of course, if flame is present due to a stuck open gas valve, this keeps the blower and inducer running. However, if the sensor is cracked and a signal is produced when there is no flame present, you are in a lock out and the blower and inducer are running.
Thank you for the comment on the site
Hi Mike,
Great article, I just had to write in with my recent experience with a flame sensor. When I arrived to a no heat call I found the diagnostic flash code was telling the the furnace was “locked out” due to too many flame recycles. I reset the power to the furnace and with my meter set to microamps(uA) and my leads in series with the flame sensor I waited for the furnace to light. I fully expected to see less than 1uA. Instead I got a reading of 11.7uA. I thought this was strange and unusually high. I put a new flame sensor in just to see if it would be different and sure enough with the new one I was getting a reading of 5uA then after a few moments 3.2uA. What could be happening that I was getting such a high reading on the first one???
Mike
the flame sensor always has power to it. If there is a hairline crack or other such flaw in the porcelain, you could be creating a signal to ground.
Hi this is a great blog.have a question my heater unit is keeping turning off in auto could u help me to find the problem thank u
with the sophisticated controls in today’s systems, it is always best to contact your installer or service company. They can do the proper diagnostics and fix the issue.
Hi Mike,
Great article, it explains everything. My issue here is I get only between 2.0 and 2.4uA. 1.5uA is the cut off for no flame. My sensor is clean, I replaced with new and get same readings. I suspect sensor is not fully in the flame, hard to see. In your list, you include manifold pressure, would I be right to guess too much pressure might be shooting the flame too far past the sensor? Is there an adjustment for this pressure on the main gas valve? PS. What I’m seeing is the system occasionally redoing the ignition sequence 2 or 3 times and even more occasionally, locking out completely (which I guess happens after 4 tries).
Josh
you need to look at the igniton control. Not all controls use 3 micro-amps as a nominal signal. some of the older white Rodgers controls used a nominal 2.5 signal..
If you control should be reading 3 to 3.5 nominal, then too much gas pressure could lift the flame off the burners causing a weak signal. Also, make sure you have a very good ground to the unit.. Lastly, if it seems to take a while for the burners to light, you could have a weak hot surface igniter.
Thank you for your response.
The manual for my furnace reads “3.7uA normal”, but that might be misinformation. It is an older White Rogers system, so your experience is likely to be correct.
In watching the cycle, I think the igniter is working fine. It glows bright for 10 seconds, the gas valve clicks, in less than a second there’s flame. Then, when the problem was happening, the flame would remain for about 7 seconds and the main valve would click off again, extinguishing the flame. I am going to redo the ground between the control board and the chassis. PS. Since I’ve played with the Flame Sensor, it has been operating without fault. There was very little room for movement of the flame sensor, but I pushed it as close to the center and back (nearest the flame) as possible. I see the flame appearing past the sensor but not engulfing it. It’s strange to describe. There is only a bit of flame just below the nozzle that touches the tip of the sensor. I would say just 1/4″ of the tip is touching flame. I was hoping to see 1″ of the sensor in contact with the flame. Many thanks again for your valued advice and experience.
Josh
Thanks for this great article.
I’ve been having issues with the flame sensor on my natural gas high-E furnace. I’ve had to clean it every two weeks because the flame would get puffed out right after the pilot came on. My first hunch was that there might be a problem with how well my furnace is burning gas, or a blockage in the air intake. However, the flame is a beautiful blue and the air intake seems ok. Any thoughts?
first thing- is it the sensor shutting off the burner? What is the microamp signal? is it possible a pressure switch is “bouncing” and shutting it off?
What is the pilot/manifold pressure? I the pressure is too high, the flame could be lifting off the pilot burner, losing “ground” and causing the microamp signal to drop and shut off.
A nice blue flame is great but there are a lot of other factors that could cause the lockout.
Thanks for the reply.
While I have not measured the current or the pilot pressure, one observation I can share is that cleaning the flame sensor solves the problem. So, when the flame seems to be extinguished shortly after pilot, I turn the furnace off, clean the sensor and turn the furnace back on, and this solves the problem (ie the flame stays on after pilot). This leads me to think that excess soot is culpable. Is this a safe assumption?
If you have to keep cleaning the sensor — where is the “dirt” coming from? Are you using inside air for combustion? Is the furnace in a laundry room? You may need to find a ‘clean” source of combustion air to solve your problem. Also, when you clean the sensor — be sure to use fine steel wool. Do NOT use any type of sand paper or emory as they can leave a gritty residue on the sensor causing it to erode the signal quicker.
The furnace IS inside a laundry room. Why is that important? What can I do?
I am not using inside air for combustion. The air intake is not blocked.
all I can say is if you are using OUTSIDE air for combustion, then wither there are contaminants in the gas or you are bringing in outside air that has contaminents. Laundry rooms, when that is the source of combustion air, causes a lot of problems from the soap in the air and chemical in the air.
laundry rooms are a source of contaminents from soap and chemicals in the air. If you unit is using outside air for combustion, then either there are contaminents in the gas or in the outside air causing the sensor to become “coated”. Try changing the snsor if you haver used something other than steel wool to clean it and see how that works.?
I actually tried this two months ago and the sensor lasted two months before failing again. Based on this result and what you’re saying about contaminants, it seems to me that the quality of combustion might be to blame. Is there anything I can do?
see if your contractor can “isolate” the furnace air for combustion or, at least, bring in some fresh air for combustion to the vestibule of the furnace
Thanks for taking the time to respond. I have an air intake which brings fresh outside air to the vestibule. As far as I can see, nothing is blocking the intake.
Then I would recommend contacting your heating service company and see if they can find the source of the contaminents affecting your flame sensor causing you to have to clean it so often.
Thanks for the great article.
I have two questions.
1.What kind of flash sequence is expected of a dirty flame sensor?
2.Should we proactively clean the flame sensor or only when we encounter a problem?
My York diamond 80 furnance, runs well for a cycle or two. Then it doesnt start for next cycles.
I switch the power to the furnance off for 3-4 hrs, It starts to work again.
Appreciate the help.
Hari
depending on the unit control board, usually a flash 7 or 8 could be a dirty sensor or other ignition problem. You could have a weak igniter not lighting the burners all the time. You could be bouncing on a pressure switch. There are a lot of reasons why it does no=t stay lit. As I comment on a lot of posts, it is always best to have a professional check it out.
Once every 24-48 hours or so, the York Diamond 80… after normal heat cycle, fans remain on. Flashing code indicates “Flame has been been sensed when no flame should be present”
Cycle power and it goes right back to heat. Replaced flame sensor and checked wiring for grounds. I was on line to buy a new control module. Do you agree it sounds like the module?
There is not enough information to determine if you need a new module. If you have the flash code saying “flame sensed when it is not supposed to be there”, you could have a gas valve the is “slow closing” causing that flash code. You could have a cracked sensor ‘bleeding” off enough voltage to simulate flame present. You could have a bad sensor wire. These are just a few of the possibilities for that flash code.
Flash codes are there for technicians to “guide” them and lead them in the correct direction. They are not the “repair code”. A good technician will use the codes (and those possibly stored in memory) and then use his meters and tools to find and correct the problem.
It turned out to be a loose hose from the pressure switch
I have been reading your blog for some solutions and I hope you can help. its a flame sensor that has me baffled and its off of an older York rooftop unit. DL- 10n18ataaa1a. I read that York was your thing. Anyway it’s giving me the old 7 second flame out deal. The micro volt measurements were good but I bought a new sensor anyway to no avail. The ground is good, the neutral is good, and the the draft motor, and vacuum switch is good. Could it be the control board?, or anything else?
what is the MICROAMP reading from the sensor? what is the inlet and manifold gas pressures when firing? are the burner crossovers keeping flame on the sensor? check these things out first. is there a “wild leg” in your line voltage (110 -110 – 220)? The control circuit should not be on the high/wild leg.
The micro amp reading goes to 4.7, however the reading is not steady. It starts low like 1.2, then it eventually reads 4.7 before it shuts down. Flame is always on the sensor. I don’t have a gas pressure tester, but I guess I need to invest in one. Will check to see which line the control circuit is on. I won’t see the unit until Tuesday, since tomorrow is a holiday. Will let you know, and thanks for the assistance.
That microamp signal is too high. when first firing, gas pressure should be at full fire — 3.5 IWC and then drop to low fire at 2.1 IWC after 60 seconds if w2 is not present. disconnect w2 off the board and see what happens. are there any flash codes on the main board? on the ignition control? does it just keep recycling?
The flash code on the board indicates bad sensor, bad gas valve, low pressure.
so — what did you find?
The gas pressure high fire was at 3.0, I raised it to 3.5. Doesn’t stay on long enough to measure low fire. After 3 cycles of trying to fire, the code reads flame sensor, gas pressure, gas valve. Since I have changed the sensor, I have ruled that out. The gas valve opens, has good pressure, and when the gas shuts down I lose the 24 volts so the valve so it’s good. I checked the volts coming in and it’s on the 110 side of the circuit.
I guess I could bypass the pressure switch to make sure it’s staying in tact.
what are the actual flash codes on the main board and on the ignition control?
There are six flashes and it reads Ignition lockout: gas valve, gas pressure, flame sensor. I didn’t get a chance to look at it today cause it rained all day.
I would try running a new wire from the sensor to the ignition control. I it still locks out, then most likely it is the ignition control
Ok it was the ignition control…I appreciate the help
Glad I was able to help
I have a Colmar boiler. When I check for voltage across flame sensor I am only getting 9.5v. When I test for current I get zero. System stops after 120 seconds.
the control with a flame sensor looks for a microamp signal to prove flame. You would need to check with the boiler manufacturer what their minimum microamp signal is to lock in the control. If it proves flame and then drops out, it could be gas pressure related, or any number of other factors. The boiler manufacturer would be your best source for technical support.
Can a flame sensor rod be cut to length. I was advised that mine needed to be replaced, but the replacement part is much longer after the bend than the flame sensor rod that was previously installed. The new part is about 3 inches after the bend where the old one was only about an inch. Just want to make sure i am getting good advice.
it is always best to use the manufacturer’s OEM sensor, you should be able to cut it down. Be careful not to crack the porcelain while doing it. I recommend using a “file” to cut the sensor.
Thanks….apparently this is the new OEM replacement? From what I understand lennox doesn’t make the original anymore. I was nervous about it needing to be cut down, especially after realizing that I had been wrong in my understanding of this as a simple heat sensor, and looking at the manual which showed such small clearances for where the sensor should terminate regarding the flame. I appreciate your quick response. Does the cut itself matter? I.e. How clean the cut is? And does where the probe sit in the flame effect the function? I was wondering if maybe a wrong sensor had been previously installed and that’s what started the problem in the first place? Thanks again
The position is “critical”. I would check with Lennox on whether the one you have is correct or if the previous one was incorrect before making any modifications to the sensor
I have an issue where only in the morning, york furnace will continue to blow after flame goes out. I get 8 flashes which is “lockout caused by loss of flame. This has been happening intermittently for over a year. I replaced flame sensor and limit switch. Can the pressure switch cause this? Also any idea why this only happens in the morning? I dont see an air intake on my unit and the PVC out seems to blow very strong outside so i doubt a blockage.
8 flashes indicates flame lost 5 times during a heating cycle. It is a 1 hour iock out and should come on again in 1 hour. For some reason the flame sensor is losing contact with the flame or the control board. What is your microamp signal from the sensor? You could have a loose or corroded connection between the sensor and the board. What is your manifold pressure? Could be an intermittent gas valve?
Gas pressure, microamp signal, gas valve amps all need to be checked and verified.
Lastly, it could be a “bouncing” safety opening intermittently causing the board to think it is a flame failure. Pressure to the pressure switch needs to be checked. Temperature rise through the furnace needs to be checked.
If you are not a contractor, I would suggest calling a qualified heating contractor to make the proper diagnosis and verify all the possibilities mentioned.