Furnace Starts Then Stops After Ignition.

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Old 09-12-20, 12:32 AM
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Furnace Starts Then Stops After Ignition.

Luxaire Furnace Model PCUHBLD12N100C

The furnace starts properly, ignites, gas valve opens, and it works for 4 seconds. Then the flame turns off. The indicator light is slow blinking amber, showing a normal operation with a call for heat.
I already replaced the flame sensor with a new one. Nothing has changed.
At the end of last winter, I replaced the limit switch because I was getting a code (4 blinks as I recall). There was a problem then but the weather got warm and I forgot about the problem. The furnace would work properly for a while, and then it would blow cold air. If I turned it off then on, it would blow warm again. Since I could access the power to it easily, I ignored the problem. Now, it seems something else is going on.
This video https://www.youtube.com/watch?v=b0sAGt-1niU&vl=en only gives one possible problem if it fails after ignition, the flame sensor.
I'm thinking maybe the blower fan motor is dead. I didn't check if it was running. Maybe that's the problem.
I would appreciate any suggestions.
 
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Old 09-12-20, 05:32 AM
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Hi, what do the LED flashing codes tell you is the problem, there must be a legend there some place.
Geo🇺🇸
 
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Old 09-12-20, 09:38 AM
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As I said, I am currently getting the normal operation code, a slow flashing amber light.
SLOW AMBER FLASH: Normal operation with call for heat.
Luxaire furnace codes error flash codes.
 
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Old 09-12-20, 10:02 AM
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If the board is showing normal.... you need to set the thermostat to heat and see what happens.

You need to make sure the flame sensor rod is actually engulfed in the burner flame.
If you think you have a blower problem..... put the stat in FAN ON..... does the blower work ?
Put the system in A/C...... does the blower work ?
 
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Old 09-12-20, 10:12 AM
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The thermostat (my dog keeps begging me to scratch her. She is pushing and rubbing at me while I type, which makes it difficult to type. It's all in good spirits, but she's interfering with my work.) .. Anyway, the thermostat only has 2 wires going to it, and it doesn't have a fan-only function, nor is there any cooling system. It's only a heating furnace. But there should be a way to run the main fan motor by itself.
I really thank you for helping me. I'm desperate to get this machine running properly. I have experience repairing lots of stuff, but I'm not a regular heating & cooling guy.
 
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Old 09-12-20, 11:09 AM
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Hi, post a pic of the control board, and the wiring Dia. if available, there should be a fan limit device there somewhere, when the temperature rises it will turn the blower on, but thats not the reason it wont fire up.
 
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Old 09-12-20, 11:16 AM
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Yes, the flame sensor is engulfed in the flame. The flame sensor is in the same position as the one in this video: https://youtu.be/JjjOVoRiMKc?t=55
 
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Old 09-12-20, 11:27 AM
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Old 09-12-20, 11:42 AM
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https://photos.app.goo.gl/G4nHyAo82SBufxDE6
https://photos.app.goo.gl/e2t2RipR24vyhH2M7
What part of the control board do you want to see?

There's no wiring diagram on the furnace. However, 4 years ago when we started living in this house, I replaced the control board. It worked fine after that until the last cold season. Then, as I said, it would blow cold air after a while of being on. I replaced the limit switch (upper right) because it was giving a 4-flash code. It got warm, and I forgot about the problem.
Now, it's cycling just like in the previous video I posted. The flame goes on for 4 seconds then it goes off.
Essentially, the furnace is behaving just like it would with a bad flame sensor, but I replaced that with a new one. There must be an answer. I'll have to test and check everything. I don't expect someone reading this to figure out what's wrong by the limited info I'm posting, but there is a best next step.
What is my next step in fixing this furnace?
 

Last edited by Nehmo Sergheyev; 09-12-20 at 12:58 PM.
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Old 09-13-20, 11:01 PM
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I just tested the pressure switch. It's normally open, and if I use my lungs as a vacuum source, it will close as indicated by a test meter. I suppose I could do further tests, like to test if the vacuum coming off the draft inducer is sufficient to close the switch, but I wanted to post now. I have a question;
During this process, the main blower motor doesn't come on. Should it? What is the sequence of actions? It seems the blower fan would be the last to come on, but I don't know. Here's the URL for the board https://climate.emerson.com/document...us-1645268.pdf
From that pdf, I get this paragraph under Operation:
NORMAL OPERATION – HEAT ON When the thermostat calls for heat the module verifies the pressure switches are open and energizes the inducer (high speed) and optional humidifier contacts. When the low pressure switch contacts close a 15 second pre-purge begins. After 15 seconds the inducer switches to low speed and the 120 VAC ignitor is energized. The ignitor warms up for 17 seconds and the gas valve is energized on low fire. Flame must be detected within 4 seconds. If flame is detected, a 45 second heat, fan on time delay begins. This allows the heat exchanger to warm up before energizing the circulator on low speed and (optional) Electronic Air Cleaner contact. When the thermostat (or module) initiates second stage the inducer is energized at high speed. This closes the second stage inducer pressure switch then energizes the second stage on the gas valve and then the high heat circulator speed

My furnace turns off after 4 seconds of the flame being on.
 
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Old 09-17-20, 12:29 PM
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According to this video https://youtu.be/Q1NlMP3rtSQ the flame sensor should be producing 3 mico amps when the meter is attached in series between the flame sensor and the control board. With the new flame sensor, I'm only getting 1 micro amp. When the flame ignites, it rapidly rises up to 1 but no more and then quickly shuts down. So, does this mean the new flame sensor is bad?
 
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Old 09-17-20, 01:06 PM
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Hi, try repositioning it a bit, also check to make sure you have good grounds.
Geo
 
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Old 09-17-20, 03:47 PM
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Using the second new flame sensor, the furnace still did not work. I also tried heating the flame sensor with a propane torch. Still, no improvement. However, measuring the output of the second new flame sensor gave the same result as the first new one, that is, only 1 microamp. Could the new ones from Arrow (a local heating & cooling supply) all be a bad batch? I'm going to try another meter.
And I'll try another flame sensor from an abandoned furnace I know of.
I did everything I could to make sure the connections were good. I re-attached the ground wire from the board, and I sanded the place where the flame sensor sits. I squeezed the female spade connector that goes to the flame sensor to get a tighter grip.
 
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Old 09-19-20, 12:21 PM
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I learned the flame sensor is just a piece of stainless steel wire, and it works simply by conducting a small current through the flame. This rectifies the current like a vacuum tube rectifier. If there is a flame, the ions in the flame conduct electricity, and the current can be measured.
In my situation, this means the flame sensor isn't defective if it shows a low current of 1 microamp, but it's possible the board isn't supplying enough voltage to the flame sensor. I measure about 16 volts between the terminal on the flame sensor and the ground. (Ground and neutral correctly measure to be the same.)
That's the question now: What should the voltage be measured at the terminal of the flame sensor?
 
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Old 09-19-20, 12:25 PM
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If you look at the top of page 2 in this pdf for the control board https://climate.emerson.com/document...us-3583550.pdf it seems to say that 1 microamp is enough to detect the flame.
"Flame Current Requirements: Minimum current to insure flame detection: 1 a DC*
Maximum current for non-detection: 0.1 a DC*
Maximum allowable leakage resistance: 100 M ohms
*Measured with a DC microammeter in the flame probe lead"
 
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Old 09-21-20, 01:53 PM
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I sent away for a new control board. I now find a video saying the voltage at the flame sensor is "usually 90 volts but varies by manufacture" https://youtu.be/JE3MzApSN7E?t=83 . I'm convinced the 16 volts I measure at my board is inadequate. The low voltage would account for the low amprage, 1 microamp, I measure through the flame sensor. It seems getting a new control board is the right move. Once I install it, I'll know for sure. Of course, there's not much else to replace.
 
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Old 10-16-20, 04:29 AM
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I got the new board, but before I installed it, I thought I'd try one more thing. I removed the metal parts surrounding the flames and cleaned them. Actually, I only removed the last (left from my POV) 2, and I could only clean the surface where the flame comes out (the part with several holes in it).
Now the furnace went through the process, but the circulation motor didn't turn on, and the limit switch killed the flame. The problem was the capacitor, which I replaced. Now, the motor works, but the limit switch still goes on after a few minutes. Then, of course, the flame goes off. Then after a few minutes of the circulation motor working with no flame, and the 4-blink (limit switch open) code showing, the furnace returns to normal, and the flame ignites.
Thus, it is now short cycling a few minutes at a time with the limit switch causing the problem.
I suspect somehow the vent tubes to the rooms got clogged with something, and this is causing the flame box to overheat. (The filter is removed, btw.) We do have animals in the attic, so it's possible. I investigate it tomorrow.
 
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Old 10-16-20, 06:03 AM
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Hi, it seems that if the LED code you are getting indicated a Hi limit switch problem, then the burner is firing enough to create a Hi temperature, I would suspect a problem with air flow or possibly a venting problem, how long has this been going on?
Geo🇺🇸
 
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Old 10-16-20, 11:27 AM
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The 1-story old house has a basement furnace and a vertical shaft that spiders out to several 6" tubes going to the rooms in the "attic". It's not actually the attic but a 2' space between the ceiling and the attic, making the tubes difficult to reach. Also, I know we have had raccoons and cats up there.
Yes, I suspect circulation problems too.
The house is much larger than we need, so I closed some of the tubes going to unused rooms. I removed the filter to improve circulation.
I'll have to figure out a way of checking the tubes for blockage. It's hard to believe an animal made an nest in there, but we'll see.
 
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Old 10-16-20, 12:59 PM
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I have now examined the vertical tube to the spider, and there is no blockage.
The circulation motor does not get excessively hot, and it seems to run properly.
The process is flame on for 2 minutes with normal code from the lamp. Then, the flame dies, and the code shows the limit circuit open for 3 minutes. It recovers to normal, and the cycle begins again with the flame on and the lamp showing normal operation for 2 minutes.
 
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Old 10-16-20, 10:35 PM
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I believe I've solved the final problem. The cooling coils were clogged with dirt. I haven't put it together yet; I wanted to post.
 
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Old 10-17-20, 01:36 PM
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If anybody is still following the thread, thank you.
I put the furnace back together sans cooling coils. Now it works for a much longer period, perhaps 15 minutes, but then again it stops. This time the code is 11 red flashes: Open Limit circuit, blower failure. This is similar to code 4, which is also an open limit switch condition without the blower remark. See page 8 of the board's installation PDF: https://climate.emerson.com/document...us-3583550.pdf
If I then turn the furnace off for a cooling period -then turn it on, it again works normally with the flame on for 15 minutes. The cycle repeats.
I suspect the motor for the circulation blower is somewhat bad. It, apparently, works for a while but then fails.
 

Last edited by Nehmo Sergheyev; 10-17-20 at 03:39 PM.
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Old 10-18-20, 02:43 PM
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Here's a question: There are 2 error code conditions that indicate an open limit switch circuit, 4-blinks and 11-blinks. The 11-blink one adds the comment "blower failure". So how does the control board know the blower is not working? IOW, how does know the difference between the 2 error conditions?
 
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