Main burner won't stay lit

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Old 09-21-08, 04:29 PM
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Main burner won't stay lit

Heating Experts,

My gas conversion steam boiler is giving me trouble and I'm running out of ideas. The call for heat works, the 30sec purge happens and the main burner lights up but won't stay lit. After 30s or so it shuts off and the cycle repeats. I think this means the flame detection fails.

Based on the following list of troubleshooting steps the user manual suggests for this condition (my results after =>), I think I need to verify the sensor position and make sure the flame covers the hot surface igniter/sensor. But I don't know how. The sensor is not visible from the eye glass / access trap of the boiler. I'm weary of sticking a mirror-on-a-stick in there due to the high temp and proximity to the flame. How do you pros do it? I've removed the sensor/igniter to insure that it gets very hot even after one 30s main burner cycle, but I suppose that is no proof that the flame covers it. The sensor itself is sorta Z shaped. Since the rods can be pivoted, I tried with the the sensor both vertical and horizontal with no luck.


Troubleshooting:

1- Check continuity of ground wire => looks good with multimeter

2- Confirm that hot and neutral 120V wires are connected to L1 & L2 => looks good

3- Check that burner flame covers igniter/sensor => don't know how

4- Check igniter/sensor or flame rods for contamination or damage. Replace, if necessary => igniter replaced ($$!), didn't help. Rods (2) look OK, but don't know how to tell nor test.

5- Check insulation on sensor leads and check sensor position => insulation looks good, how do I check the position?

6- If checks are OK, replace module => module already replaced, didn't help.

Not sure if it matters, but the conversion is likely 7-20 years old and the boiler itself is probably 40-50 years. Thanks in advance for any thoughts,

JP

-Carlin Model G3B Power Gas Conversion Burner
-Honeywell S8910U Universal Hot Surface Ignition Module
-Uni-Line (41-413) hot surface ignitor
 
  #2  
Old 09-21-08, 06:30 PM
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Main burner problem

I'm not familiar with that burner. Does it use the hot surface ignitor for flame sensing or have a separate flame rod? If a seperate flame rod, remove & clean it with a Scotch Brite pad. Do not use sandpaper or steel wool unless the steel wool has been de-oiled by soaking in alcohol or other solvent. Some pics would help.
 
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Old 09-21-08, 07:00 PM
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Hi Grady,

Yes, the ignitor is used for flame detection also. Since it's made from brittle material (carbide?) I'm not sure that cleaning via abrasion is indicated in my case, though I imagine it would if it were a separate rod. On this picture, the ignitor/sensor is model 41-413. It's connected to the burner via two long metallic rods, themselves terminated with electric connectors that link the assembly to the control module. These two rods go through a rubber grommet on the far end of the ignitor/sensor, which has the dual effect of keeping them apart and away from the walls of the burner assembly.

Also, I just discovered the Status LED on the control module. When the main burner stops, it sometimes blinks once, sometimes twice. Once would indicate failure of flame detection. Twice would indicate weak flame rectification signal, whatever that is.

There really isn't much physical adjustment that can be done on this burner, but if I pull the ignitor/rods/grommet assembly back by the thickness of the grommet (~1/4 inch), the burner seems to stay lit longer. I believe the pullback has the effect of exposing less of the ignitor to the flame. Both based on this page, which explains how flame rods work, and the module user manual, the amount of sensor exposed to the flame should be not greater than 25% of the amount of "burner" also exposed to the flame. Something about ionization and micro-amp currents...

I've been trying to think what might have changed over the past 5 years when this boiler has been working fine. Perhaps the metallic burner somehow lost mass & surface and now the ratio is off? Maybe I sprung an air leak and the flame quality is off, which could put the sensor in the non-blue section of the flame?

I'm going to go make some time measurements to verify my "pullback" findings and snap some pics.

Thanks,
JP
 
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Old 09-21-08, 07:18 PM
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Hot surface ignitor

You are correct. Don't try to clean it. Hot surface ignitors have a reputation for developing very small cracks which will not effect ignition but will cause the ignitor not to sense flame. This is why hardly anbody uses the ignitor as a sensor anymore. They all use a separate flame rod.

Here's a link to the instruction sheet which comes with the ignition module. There is troubleshooting at the end. If you follow the flow chart, you can probably find the source of the trouble.
http://customer.honeywell.com/techli...it/69-0845.pdf
 
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Old 09-21-08, 08:31 PM
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Yep, I got the instructions with me. Which is why I suspect a flame sensing issue.

Here are some pics (hopefully this is within what's acceptable for this board).

First of the whole thing, boiler and burner

Then, of the burner itself as viewed from inside the boiler. Sorry for the low res, I had to drop a video cam in there. Note how the ignitor/sensor is "fully extended" or protruding as far as it'll go, which is its normal position. The burner looks good to me, so here goes my rust theory.

Here's the same picture, but with the ignitor slightly retracted, as I was trying to explain. I'll admit, it's pretty subtle.

Now the same pair of pictures, but seen from the outside. First, the ignitor leads in their normal position:

And here when slightly pulled out.
 
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Old 09-22-08, 11:57 AM
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I think we can forget about adjusting the ignitor/sensor and all that (nonsense). The more I play with the burner, the more I realize none of this is meant to be adjusted.

So I'm now leaning toward flame "quality". I accidentally temporarily obstructed part of the air intake and the burner went out immediately. I then looked into said intake and it was somewhat dirty. I only had time for one test after cleaning it but it worked, so I'm cautiously optimistic that by cleaning the air flow, the flame quality will improve, along with the flame rectification signal.

This hypothesis would address two points. First, the intake has likely gotten dirtier over the past 5 years. Second, when I was pulling back the ignitor while chasing the red herring, I was perhaps providing more air to the system.

I'll run more tests tonight and snap a pic of the intake, which, incidentally, is adjustable.

Stay tuned,
JP
 
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Old 09-22-08, 06:38 PM
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The burner has stayed lit 2 for 2 after the air intake was cleaned. I'm cautiously optimistic. Though I can't believe that such a small tweak to the airflow would have such a large effect. Anyway, as promised, here's a shot of the intake:

 
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Old 09-22-08, 06:54 PM
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Thanks for the Pics

The pics were certainly a help. It seems the flame sensing is NOT thru the HSI as I originally thought but actually via a flame rod. The flame rod is best cleaned with a Scotch Brite pad. Check ALL of the grounds, all the way back to the electrical panel.

Flame quality certainly will cause problems & I agree that you may have found the culprit. Clean the air pathway very well including the burner's blower wheel. If you still have the manual which came with the burner, check the specs on the flame rod for clearances. I have been trying, unsuccessfully so far, to find the flame current specs for the module.
 
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Old 09-22-08, 07:55 PM
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Grady,

Are you saying the two "hooks" shown in the 3rd and 4th pictures are actually flame detection rods? If so, how is the measurement made? I would have expected to see an extra electrical wire or something, but there seems to be none. Unless the module measures current over the ground path.

JP
 
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Old 09-22-08, 08:18 PM
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Hooks

I belive them to be flame sensors. They do come out of the porcelains with the "spark plug" boots on them, do they not?
Please trace the "spark plug" wires & tell me to which terminals they connect. I suspect one will go to "sense" & the other maybe to a ground terminal.
 
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Old 09-25-08, 08:18 AM
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I went in and cleaned the blower wheel, which took some doing for it is not easily accessible. The wheel was about 3/4 clogged (yikes!) The boiler/burner has been operating well since, so it looks like air pathway was the culprit.

Now for a confession. You know those two "hooks" we theorized might be flame sensors? Well, they are not. What we see is the back of the work light I had shoved in there to be able to snap pics. Enough said about that...

Thanks a bunch for the assistance.
JP
 
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Old 09-25-08, 06:00 PM
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Red face Work Light

Oh well, they looked like they could be flame sensors.

Glad you got it up & running as it should. That blower wheel should be checked & cleaned every so often. Normally once a year is more than ample unless there's a lot of lint, pet hair, etc. in the area.
 
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Old 10-28-08, 04:55 PM
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I have a gas-powered baseboard hot water heating system. I had problems with the pilot staying lit in the boiler, and a plumber changed the thermocouple and, when that didn't do the trick, replaced the gas valve. It worked with no issues for 5 days, and then today the pilot went out. It would re-light,but then go out after about 10 minutes. After re-lighting a couple times, now the pilot actually stays on, but the burner is turning off without the set temperature being reached. I cut the entire system off with the emergency switch (as I had seen the plumber do to test the pilot), and the pilot stayed on. When I turned the system back on, I thought there was some noise coming from the exhaust/pipes leading to the chimney flue. Could a block be causing my lighting problems? And could it also be the air intake? Would a gas powered boiler have an air intake and, if so, where would it be so I can try to clean it? Any help would be greatly appreciated -- I'm completely frustrated!
 
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Old 10-28-08, 10:23 PM
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Originally Posted by BenderHeel View Post
...I thought there was some noise coming from the exhaust/pipes leading to the chimney flue...
What kind of noise ? Like squirrels gasping for breath ? Can't say it hasn't happened ! One year for some reason, I had like 4 starlings scratching around inside my boiler. (I fixed the screen on the chimney after that!)

But, yeah, a flue blockage could cause the boiler to shut down. There are safety switches in a gas boiler that do that... There's a 'rollout switch' a 'spill switch' (aka 'blocked vent switch') ... yes, your damper could be causing the problem... there is a switch in there that activates when the damper is open... that switch could be 'flaky' ...

Lotsa things could cause this ...
 
 

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