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Pilot light gets very small after burner ignites??

Post in 'It's a Gas!' started by 0007, Dec 19, 2008.

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  1. 0007

    0007 Guest

    I had a single burner natural gas Desa B-Vent fireplace, Model P324 that used to work just fine until I upgraded to a dual burner and new log set. The pilot light works fine but when the burner comes on, the pilot light gets very small and the flame no longer touches the thermocouple. It runs for a few minutes and then the burner shuts off and the pilot light gets big again. I adjusted the pilot light set screw to try and increase the flame size but that did not help. The fireplace also has a whistle to it that it did not have before. What is causing this problem? Could it be the orifice size? The original single burner had a small #49 orifice where the new burner came with a #20. The directions said it was supposed to have a #26 orifice but I think they packed the #20 by accident which was for a bigger log set that I do not have. I've been searching for hours for a solution but can not find anything. Any ideas? Thanks

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  2. R&D Guy

    R&D Guy New Member

    Joined:
    Apr 25, 2007
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    375
    Sounds like you don't have enough line pressure. The larger orifice puts a larger demand on the gas line and it sounds like the line pressure is dropping when the burner comes on.

    The whistle sound is likely the flexible gas line singing due to the increased flow, this is a sign that the line size (diameter) is too small for the CFM running through the line which could be part of the problem.

    Also, I wouldn't run the unit if you believe the orifice is several sizes larger than what is spec'd. This could cause the unit to burn much hotter than it was designed and lead to a fire.

    Does your unit turn down to a low setting? If so turn it down to low and see if the pilot is any better, but seeing how the unit ran fine until the BTU's were increased, I'd say it's the increased draw that's killing your pilot.
  3. 0007

    0007 Guest

    I did try turning the gas shutoff valve way down to see if that would help out the pilot light and it does not have any affect on it.

    It looks like the gas line coming into the regulator is 1/2" and then comes out as a 3/8" to the burner. I'm not sure if this is possible, but should I replace the flexible 3/8" line with a 1/2" line? That would be a fairly easy fix if it gets rid of the whistle sound and fixes the pilot light problem! I can also try the smaller #26 orifice it was supposed to come with but I will need to go out and buy one. Do they sell these at Home Depot? I'd rather not wait 1-2 weeks for the manufacture to get around to sending me a new one over the holidays.
  4. 0007

    0007 Guest

    Another interesting thing I noted is the builder who orginally installed the fireplace used some kind of expoxy on the non-flared fittings that I have found impossible to remove. I've never heard of using anything like this. What is this stuff and how the heck to you break it free!?
  5. R&D Guy

    R&D Guy New Member

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    So I am clear, this "shut off valve", is this the same valve that has the pilot leads and other wire connected to it, or is this a separate red knob valve?
  6. 0007

    0007 Guest

    The "shut of valve" I am referring to is a keyed one that is built into the wall. There is a seperate red knob just prior to the inlet of the regulator but I did not touch that one.

    I did wonder about the reverse thread theory so I took a close look at the threads and they are normal threads.
  7. DAKSY

    DAKSY Patriot Guard Rider Staff Member

    Joined:
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    Loc:
    Averill Park, NY, on Burden Lake II...
    The whistle is a harmonic & is generally not caused by the SIZE of the orifice hole...
    If your orifice is not drilled cleanly, the burrs left on the hole can cause a harmonic when the pressurized gas flows by...
    Your local plumbing/heating supply store should have orifice drills, if you need to increase the bore,
    & if it's already too big, they have kits to repair orifices...
    If you have a dog, the harmonic's probably driving him NUTS!
    I think I'd check with the manufacturer to make sure there's not something else that needs replacement when you change to a larger burner...
    Maybe the valve regulator needs changing, too?
  8. 0007

    0007 Guest

    I installed a larger orifice today and that helped. The pilot still gets smaller, but not nearly as much. The whistle sounds has also gone away. The only problem now is after about 20 minutes, the fireplace went out. What should the thermocouple read when the burner is off and when the burner is on? I measured ~350 mV with no burner and then ~150mV when the burned comes on. Is this normal?

    I also noticed on the regulator there is a "Inlet Pressure Tap" and a "Outlet Pressure Tap". What do these do? Perhaps I need to increase the outlet pressure?
  9. DAKSY

    DAKSY Patriot Guard Rider Staff Member

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    Loc:
    Averill Park, NY, on Burden Lake II...
    Hey, 0007

    <>I installed a larger orifice today and that helped. The pilot still gets smaller, but not nearly as much. The whistle sounds has also gone away. <>

    Sounds like you're getting closer...I bet the whistle WAS caused by the drilling of the other orifice...

    <>The only problem now is after about 20 minutes, the fireplace went out. What should the thermocouple read when the burner is off and when the burner is on? I measured ~350 mV with no burner and then ~150mV when the burned comes on. Is this normal? <>


    Actually, what you're reading should be the THERMOPILE (T-P)...
    The thermocouple reads down around 25 - 30 mv...
    But your T-P readings are WAY low...On the pilot I like to shoot for 550 mv - IF I can get em...600mv MAX...If you get too much higher, the T-Ps (I know they're rated for 750mv), tend to fail early...
    If you're at 350 & pull about 200 off to run the burner magnets, you're near the bottom...they'll generally close down at 100 - 125 mv...
    Replace the T-Pile...

    <>I also noticed on the regulator there is a "Inlet Pressure Tap" and a "Outlet Pressure Tap". What do these do? Perhaps I need to increase the outlet pressure?<>

    You're not gonna increase the pressure thru the ports. They're there to verify you have the correct line pressure coming in & going thru the regulator on the valve...You'll only increase pressure by putting in larger gas lines (NG) or by jacking up the pressure regulator at the tank (LP)...

    Speaking of which, how is your unit fed? If you're on NG & you have a 1/2 inch line in excess of 30 feet, you're probably not getting enough VOLUME of gas to the unit...
  10. 0007

    0007 Guest

    I just conducted an experiement which brings up an interesting question. I disconnected the thermopile from the TP and TH/TP screws on the regulator and connected a AAA battery instead which was providing about 1000mV. When I started the fireplace, my reading droped to 150mV which is the same as when the thermopile was connected. After letting it run for the last 20 minutes with the AAA battery, I still measured 150mV but when I measured the thermopile (which is no longer connected to anything) it was reading 5000mV. Is there some sort of failsafe that shuts the gas off if the thermopile gets to hot??
  11. 0007

    0007 Guest

    DASKY - I'm running 1/2" line and it's probably in excess of 30 feet from the source. It's natural gas.
  12. R&D Guy

    R&D Guy New Member

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    The inlet and outlet taps are for measuring pressure with a slack tube and a low pressure manometer. If you unscrew them gas will come out so I'd leave them alone. When you say you installed a larger orifice, do you mean in the pilot or the main burner orifice? Sounds like your orifice was already several sizes too large, again you are risking a house fire among other things using the wrong orifice size.

    The gas shut off valve on the wall needs to be in the wide open position, same with the red knob'd shut off valve. This is your best chance of getting enough pressure at the valve. Think of it this way, you are now running 2 sprinklers off the same garden hose, in order to make sure they have enough water to work you need to follow the garden hose back where it connects to the house and turn the water on full blast. Make sense?
  13. R&D Guy

    R&D Guy New Member

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    That's just because it wasn't connected to anything, there was no draw.
  14. 0007

    0007 Guest

    I'm referring to the main burner orifice which according to the directions should be a #26. It shipped with a #20 for some reason. I was able to get a #31 and a #49 to test. The #49 was very weak, hardly any flame. The #31 worked pretty good and eliminated the whistle that I had with the #20. The #31 should be safe since it is smaller than the #26.


    Gotcha!

    My experiment ended up with the burner shutting itself off with the AAA battery connected after 25 minutes. Pretty much the same result as when the thermopile was connected. What in the world causes this???? This is driving me crazy! :bug:
  15. 0007

    0007 Guest

    Even after my battery experiment you think it's a bad thermopile? I just ran up to Home Depot to get a new thermopile but they do not carry them. I will have to wait until tomorrow to find someone else that carries them.
  16. R&D Guy

    R&D Guy New Member

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    Some units have a limit switch which will turn the unit off it the unit gets too hot. That's likely why your unit turn off right around 25 minutes after starting. Glad to hear you were referring to the drill # as larger, it's actually the opposite - larger the number, the smaller the orifice, but thanks for clearing that up. :)
  17. 0007

    0007 Guest

    Hmm...where is the limit switch located? Is there something I can monitor to see if the limit switch is getting triggered? I do have a digital probe I can use to monitor temps if need be.
  18. R&D Guy

    R&D Guy New Member

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    I just looked up your manual online and it does have a limit switch. I only skimmed the manual, but look on page 26 and see item 34. It's a limit switch. Also look at page 31 and note the wiring diagram. As with every B-vent there is a vent safety shut-off. Take a look at that diagram, could be they are referring to the same limit switch shown on page 26, but I haven't taken the time to figure that out. I'd take a look at that wiring diagram and find that vent safety shut off wiring. If you have a multi-meter you can do a continuity check after the unit shuts off to see if this is whats turning the unit off.

    Now if you choose to bypass it to investigate you need to realize that like using the AAA battery, you are bypassing a HUGE safety feature built into the unit to keep this unit from killing you, your family and or burning your home down. The whole point of the vent safety shut off is to insure the exhaust does not back draft into your home filling it with poison gas.

    A link to your manual:
    http://12.153.20.74/manuals/108660-01F.PDF
  19. 0007

    0007 Guest

    Thanks, I will see if I can track that switch down. All my test are closely monitored and for sure will not permanently bypass anything! You know what temperature that switch gets triggered at? I'm monitoring temp right now up by the vent to see where it's at when it shuts down.
  20. R&D Guy

    R&D Guy New Member

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    I don't know the temp for this specific unit, although if you call Desa's Tech Support they should be able to tell you. Either way a continuity check of the system will at least tell you if this is what's turning your unit off.
  21. DAKSY

    DAKSY Patriot Guard Rider Staff Member

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    Loc:
    Averill Park, NY, on Burden Lake II...
    That could be the problem. Rule of thumb is to have 3/4 inch pipe for anything over 30'. Otherwise you're just not gonna get the volume.
    If you wanna check, see if you can find a manometer, & attach it to the inlet pressure port, after you loosen the screw...
    Should be about 7" W.C. coming in. If it's not, you answered the problem..
    If it IS at or above 7" W.C., turn the regulator to high & check the manifold (outlet) side to see if you're getting 3.5" W.C.
    If You're not, then the regulator is shot.
    As far as the limil switch temperature, I've seen them open as low as 275*F...& as high as 450*F
    You can test the switch - If you can find it - with a continuity tester...Set it to beep & jump the spade connectors... It will beep if the switch is closed (normal)...put a flame - a lighter will do - under the metal disc & hold it til it opens...
  22. 0007

    0007 Guest

    Many thanks R & D Guy and DAKSY! Problem is solved. It was the limit switch triggering. I tracked it down and monitored continuity. I guess that #31 orifice is still too big causing the fireplace to get too hot. I used the shut off valve to limit the gas to the burner which gives me a smaller flame but it's been running for over an hour now with no problems. I will check around to see if I can get maybe a #40 orifice, or something close, so I can leave the shut off valve all the way open. I figure #31 runs too hot and #49 doesn't really have any flames so something in the middle would probably be just right. I'm not sure where they came up with #26! That must be for people who want a raging fire for just 10 minutes. I think I will also replace the thermopile because it seems a little weak. What was supposed to be a simple unscrew the old burner and screw a new burner in has turned into a several day project. I know more than I ever thought I would about gas fireplaces now! Thanks again for your help troubleshooting! :)
  23. R&D Guy

    R&D Guy New Member

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    Well good. I'm sure DAKSY will agree, finding the cause is often the hardest part of solving the problem. Does your unit have a fan? If so running the fan will likely mean that your unit will stay on much longer.

    I'd call the mfg and ask their tech department what can be done about this. It's hard to image that they are selling a burner option that works this poorly, but could be they had a batch that was assembled incorrectly and they have a fix. Could be that you need to open your exhaust restrictor more.
  24. 0007

    0007 Guest

    I do not have a fan although I have the option to add one.

    I attached a pic of my final project. I built a fireplace surround out of 8/4 African Padauk, installed the Blue Pearl granite tile and installed the new burner and log set (which was not so easy as you all know).

    [​IMG]
  25. stoveguy13

    stoveguy13 Minister of Fire

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    are you sure your fireplace is large enough for the log set you bought if it is to large it cloud over heat with out air flow around it you may want to check with the maker of the log to to make sure it will work ok with a smaller orifice you could cause your self some trouble over time just by guessing what size to down size to.
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