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pilot light problem

Del Member Posts: 52
Hi everyone-was on a call earlier today where the pilot would not stay lit. Unit was an older Utica boiler with the pilot burner in the center of 2 burners.  The pilot burner has a left and right outlet...the right side hits the thermocouple, and the left hits the main burner. Seems to be a bad design, as it is hard to get it to hit the thermocouple due to the flame wanting to lift.  After testing the thermocouple and cleaning the orifice, I replaced the gas valve (honeywell 24V) and everything seemed good to go.  Tested ignition 3 or 4 times, then let the boiler heat up to 180F.  Right when the main flame went out, the pilot was still lit, but it got very small to the point where it would not hold the valve open and the pilot went out.  After cooling down for a few minutes, I lit the pilot again and it was a good steady flame, enough to hold the valve open. I Tried to adjust the pilot to get a bit more flame, but was unable to get the flame to grow much.  I then tested the manifold in/out pressure, and everything was within tolerance (3.5", 8.5").  It would appear to me that the gas valve I put in is defective, as it is not letting the pilot gas through fast enough when it closes the main valve.  My other thought was that the pilot burner might be constricting when it heats up, then opening as it cools off, but I doubt it.  The plan for tomorrow is to try another pilot burner quick, and then try another gas valve.  Anyone have any other ideas?  Thanks!



  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    Watch the incoming pressure on burner shut down.

    Could be that you have a defective pressure regulator upstream. The pilot pressure is unregulated. In other words, if you have 6" at the inlet to the gas valve, your pilot pressure is 6".

    I have seen zero defective new gas valves in my 36 years... But have seen MANY defective pressure regulators, on both ends of the scale.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Del
    Del Member Posts: 52
    good idea

     It seemed to me that the pressure was having a hard time coming back up after the burner ran for a while, and only after the burner ran for a while (over 10 minutes). Once I let it sit (build up pressure) it worked fine.  Also, I could not raise the pressure of the pilot light, and the orifice was clean.  I usually see problems like that in the morning after a cold night.....the gas meters freeze up.  Is that something the utility would deal with?  I'm near Albany NY, National Grid territory. 
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    Confirm the problem first...

    THEN call the utility and have them provide an emergency service call.

    Make certain that someone hasn't partially closed off a gas cock at the meter or in between the meter and the appliance.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    edited January 2011
    Do you live in a high gas ppressure area?

    You can tell because in addition to your meter you will have regulator near the meter that reduces pounds pressure to 6" W.C.

    If you are in a low pressure area there will be no regulator at the meter. Low pressure areas are more susceptible to having pressure problems when it is cold.

    Utica boilers are known to run at rather high temps in and around the pilot\/thermocouple and cause pilot problems.

    You need to have someone go through a thermocouple testing procedure which I will post here for you.
  • Tim McElwain
    Tim McElwain Member Posts: 4,588
    Here is the procedure



    I am often asked about troubleshooting a thermocouple on gas systems. This will be a permanent reference that will give a step-by-step procedure.


    A thermocouple is a device used to satisfy pilot safety on many 24-volt gas systems. The thermocouple is a device made up of two dissimilar metals. They are joined together at the tip (Hot Junction). When heat is applied to that hot junction a small millivoltage is created. This develops because of temperature difference between the hot junction and what is called the cold junction. The flame has to envelop the upper 1/2" to 3/8" of the thermocouple and the tip should glow a "dull red". If the flame is adjusted to a sharp flame it will glow "cherry red" this will cause the tip to be welded and eventually the thermocouple will fail. The flame should be adjusted to a soft blue flame, not roaring or lifting. The normal millivolt output is 25 to 35 millivolts, on some you may even get up to 35.


    The other part of this safety pilot system is the electromagnet (power unit). It is if you will the LOAD and we can say the thermocouple is the SOURCE. The electromagnet is made up of a coil of wire and "U" shaped iron core. When the thermocouple is heated and the millivolts generated the coil will be energized and create a magnetic field. The magnetic field will cause the "U" shaped iron core to be magnetized; it in turn will hold open a seat allowing gas to pass through.


    When this system malfunctions it typically causes the pilot to go out and the gas will not flow. The first thing that should be done when arriving at a pilot outage situation is to do some visual checks.









    Once those things are addressed it is a good idea to take some millivolt readings. It should also be mentioned that many times it is the policy of some to replace the thermocouple on a call and clean the pilot. It is not a bad thing to do, however it is statistically about 85% of the time it is the thermocouple giving the problem. It is the other 15% of the time that taking readings can solve other problems.


    You need a multimeter with a DC volt scale, as the millivolts generated are DC volts. There are four readings we are going to take they are


    OPEN CIRCUIT - this is taken with the thermocouple disconnected and the meter leads attached to the outside of the thermocouple and the other meter lead attached to the tip of thermocouple. The pilot-on-off knob will have to be held manually to take this reading. This measures the output of T'couple the readings must be above 17 to 18 millivolts.


     * CLOSED CIRCUIT - This measures the millivolts used by the coil in the electromagnet. A rule-of-thumb is this reading should be roughly half of the open circuit. It is taken using an adapter screwed into the magnet and the thermocouple screwed into the adapter.


    CLOSED CIRCUIT LOAD - This reading is taken the same as the previous reading except the burner is now on. With a proper flame this reading should be about the same as the previous reading. With a lifting main burner flame or excessive drafts or chimney pull, this reading may reduce from previous reading (flame being pulled away from the thermocouple). With the cold junction being heated this reading may increase. If the "cold junction" is heated excessively it will break down.


    DROP OUT - This is the final reading. It requires the pilot to be blown out. It measures the ability of the magnet to hold under reduced MV input. A good unit should drop out below 6 MV's - normal is 1 to 2 MV's. The allowable "drop out" time is 180 seconds yes three minutes. It is more likely to be a minute and half to two minutes. There will be an audible "click" when the magnet shuts down.




    A normal set of readings


    OC- 30 millivolts

    CC- 15 millivolts

    CC(load) -15 millivolts

    DO- 1 millivolt


    The best way to be able to diagnose these readings is to use MILLIVOLT CHARTS these can not be displayed here but I can provide them if you e-mail me.


    Thermocouples from different manufacturers vary as to their dependability. The only thermocouples I recommend are made by Johnson Controls. The K15 and K16 series are the best. If you are having durability problems then use the K16RA, which is a nickel-plated high ambient or corrosive environment thermocouple. The Husky (K16) will fit most applications and for those that it does not the Slim Jim (K15) will fit.


    To repeat the adjustment of the pilot flame to envelope the upper 1/2 to 3/8 of the thermocouple is important, the flame should be a soft blue flame not roaring which will cause the tip to glow a "dull red" versus "cherry red".


    The combustion condition (excessive temperatures) in the chamber is also an issue and this will require a combustion test and draft measurement to insure that excessive temperatures are not being applied to the pilot. In some cases on water heaters it may be necessary to alter the pilot adaptation to get better quality performance. This however should not be done unless you have had proper training.


    The possibility of the equipment operating in a depressurization environment will certainly lead to thermocouple failure. In addition if the equipment is flued together with a "fan assisted" furnace or boiler this can lead to problems. There are solutions to this also but training is required.


    The thing that I find is often a problem is the environment in which the equipment is operating. Many times corrosive chemicals and airborne contaminants are being drawn into the air gas mix and a chemical reaction takes place. This again requires attendance at a training session by a professional combustion person to help you to see the various affects this will have.


    Last of all the failure to put all the doors and covers back in place on equipment. The failure to do this will cause an alteration in combustion air and the flame stability is affected.


    The design of some equipment is also a problem. When there is high demand for heat (very cold weather) the temperatures that are created in the chamber have an adverse affect on the pilot and thermocouple system. The addition of the K16RA thermocouple can offer some assistance toward extending the life of the thermocouple in this situation.


    Insufficient air for combustion and dryers operating in close proximity to equipment also lead to problems.


    Last of all and this is not directed at any one in particular but just plain lack of service personnel and installers knowing what they are doing.


    My book "Circuitry and Troubleshooting" Volume II addresses many of the things in question here.

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