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Pilot Orifice Sizes

JStar Member Posts: 2,752
I replaced a faulty gas valve today. New thermocouple. I was having trouble getting high enough mV under load. The pilot would intermittently drop out. Adjusting the thermocouple made no difference. Pilot adjustment made no difference. Gas pressure in was 5.5 " WC. It would start out at 13 mV, then the burners kick on and it drops out at 8 mV.

I wound up (very carefully) reaming out the pilot orifice a little bit. I know it's not proper practice but it worked...for now.

So I'm just wondering what you would use to check the orifice size, and how you would determine the correct size for a replacement. Is it just trial and error?


  • Tim McElwain
    Tim McElwain Member Posts: 4,481
    As for orifice sizes

    for natural gas pilot orifices run from .014 to .026 and for LP from .006 to .013. Most natural gas pilots on heating using thermocouples are .018. They can be purchased from most reputable heating supply stores. The orifice size is usually stamped on the orifice somewhere.

    I am concerned about your drop out on the new valve of 8 that is too high and says you have a faulty magnet assembly in the new gas valve. The reading should be between .75 to 6 millivolts maximum.

    What was wrong with the other gas valve that caused you to replace it? What is the make/number of the gas valve old and new, also the pilot and thermocouple

    Here is a procedure that may help you: You can also find it along with millivolt charts by clicking "resources" above then "Library" then look for "Tims Closet".



    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.

  • JStar
    JStar Member Posts: 2,752

    I replaced the original valve because the circulator pump leaked oil all over it, and caused it to short out and burn up the aquastat transformer.

    I also thought that the drop out number was too high. After I made the orifice adjustment, the mV stayed the same but without shutting the valve down.

    I don't have the model numbers right now.

    The pilot seemed very weak. It didn't fully engulf the thermocouple. That's what lead me to the orifice sizing.

    By the way, this was a Utica MGB175HD.
  • JStar
    JStar Member Posts: 2,752

    Just found this service bulletin from Utica...

    They upgraded orifice sizes from 0.014 to 0.018 in 2007. My boiler was a 1998.

    So, now my other question. Is there any harm in using different pilot sizes if you're having issues, as long as they operate correctly (proper flame and coverage)?
  • Tim McElwain
    Tim McElwain Member Posts: 4,481
    Any alterations

    or actual changes of the pilot require a "Pilot Safety Turndown Test" to be done. If you are not familiar with how to do that e-mail me at [email protected] and I will send you the procedure.

    I also recommend a high temperature Baso Gas Controls Thermocouple K16LRA (used to be Johnson Controls)

    Utica boilers are tough on thermocouples
  • JStar
    JStar Member Posts: 2,752

    The pilot adjustment made no change to the pilot flame, so a turndown test could not be completed in full. Even fully front seated, the pilot did not change. I did cycle the burners several times, both cold and at peak temperature.

    Theoretical question...Could you pinch down one side of a pilot hood to allow the flame to favor the side nearest the thermocouple? Assuming the main burners light properly...
  • Tim McElwain
    Tim McElwain Member Posts: 4,481
    A brand new gas

    valve that will not adjust pilot gas flow tells me there is something wrong with your new gas valve. Whose make and number is it?

    Do not alter a pilot in any way as you can affect not only its ability to operate correctly but it is unsafe. Put in a new pilot.
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