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Utica boiler Thermocouple problems

Wormhole Member Posts: 3
Hey guys I'm new to this site so I apologize in advance if my topic has already been posted. I have a 105,000 BTU Utica gas-fired steam boiler that was installed 6 years ago tomorrow. Heres my problem, I fired up my boiler for the first time earlier this month, and noticed that the pilot flame was large and a light orange color. I don't remember the pilot flame being that noticeable. My friend, who has an hvac company with his father said that it wouldn't be a problem but that the thermocouple would need to be changed sooner rather than later. Another friend of mine whose in heating said that the only problem would be that it would use more gas as its feeding a larger pilot flame But later on in the night I noticed that the boiler was cycling on and off in a matter of about a minute. The low water cutoff light would start blinking, as if there was no water, and then reset, then the boiler would start up as if to heat the house but only for a few seconds, then it would click off again, And then repeat. I called a local hvac company for a troubleshoot house call, and they also noticed the intensity of the pilot flame and the only thing they said was that the gas valve needed to be replaced. My question to anyone that can help is, could a faulty thermocouple cause the symptoms I described? If not, what are the signs of a faulty thermocouple? Is it safe to have such an intense pilot flame?

Thanks for your help.


  • Unknown
    edited December 2010
    Thermocouple woes,

    This is a fantastic procedure for checking thermocouples.

    Please note, this document is courtesy of Tim McElwain!

    Sorry,,, it wont go!! AARRGG.

    I thought I could post a pdf from my files??
  • Ironman
    Ironman Member Posts: 6,869
    Sounds Like...

    Your boiler is low on water. Does it have a manual or automatic fill valve? The thermocouple won't cause what you've described, but you may have a water leak. An orange flame is caused by rust particles mixing with the flame. A yellow flame indicates too much gas / not enough air.

    You should post this on the main wall or strictly steam section, you'll get more traffic there.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    Wormhole for what it is

    worth for perhaps future reference 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.


  • Wormhole
    Wormhole Member Posts: 3
    Hey Guys

    Thanks for all the info.

    @ironman: It has a manual fill valve, but at the time of the problem, (cycling on and off), the water in the glass tube was at the level that it was supposed to be. Also, to correct my self, the pilot flame is yellow not orange, I took a look at it again last night and it is yellow.

    @Tim M: In the "Thermocouple testing procedure" it mentions that a depressurization environment could cause thermocouple failure, what qualifies as a depressurization environment? Also When my friend did the tune-up/repair, he had a manual air blower, which you squeeze to get a burst of air; I remember he placed the tip of the blower to the tip of the thermocouple and blew into it as if to clean it. Could that have caused damage to the tip resulting in a yellow flame?

    @everyone: Is the K16RA thermocouple universal, and do you recommend just changing the thermocouple as a place to start?


    PS I apologize for the corny nickname; I was expecting my first name to show up.
  • Ironman
    Ironman Member Posts: 6,869
    Boiler Surging

    It sounds like your boiler is "surging". Several things can cause this, dirty boiler water or high pH level are the most common. The thermocouple has nothing to do with the boiler short cycling on the low water cut off. You need to call a steam pro to check it out. The average HVAC tech will be clueless.

    Check the "Find a Contractor" tab above or post your city/state and maybe someone on here can give you a recommendation.

    Also, I'd recommend re-posting this on the steam section. Your main problem is a steam problem.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Wormhole
    Wormhole Member Posts: 3
    Makes sense

    The water in the glass tube is clear when the boiler is off, but when its starts up and fully gets going the water literally turns black. Not knowing much about boilers and maintenance, I always thought that this was normal. is this a major problem? also, is there anything I could do about it for the time being in order to get the boiler running without any interruptions?
  • Ironman
    Ironman Member Posts: 6,869

    The boiler needs to be skimmed. Again, I'd recommend calling a steam pro.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
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