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Thermocouple Testing Procedure

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Author
Tim McElwain
Published
July 16, 2009
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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.

1. IS THE PILOT LIT?

2. IS THE PILOT CLEAN? (NOT YELLOW)

3. IS THE PILOT HITTING THE UPPER 1/2 TO 3/8 OF THE THERMOCOUPLE?

4. ARE CONNECTIONS TIGHT?

5. IS TIP DAMAGED?

6. IS THE COLD JUNCTION BEING HEATED BY THE PILOT OR MAIN BURNER FLAME?

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, 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 MVs - normal is 1 to 2 MVs. 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.

THE CLOSED CIRCUIT READING REQUIRES A SPECIAL ADAPTER THAT SCREWS INTO THE MAGNET ASSEMBLY TO ALLOW CONNECTION OF THE METER. AN ADAPTER CAN BE PURCHASED FROM ANY ROBERTSHAW DEALER THE PART NUMBER IS 10-038 THERMOCOUPLE TEST ADAPTER.

A normal set of readings

OC- 30 millivolts

CC- 15 millivolts

CC (load) -15 millivolt

DO- 1 millivolt

The best way to be able to diagnose these readings is to use MILLIVOLT CHARTS these cannot 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.

Lastly, 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.

Tim McElwain

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Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915
E-mail gastc@cox.net