Pilot Light Still Won’t Stay On after Changing Thermocouple

Thermocouple Overview

A thermocouple is an electrical generating device that consists of dissimilar metal wires fused at one end and connected to a voltage sensing device at the other end.

When the joined end is placed in a warmer source than the measuring end a small voltage is induced in the wires. The voltage generated varies by temperature but is measured in mV (millivolts).

The most common type of thermocouple for home use is the type K. It is a Nickel-Chromium / Nickel-Alumel combination and has an accuracy of 2.5oC. The voltage generated varies by temperature but is measured in mV (millivolts).

Gas water heaters and gas furnaces and fireplaces are the most common places to find a thermocouple of the type K. They are available in different lengths with the 18” or 24” being the most common.

Since these are inexpensive, it is a good idea to keep a spare on hand for the late night weekend replacement of a faulty one. This could mean the difference between being cold or toasty warm as the gas appliance will not operate without a working thermocouple.

It is a good idea to learn how to change out a thermocouple. A minimum amount of tools are required and the procedure is not beyond the abilities of a DIY person.

Torx bit screwdrivers, small adjustable wrench, nut drivers, flat blade and Phillips screwdrivers, small open end wrenches, pliers, and a soapy water solution are the tools that are needed.

The main requirement of a thermocouple change is to get the replacement into the pilot flame in the right position to receive a certain amount of heat to maintain the voltage required to keep the gas valve open.

Multiple thermocouples can be connected in a series to become a thermopile which will then generate much more voltage making for better control. Many central heat furnaces rely on thermopiles to provide the mandated safety practices.

Pilot Light

A pilot light is a very small flame that burns continuously and maintains heat on a thermocouple to keep a gas valve open. When the thermostat calls for heat and the main gas valve opens the pilot will light the main burner.

If the main burner does not light the gas valve will close and shut off the gas supply thus saving you and your house from a potential explosion and/or fire.

If you have lit the pilot light and it will not stay burning, the problem could be the thermocouple. It must be generating enough voltage to keep the gas valve open so as to have flame for heating. The position of the thermocouple in relation to the pilot light flame is critical to have it in the correct position to generate enough voltage.

If you are replacing a thermocouple, take special note of its position in the holder prior to disassembly, so you can place the new one in a similar position.

Thermocouples can and do fail and it seems like the most inopportune time such as during a blizzard on a late weekend night when service personnel are unavailable.

If you are having this problem, check the adjustment of the thermocouple in the pilot flame. The positioning of the thermocouple is critical to get it into the pilot flame so it can get enough heat to generate the voltage necessary to keep the gas valve open so the burner can get gas to operate.

Should the pilot go out, the gas valve will close and stop any escaping gas thus saving your family and home.

Another possible cause of pilot light failure is a blockage in the pilot light gas feed tube which can be cleaned out using various means.

The main gas valve should be shut off before attempting any repairs to a gas fueled appliance. Safety should always be the utmost importance.

After repairs are made, turn the gas valve on and test for any leaks using a soapy water solution by spraying or brushing it on the fittings that have been reconnected. Look for bubbles in the soap solution as an indicator of a gas leak.

Many of the K type thermocouples have a gas tight connection to the appliance gas valve which needs to be tight enough to seal the connection.

Torch tip cleaners that are available from welding supplies retailers can be used to clear debris from the small orifice in the pilot gas supply tube.

A wire snipped from a wire brush can work in an emergency situation. Take care to not distort the orifice opening as this will make the pilot not have the correct burn pattern that is necessary to keep the thermocouple heated properly.

Compressed air blown backwards through the pilot orifice can clear it. This operation usually requires removal of the pilot light assembly to be able to reach the necessary places.

Care should be taken on newer appliances that are equipped with a Piezo igniter so as not to damage the Piezo igniter or its wires.

These igniters are the ones with a push button on the main valve and are used to light the pilot. The pilot is placed in an inaccessible place to light with a match thus requiring the Piezo igniter. These units can have the pilot light lit without removing or opening any parts of the appliance, thus being a safer alternative to the older units.

One special consideration for these newer appliances is to keep the air intake vents clean as a shortage of fresh air will cause the pilot to go out. These vents should be vacuumed about every three months or more often if you have “dust bunny” problems.

On a water heater these vents are near the bottom and are a series of small holes that allow fresh air into the combustion chamber to support the flame.

They also have a small glass vial under the burner that props open the air intake shutters and it is designed to shatter and close the shutters on the air intakes if the combustion chamber of the appliance overheats, thus depriving the combustion chamber of fresh air.

Broken glass shards on the bottom of appliance under the burner is the tip off to this failure. It is almost always caused by stoppage of the fresh air vents.

This repair can be accomplished by an experienced DIY person, but is best left to the gas appliance technicians. They will know what other steps are necessary to take when this failure occurs.

Another style of control has a thermal breaker in the wiring for the thermocouple that will trip when the combustion chamber overheats and shut off the gas flow.

This thermal breaker is resettable using a push button on it. Push the button to reset the breaker and attempt to light the pilot.

You can test this breaker with a VOM (volt ohm meter). Detach the wires on the breaker and using the ohms function test it for continuity.

If no continuity, press the reset button and test again. This breaker is located in the face of the burner attachment plate and the reset button shows to the outside of the appliance.

How to Light a Pilot

To light a pilot, turn off the gas valve on the control unit. Wait about 5 minutes for the valve to reset. Turn the knob to pilot and press down on it. This will override the safety features on the pilot and allow gas to flow.

If it is a match light type, light the pilot and continue depressing the knob for at least a full minute to allow the thermocouple to warm enough to hold the gas valve open.

Slowly release the depressed knob and hope the pilot stays lit. If it does, turn the knob to run or on and the burner should light. If the pilot does not continue burning, try again and if no luck, you will need to get deeper into trouble shooting the problem.

The process is similar for the Piezo igniter pilot. Depress the knob and then press the Piezo button on the gas control valve a few times. The pilot should ignite from the sparks from the Piezo igniter. Continue as previously explained.

Most gas appliances have pilot lighting instructions printed somewhere on the appliance in easy view.

Pilot failure can be caused by shorted wires on this type of safety system. A thorough check of all related items including components and wiring is necessary when trouble shooting the pilot light failure.

The magnetic solenoid for the valve hold open can fail, though very rarely. A few gas controls are designed so the solenoid can be replaced, but on most, it is integral to the gas control valve and require a replacement of the valve assembly.

Troubleshooting Guide

Visually inspect the pilot assembly to see if the thermocouple is in correct contact with pilot flame for proper heating.

Using a VOM that has a mV scale you can test the output of the thermocouple in millivolts. Usually a millivolt reading of near 30 millivolts will be enough voltage to hold the solenoid in the open position. The K type thermocouples are rated at 30 mV.

Due to the low cost of a thermocouple, substitute a new one in place and see if the problem is solved.

If you have a thermopile, the cost is more and you really should check it for millivolt output before replacing. Thermopiles output a greater voltage and you should be aware of the required amount of millivolts when testing.

If flame is low a pilot light assembly cleaning is indicated. Removal from the unit will assist in this process.

Compressed air cam be used to blow backwards through the pilot gas feed tube and will most likely blow out any restrictive debris. A small wire clipped from a wire brush can be used to clear a clogged orifice.

Take care not to distort the orifice hole as it has a critical shape that is necessary for the correct flame pattern to heat the thermocouple.

Corrosion at the fusion joint of the dissimilar metal wires can cause low or no output from the thermocouple. Any electrical connection can have the same detrimental effect from corrosion. Visual checks for corrosion are a good idea.

A corroded or soot covered thermocouple can prevent the flame from properly warming the thermocouple preventing it from operating correctly. It can be cleaned using steel wool or fine grit sandpaper.

Another pilot light problem to consider is wind currents. Have you changed anything that could increase wind currents in or around the appliance and cause the pilot flame to blown away from the thermocouple. This will cool it enough to prevent it from generating enough mV to keep the pilot solenoid open.

Pilot lights and their circuits can operate for many years continuously providing their safety operations without any problems.

You can come to expect them to continue with their uninterrupted service and then when the most inopportune time occurs they can fail and leave you without the services of your gas appliances.

Personal Note From Author

It would be in the best interest of any DIY person to get the knowledge and tools necessary for doing pilot light service in such an emergency.

The author has kept extra thermocouples on hand for the last 50+ years just for such occasions. I have multiple gas appliances that use the type K thermocouples. This preparation has served me well over the years and kept me in the “warm” many times.

A thermocouple change and pilot flame adjustment only takes a few minutes and a minimum of tools to accomplish. A spare thermocouple of the type K is not at all expensive. If your spare is too long, you can coil the excess and use it that way.

It has been several years since I have had any pilot light problems, but I know they can strike anytime. I am prepared to take care of any pilot light issues.