How to Replace a Water Heater's Thermocouple

Replace your water heater's thermocouple the right way with this step-by-step guide.

Have you noticed that the pilot light on your gas-powered tank water heater won’t stay lit? Before you run out the door to look for a new water heater, we’re here to help. It might be due to a bad thermocouple.

With the right knowledge, water heater thermocouple replacement is an easy job. In some instances, cleaning the thermocouple could be enough.

What Is a Water Heater Thermocouple?

A thermocouple is what controls gas flow within a gas-powered water heater. It’s also referred to as flame sensor, but this is mostly in new water heaters.

The thermocouple is a component of the pilot burner. It works with the gas control valve, which is on the exterior of the unit. Essentially, it converts heat from a flame to small electrical currents.

This allows the thermocouple to work similarly to a switch, controlling the gas flow from the gas valve. A thermocouple is an essential safety feature of a gas-powered water heater. If it doesn’t sense a flame, it will stop the gas flow to the burner.

A thermocouple consists of two parts. The first is a small probe, about one inch long and ¼ inch in diameter, positioned directly above the pilot orifice. Whenever the pilot light is on, the probe is heated by the flame.

The other part is a copper tube. This is connected to the front or side of the gas valve with a small nut.

Thermoelectric Effect

The thermocouple we know today is based on the Seebeck effect, which was named after a German physicist who discovered it in 1821 (1).

The effect works with any two types of metal. However, the metals most commonly used in water heater thermocouples are stainless steel and copel — an alloy of copper and nickel.

Electronic Ignition vs Standing Pilot

There are two main ignition systems available: standing pilot and electronic ignition. How you should replace the thermocouple depends on the type of system your unit has.

If you have a standing pilot system, all you have is the pilot and the thermocouple with the burner assembly. Electronic ignitions include the same parts but they also have an electronic igniter.

An easy way to check whether you have a standing or electronic system is to look to see if there’s a pilot flame. Standing pilot units always have a small flame burning. Electronic ignition units rely on the thermostat to decide when the burner should light.

How Do You Know If You Have a Bad Thermocouple?

When you turn on a gas-powered water heater, you hold the gas control knob for about 20 to 30 seconds. This gives the thermocouple time to heat up. If the flame first lights, but then goes out once you release the knob, the thermocouple is bad.

If the pilot light is burning but is weak and orange in color, there could also be a problem. There might be an obstruction in the pilot tube, preventing the pilot light from heating up the thermocouple.

If the pilot won’t light at all, this might also be due to an obstruction in the pilot light tube. In these instances, the problem should be solved after removing the obstruction.

How to Test the Thermocouple

If you’re still unsure whether or not you need to replace the thermocouple, we suggest you test it. Since the thermocouple creates small electrical currents, you can use a multimeter to test if it works.

Start by unscrewing the thermocouple from the gas valve, then start the pilot — you may need a friend to hold the gas control knob in.

Set your multimeter to millivolts and wait about a minute until the probe is hot. When you’re sure it’s hot, place one lead on the thermocouple shaft and one lead on the connector.

A working thermocouple will generally get a reading between 25 and 35 millivolts (2). However, if you get a result of 25 millivolts or less, the thermocouple has malfunctioned and needs to be replaced.

How to Replace a Water Heater Thermocouple

Once you have determined that the thermocouple is faulty, it’s time to replace it. The replacement method varies slightly for each system, but we’ll explain the differences.

1. Removing the Cover Panel

Before starting, it’s important to turn off the heater’s gas control valve. You do this the same way whether you have a standing pilot or electronic ignition.

You now need to close the shut-off valve on the main gas supply pipe. If you have a standing pilot, you can then begin to take out the cover panel to reveal the burner assembly.

If you have an electronic ignition, you’ll have to remove the burner assembly manifold cover panel. Start by removing the fasteners that secure the manifold top plate.

Determined by the manufacturer, these could be screws, nuts or even specialized fasteners.

Next, you’ll have to take out the burner reservoir tube. Then gently push down to get the pilot tube, manifold, and thermocouple circuitry out.

Be Gentle

When removing the manifold and burner parts from the burner combustion chamber you could damage internal components. Check around the base if it doesn’t loosen easily — there could be more screws.

2. Disconnect the Control Valve

Use an open-end or adjustable wrench to release the thermocouple connection from the control valve. You also need to disconnect the pilot supply tube and main burner supply tube. If you have an electronic ignition, the wire to the Piezo igniter also needs to be removed.

3. Removing the Thermocouple

Once you have the burner assembly out, slowly but firmly remove the faulty thermocouple from the mounting bracket that it’s on. Take care not to damage any parts.

To completely remove the thermocouple from an electronic ignition system, you’ll have to pull it through the grommet seal.

4. Install the New Thermocouple

Now, it’s time to install your replacement thermocouple. This varies depending on the type of system you have, so let’s look more closely:

For Electronic Ignition Systems

Insert the new thermocouple through the grommet seal and the opening of the assembly manifold cover.

For both electronic ignition and standing pilot systems, you’ll have to place the thermocouple inside the mounting bracket. Push it gently until it’s fully in position or clicks, indicating that it’s in place.

Place the thermocouple so the pilot flame covers about half an inch of the thermocouple tip.

The burner assembly has to be reinstalled in the burner combustion chamber. Make sure it’s properly in place.

An electronic ignition system also requires a gasket around the manifold top plate. If it’s still in good shape you can reuse the same one. If it looks damaged or old, replace it with a new one.

5. Reconnect Gas Control

Start by reconnecting the pilot and burner supply tubes, as well as the thermocouple, to the gas control valve. Make sure the thermocouple is only a quarter turn more than hand-tight. If you have an electronic ignition, the Piezo igniter needs to be reconnected now too.

Next, open the gas shut-off valve, then switch the gas control valve to “ON”. If you have a standing pilot system, you’ll have to light the pilot before turning the gas control valve on.

Set your desired temperature at the thermostat.

6. Check for Gas Leakage

Whenever you’re dealing with a gas-powered water heater, it’s crucial to make sure there are no leaks. The best way to check is by using a water and soap mixture.

Cover all the fittings with the soapy mixture and look out for any air bubbles. If you do notice leak-bubbles, re-check and tighten all fittings. Check again until you’re sure there’s no leak.

How Much Is a Thermocouple for a Water Heater?

A thermocouple is an inexpensive but important part of a gas-powered water heater. If you choose to have a plumber do the job, it could cost up to $150.

However, if you choose to follow our guide and do-it-yourself, you’ll only have to pay for the replacement thermocouple — generally around $20.

You can buy them at your local hardware store, plumbing supply store, or home improvement store.

We highly recommend you bring your old thermocouple to the shop. It’s important to get an exact replacement, and that can be difficult without the actual item on hand.

Keep It Lit

If you’re having trouble keeping the pilot light lit, or getting it to light, it could just be a blockage that’s easily resolved. If not, then replacing the thermocouple could be what’s needed.

Have you attempted to replace an old thermocouple? Share your tips or comments below — we love hearing back from you.

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About the Author

Peter Gray

Peter has been a homeowner for 35+ years and has always done his own repair and improvement tasks. As a retired plumber, Peter now spends his time teaching others how they can fix leaks, replace faucets, and make home improvements on a budget.

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