Water Heater Thermocouple Replacement

When you have no hot water it’s a pretty good bet you need a water heater thermocouple replacement.

So if you’re asking yourself,  “Why did my water heater stop heating the water” you’re at the right place.

This is a moderately difficult repair for the DIY home owner. It’s also one I’d highly recommend you let a professional do.

There are a few ways you could harm yourself or damage your water heater further.

But since you’re still reading I’ll assume your mind is made up. So let’s get to it.

What’s a thermocouple?

Wikipedia defines a thermocouple as “an electrical device consisting of two dissimilar electrical conductors forming electrical junctions at differing temperatures.

It produces a temperature-dependent voltage as a result of How to replace a water heater thermocouplethe thermoelectric effect, and this voltage can be interpreted to measure temperature.

Wow! I’ve replaced a million of them and that scares me to death!

In layman’s terms it’s a metal rod with a small copper line attached to the under side of the water heater gas valve.

The metal rod portion is attached to the burner assembly such that it’s always in the pilot flame.

What’s it do?

The effect of heat from the pilot flame onto the tip of the thermocouple rod creates a small voltage.

This voltage runs through the thermocouple to the gas valve. That activates a switch that allows gas to the valve.

So if the water heater pilot flame ever goes out for any reason you won’t have gas filling your home.

So in short, it’s a safety sensor device.

Does my water heater thermocouple need replacing?

Knowing when to replace a thermocouple is a pretty big deal. You don’t want to go through all the time and expense of making a water heater repair only to find out it was something else.

So what are some signs that you need to replace it?

Water heater pilot light won’t light

If you’re following the lighting instructions clearly labeled on the side of the tank with no luck, then it’s probably the thermocouple.

That’s assuming the flame does initially ignite but won’t stay lit. And assuming the gas is on.

I’ve had that happen. A young couple in a new Lee’s Summit home they’re still moving into calls me after hours.

Turns out the gas was never turned on to the house. But we’ll get to that.

My water heater pilot won’t stay lit

If you’re relighting the pilot light and it stays lit for a few hours or even a few minutes, you may need a thermocouple replacement.

But it still isn’t a definite. Let’s explore some possible other causes for your water heater pilot light going out.

Other possibilities

Before you start repairing your water heater you have to ask yourself “Should I replace my water heater.”

If the tank is over 8 to 10 years old it’s a better value to just replace it. I understand that we can’t all afford that.

That’s a valid point. But depending on usage and water quality the average water heater life span is about 12 to 15 years.

So putting two or three hundred dollars into it may not be a good value if you have to replace it a couple of years later.

Let’s assume it isn’t that old or you just don’t want a water heater replacement. There are other possibilities.

A failed gas valve

The gas valve is the brain of the water heater. The gas piping goes into it and the burner assembly is attached to it. There are a few main reasons for a gas valve failure.water heater gas valve

  • Improper gas pipe installation
  • It’s really old
  • It’s defective

A gas valve replacement is a call to be made by a professional plumber. Don’t tackle this.


Every spring I get a call from a customer complaining about no hot water. The first thing I ask them is “Have you been running your attic fan.”

If they say “yes” then I’ll confirm they opened plenty of windows before turning it on.

If you don’t, air will rush down the exhaust flue and blow out the pilot light. In that case, just relight it.

And be sure you have plenty of windows open before turning on the attic fan.

Sometimes people open the house up on a windy day and it blows out the pilot.

If you have a water heater in your garage or it’s in a basement with the garage door and basement door open at the same time, the wind can blow it out.

So shut the door and relight the pilot light.

Insufficient fuel gas

Insufficient fuel gas just means it ran out of air. We plumbers like to sound smart sometimes.

But if you’ve done some remodeling recently and closed off the space around the water heater you may have starved it of air to burn the gas.

If you’ve started using the spare space around the water heater as storage you may be taking up air space.

The average water heater burns at 40,000 btu’s. It needs a minimum amount of air to not only ignite a flame but burn cleanly.

Water in the burner chamber

Water can put the pilot flame out. It sounds unlikely but happens more often than you’d think.

A ruptured tank can drip on the pilot flame. Just like a flooded basement can. On a rare occasion a tank can condensate and drip on the flame.

Some things you’ll need

Before you begin a water heater thermocouple replacement you’ll need a few things. Below you’ll find a list of tools.

  • A small adjustable wrench
  • A multi screwdriver
  • A razor knife (If you have a rubber grommet around your thermocouple)
  • A new thermocouple (Usually a 24 inch one is adequate. Home stores and hardware stores carry them)
  • Gas leak detection fluid (Childrens bubbles can be used too)

How to replace a water heater thermocouple

The newer water heaters in the last 15 years have a sealed burner chamber. So you can’t just light a pilot light with a match anymore.

That also means it’s a lot harder to replace the thermocouple. I’m only discussing these newer models because it isn’t advisable to repair a water heater older than that.

There are a lot of variations in design but we’ll be addressing the most standard design in this post.

I don’t recommend a novice try this. In fact, I’d highly recommend you have a professional plumber make this repair.

The following instructions are how I do it.

Shut the gas off

This may seem obvious but you should turn the gas off to the water heater. It’s really just a precaution.

Gas won’t come out of the gas valve unless it’s badly damaged or it’s forced through by pushing and holding in the pilot button.

You’ll smell a little residual gas from the tubing when disassembling the burner chamber, however.

Disconnect the water heater burner assembly

The next step is to remove the burner assembly from the bottom of the gas valve.

To do this, you first carefully loosen the pilot tube nut using your adjustable wrench or pliers.

Then loosen the larger main gas line and How a water heater worksthermocouple nuts. Your model water heater may have wires leading to a heat sensor switch on the outside of the burner chamber and a piezo (spark) igniter. Carefully disconnect these wires.

Remove the water heater burner assembly 

Now we have to remove the burner assembly completely. To do this you have to remove the burner plate mounting screws using your screwdriver and partially pull off the burner plate.

Be careful not to drop the screws inside the water heater. Gently pull out each tube from the base of the gas valve and pull the burner assembly out.

Replace the water heater thermocouple

To replace the water heater thermocouple grab the probe end and twist it back and forth to unsieze it and pull it off the burner assembly.

Some models have a rubber grommet plug on the burner plate that the thermocouple goes through. I usually push this grommet inward to displace it from the burner plate.

Then I carefully slice it with a razor knife from the outside to the thermocouple.

Then discard the old thermocouple and get your new one. Uncoil it. Insert the probe end of it back into the burner assembly.

If you have a rubber grommet, insert the new thermocouple into the grommet and reinstall the grommet.

It should just push back into the burner plate.

Install the water heater burner plate

Now reinstall the water heater burner plate. Be sure the burner plate is back in place as it was designed.

Some hot why is my hot water coldwater heater manufacturers have a slot that the end of the burner assembly sets in.

Be sure it’s fully inside the burner chamber. Then install the main gas supply tube, pilot tube and thermocouple hand tight only.

Be carefull not to cross thread of strip the nuts. Tighten the burner plate screws back into position with your screwdriver.

With your pliers or wrench, tighten the gas tubing and thermocouple.

Once the burner assembly is back in place, turn the water heater gas supply back on.


Relight your water heater

Following the instructions on your water heater, relight your water heater pilot light.

Test for gas leaks

With your gas leak soap bubble solution, wipe around the main gas supply and pilot tube nuts.

You’ll see bubbles. But we’re looking for growing bubbles as an indication of leaks.

If you see some, tighten a little bit more and test again.

Now that we’re done

Now that we’re done explaining how to do this, I’d highly recommend you have a professional do this.

So many things can go wrong when messing with a gas appliance. This is a relatively small repair for a plumber.

But I understand if you just don’t have the extra money. We’ve all been there.

If you do, consider having a professional plumber do it once while you watch.

I never mind a customer learning. In fact, I encourage it. But don’t gamble on this one. Your safety is more important.

And if you do decide to have a pro do it for you, call Advocate Master Plumbing.

We’re your local plumbing contractor that’s family owned and operated. There are no large corporate advertising budgets to fund.

And there are no high pressure sales people disguised as plumbing technicians. We’re the homeowners advocate.

And remember we also serve Lee’s Summit, Grain Valley, Independence &  all Jackson County Missouri.

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