How do I know if my thermocouple is bad on my water heater?

If you suspect the thermocouple on your water heater may be malfunctioning, there are a few signs you can check for to determine if it needs to be replaced. The thermocouple is a crucial safety device that shuts off the gas supply to the water heater’s burner if the pilot light goes out. A faulty thermocouple that fails to do this can lead to raw gas accumulating in the combustion chamber, creating a dangerous situation.

What is a Thermocouple?

A thermocouple is a safety device found on most gas-powered water heaters. It consists of two thin metal wires made from different metals joined together at one end. This junction between the two metals produces a small voltage when heated. One wire runs to the gas control valve while the other extends into the pilot flame. As long as the pilot flame is burning and heating the thermocouple junction, a small electric current flows and signals the gas valve to remain open. If the pilot light goes out, the junction cools and the circuit is broken, causing the gas valve to automatically shut off the gas supply as a safety precaution.

How a Thermocouple Works

In more detail, the thermocouple works due to the thermoelectric effect. When two different metals are joined and the junction between them experiences a temperature change, a voltage is produced. In a thermocouple, one of the wires is usually made from iron while the other is made from nickel-aluminum or chromel. The iron generates electrons when heated while the other metal collects the electrons at the cooler end. This flow of electrons produces a small voltage, typically around 20 to 30 millivolts. This voltage holds the gas valve in the open position. If the pilot light goes out and the junction cools, the voltage stops, breaking the circuit and triggering the gas valve to pop closed. This shuts off the gas supply as a safety measure in case the main burner fails to ignite or the pilot flame is extinguished.

Thermocouple Designs

There are a few common thermocouple designs used on residential water heaters:

• Tubular – A small diameter stainless steel tube houses the thermocouple wires. This protected design resists corrosion.
• Exposed Junction – The two thermocouple wires are welded together without any protective housing. More prone to corrosion.
• Quick Connect – Uses special connectors to allow for easy replacement without cutting and splicing wires.

Signs Your Thermocouple is Malfunctioning

Here are some common signs that indicate your water heater’s thermocouple may be having issues:

1. Pilot Light Keeps Going Out

If the pilot light continues to go out, it could point to a problem with the thermocouple. As the thermocouple tells the gas valve when to open and close, a faulty thermocouple may fail to hold the valve open as it should when the pilot is lit. However, there can be other causes for a stubborn pilot light that keeps going out, such as:

• Dirty thermopile – This device also helps hold the gas valve open.
• Weak gas flow – Clogged piping or too low gas pressure.
• Drafts – Pilot flame gets blown out.
• Damaged pilot orifice – Blocked opening reduces flame.

While not the only possibility, a problematic thermocouple should be investigated if the pilot repeatedly will not stay lit.

2. Burner Fails to Light

If turning up the temperature doesn’t trigger the main burner to ignite, low voltage from a bad thermocouple could be the reason. Too little voltage may fail to tell the gas valve to open and allow gas to the burner. Safety devices prevent the main burner from operating if the pilot goes out, so if the thermocouple isn’t giving the right signal the burner won’t start.

3. Temperature Fluctuations

If hot water seems to work fine sometimes but then goes cold unexpectedly, it could point to thermocouple problems. The thermocouple may be providing intermittent or fluctuating voltage leading to unreliable gas valve operation. The signal may drop out at times and shut off the gas supply, causing the burner flames to cut out until the thermocouple reconnects and sparks the flame again. These types of temperature ups and downs are a clue of thermocouple issues.

4. Soot Buildup

Black sooty deposits on the outside of the water heater near the draft hood may stem from a faulty thermocouple. As the thermocouple is supposed to cut off the gas supply if the pilot goes out, a malfunctioning unit could allow gas to continue flowing and ignite at the main burner. This can result in impure, sooty combustion leaving behind black residue around the draft hood.

5. Gas Smell

If you detect the scent of natural gas around the water heater, there may be a problem with gas control safety mechanisms like the thermocouple. Gas should completely shut off if the pilot goes out. A damaged thermocouple could fail to signal the gas valve to close when needed, allowing gas to accumulate in the combustion chamber and leak out. An odor of gas points to a potentially hazardous situation requiring immediate investigation.

Testing a Thermocouple

If you suspect your thermocouple is malfunctioning based on the above signs, you can test it directly using a multimeter. Here is a step-by-step guide:

1. Turn off gas supply and pilot light.
2. Remove thermocouple from gas valve.
3. Ignite lighter or match and hold thermocouple tip in flame.
4. Touch multimeter leads to ends of thermocouple wires.
5. Meter should read 15-30 mV if thermocouple is good.
6. Lower reading or no reading indicates a bad thermocouple.

This direct flame test heats up the thermocouple hot junction and checks if it produces the required voltage. Just make sure all gas is off for safety before removing the thermocouple.

Multimeter Thermocouple Testing Tips

• Use a digital multimeter capable of measuring millivolts.
• Connect meter leads directly to thermocouple wire ends, not terminals.
• Extend tip into steady part of the lighter flame, the blue cone near the base.
• It can take about 30 seconds before full voltage registers.
• Compare readings to manufacturer’s specs, usually 15-30 mV range.

This method provides a definitive check to determine if the thermocouple is faulty or still functioning within specifications.

Replacing a Thermocouple

If testing confirms your thermocouple is bad, replacement is recommended. Leaving a malfunctioning thermocouple in place is a safety risk. Follow these guidelines for installation:

Materials Needed

• Replacement thermocouple with correct terminal ends to match water heater model
• Adjustable wrench and pipe wrench
• Wire cutters
• Multimeter
• Pipe sealant or Teflon tape

Thermocouple Replacement Steps

1. Turn off gas supply and pilot light – Locate shut off valve and turn gas off to the water heater. Depress or turn gas control knob to off position.
2. Disconnect and remove old thermocouple – Use wrenches to disconnect thermocouple from gas valve. Cut wires if needed for removal.
3. Install new thermocouple – Feed new thermocouple through bracket and connect to valve. Use pipe sealant on threads. Position tip in pilot flame.
4. Re-light pilot – Turn gas back on and light pilot according to water heater lighting instructions.
5. Test voltage – Check millivolts signal with multimeter. It should read between 15-30 mV.
6. Turn gas valve to “On” position – If pilot stays lit and thermocouple voltage is correct, turn gas control knob to “On” to enable water heating.

Following proper thermocouple testing and replacement procedures helps ensure continued safe operation of your gas water heater.

When to Call a Professional

While do-it-yourself thermocouple replacement is possible in many cases, it’s recommended you call a professional plumber or water heater technician if:

• You don’t feel comfortable working with gas lines.
• The pilot light won’t stay lit after multiple thermocouple replacements.
• There are other symptoms of water heater problems beyond the thermocouple.
• You need help testing the thermocouple.
• The thermocouple wires are damaged or corroded.
• You can’t identify the correct replacement thermocouple.

Professionals have specialized tools and expertise to thoroughly diagnose issues and ensure repairs are completed to manufacturer specifications. They can also provide recommendations if a full water heater replacement is needed.

Conclusion

A malfunctioning thermocouple is one of the most common reasons for pilot and burner issues on gas water heaters. Being able to recognize symptoms of a faulty thermocouple allows you to troubleshoot problems and take corrective action. Testing the thermocouple with a multimeter provides definitive evidence of whether replacement is required. Always follow safety procedures when working with gas appliances. If in doubt, call a professional for assistance to keep your water heater operating safely and efficiently.