Furnace Pilot Won’t Stay Lit After Replacing the Thermocouple

You’ve had issues with your furnace for days now. However, you thought you had identified a bad thermocouple problem when you discovered that the furnace works when you bypass the old thermocouple. So, you purchased a new thermocouple, uninstalled the old one, and installed the new one in its place.

Surprisingly, the pilot flame won’t stay lit after replacing the old thermocouple. It comes on and goes out almost immediately or refuses to light at all. What’s the problem and what do you need to do?

Let’s begin with a short answer.

Why Won’t the Pilot Light Stay Lit After Replacing the Thermocouple?

The most likely reason why your furnace pilot light won’t stay lit after replacing the thermocouple is that you diagnosed the wrong problem. You likely thought you had a bad thermocouple when indeed the issue lay somewhere else, such as in the gas valve or pilot orifice.

Alternatively, the thermocouple was bad too. However, it wasn’t the only thing wrong with your furnace. Maybe your pilot orifice was also blocked. For instance, maybe the carbon deposits that affected the thermocouple also blocked the pilot’s gas inlet pipe.

Finally, you also need to consider that you may have installed the new thermocouple incorrectly. The pilot light won’t work if the thermocouple is installed wrongly. Let’s get to the details so you can understand thermocouples better, learn what may go wrong, how to replace a thermocouple, and what to do if the pilot light won’t stay lit after replacing the thermocouple.

What’s a Pilot Thermocouple?

The pilot light thermocouple is an electrical device that generates a small voltage when subjected to temperature changes. Standard thermocouples comprise two dissimilar metals or wires welded together and forming a measurement junction at the sensor tip. Typical metals used are copper, nickel, and iron.

For instance, Type E thermocouples, the most common on residential furnaces, comprise a nickel-chromium positive alloy leg and a nickel-aluminum allow negative leg. When the junction experiences a temperature change, a voltage is created.  

How Does a Pilot Thermocouple Work?

The thermocouple in a furnace pilot performs a safety role. It is installed such that it’s sensitive (the tip is equipped with a sensor) is dipped in the pilot flame when the pilot is lit. This way, the thermocouple helps the furnace keep track of the pilot flame and act appropriately if the gas goes out accidentally.  

How does it do this? It’s simple. One of the thermocouple’s legs is connected to an electromagnetic gas valve that sits across the pilot gas line. When the pilot flame is lit, the bimetallic thermocouple expands, pulling the pilot gas valve out of the way and allowing gas to continue flowing to the pilot light.

However, the bimetallic strip cools down as soon as the pilot flame goes out, releasing the gas valve and closing the pilot gas line.

What Causes a Thermocouple to Go Out?

The thermocouple can go bad for many reasons. However, the three main reasons are metal fatigue, oxidation, and thermal shunting.

  • Metal fatigue

Yes, metals experience fatigue. So, like any metal part, the thermocouple is guaranteed to wear out naturally at some point. For instance, temperature changes cause regular expansion and contraction in the metal, causing it to weaken with time.

This can cause the thermocouple to break after some time. Or it may start to give unusual readings.

  • Oxidation

The presence of oxygen around the thermocouple (which happens naturally) can wear down the part, causing the part to become thinner and more brittle. It can also affect electric conduction. For instance, green rot is a well-known chemical reaction in chromium metals.

 It involves the formation of an oxide layer on the furnace of chromium. Green rot can cause traditionally non-magnetic nickel-chromium thermocouple legs to become magnetic, compromising the thermoelectric voltage.

  • Thermal shunting

Finally, thermal shunting, also known as insulation resistance, refers to the property of insulating materials in thermocouples to lose electrical resistance exponentially with increased temperatures. For instance, insulation resistance at 1000°F or higher can cause errors in thermocouple thermometry, potentially resulting in a second junction in the thermocouple. Virtual second junctions typically cause volatile, inaccurate readings.

Of course, the thermocouple can also break from physical damage and suffer dangerous levels of dirt buildup that may also impact its effectiveness.

Furnace Pilot Won’t Stay Lit After Replacing the Thermocouple

The first thing that comes to mind when your furnace’s pilot light thermocouple is dead, damaged, or otherwise compromised is to replace it. But what if the furnace pilot won’t stay lit even after replacing the thermocouple? The following are the most likely reasons and what you can do about each.

  1. Filthy/blocked pilot orifice 

You should first assume that your installation is correct, but something else prevents the pilot light from staying lit. We recommend starting with a quick inspection to determine if the furnace, especially the pilot light area, needs cleaning. A dirty pilot burner or blocked orifice means the gas necessary to keep the pilot flame lit cannot come out. Thus, the pilot light cannot stay lit even if you replace the thermocouple.

Solution: Fortunately, it’s easy to clean the pilot orifice. Remove the pilot tube from the burner assembly and gently rub it with a piece of soft sandpaper. Alternatively, use compressed air to blow out the dust.

Then use a thin sharp pin to scrub out the dirt buildup inside the orifice and again blow off the dirt compressed air.

2. Faulty/stuck gas valve

We mentioned that the pilot light depends on an electromagnetic valve that opens or closes depending on whether the pilot light is lit or off. Although the expanding and contracting bimetallic thermocouple controls this valve, it can become unresponsive for various reasons. For instance, the valve can become stuck in the closed position due to dirt buildup, with carbon residue a common culprit. The valve that’s stuck closed means the pilot light cannot stay lit.

Solution: You can either clean or, if necessary, change the pilot gas valve. To do so, turn off the gas supply and use a screwdriver to remove the valve. Then clean it and the surrounding area and replace it.

If the pilot light still won’t stay lit, remove the old valve and install a new one, making sure it’s tightly screwed.

3. Draft issues

Another common challenge is strong drafts. Remember that air constantly passes through the furnace at speed. For instance, supply air must move at speed to reach every corner of your room. Otherwise, even heating becomes a pipedream.

Furnaces employ a draft hood that sits above the uppermost part of the gas furnace to help control the draft process while protecting heater operations from updrafts and downdrafts. But the draft hood may get out of position too.

Solution: Positioning the draft hood correctly is often enough to solve draft problems within the furnace. However, if the problem persists, find out if the furnace has other openings that may allow airflow circulation into the furnace and seal them appropriately.

4. Defective thermocouple 

If the three tips above don’t work, it’s time to consider that perhaps there was a problem when replacing the old thermocouple. First, ask yourself whether you used a suitable thermocouple. Thermocouples come in many types, from Type B to Type T. Did you use the right one?

On the same note, ask yourself whether you might have installed a broken thermocouple. For example, did you accidentally damage the sensitive sensor-equipped junction tip?

Solution: Refer to your manual and reach out to your HVAC technician to confirm that you installed the right thermocouple type. If so, test the device to make sure it’s working.

How to Test a Thermocouple 

Get a multimeter, set it to volts, and ensure it reads zero. Then place the tip of the installed thermocouple near a burning flame (a lighter is more than sufficient) and wait till it’s hot. From there, place one lead of the multimeter on the shaft of the thermocouple and the other lead where the thermocouple meets the gas valve.

If the reading is 25+ millivolts, the thermocouple is fine. Otherwise, it’s defective and must be replaced too.

  • Improper installation 

If you find that you installed the right thermocouple and it’s working correctly, you need to go back to the installation process and ask yourself whether you installed the device correctly. Although many people see it as an easy DIY job, installing the pilot light thermocouple is a delicate process that can go wrong easily.

Solution: We recommend removing the thermocouple and installing it afresh. Below is a step-by-step guide to install your pilot light thermocouple correctly.

How to Install a Pilot Light Thermocouple

You need a screwdriver and a replacement thermocouple for this process. However, you can also use the old thermocouple if the multimeter test above shows that it still works. Here’s how to proceed;

  1. Turn off the gas supply and shut down the furnace.
  2. Remove the access panel and access the pilot light.
  3. Unscrew the old thermocouple by removing the bolts holding it in place.
  4. Remove the clip holding the old thermocouple to the pilot light tube.
  5. Prepare the new thermocouple. Use the bending template in your owner’s manual to bend it appropriately. Alternatively, use a round object 2-3-inches to bend it. Take care not to crimp or squash the tubing.
  6. Install the thermocouple using the routing in your owner’s manual. You can coil the extra length or leave it near the connection point to the gas valve.
  7. Mount the sensor end of the thermocouple into the supporting bracket next to the pilot tubing so that it dips into the pilot flame area. A few manufacturers provide a small clip or nut to hold the sensor tip in position.
  8. Reconnect the end of the thermocouple to the gas valve where you unscrewed the old one. Make sure to tighten it with a screwdriver. However, don’t overtighten the gas valve. Also, don’t cross-thread or apply a sealant to the connector.
  9. Return the access panel, turn on the main gas valve, and switch on the furnace to see if the pilot light now stays ON. If it still won’t stay ON, it’s time to call an HVAC professional.

FAQs

How Do I Reset my Thermocouple? To reset your thermocouple, locate the small reset button in the center of the switch and press it for 30-60 seconds to give the thermocouple time to heat up. Then release the button and watch to see if the pilot light goes out.

When I Release the pilot button, the flame goes out! If the pilot light goes out when you release the pilot button, the thermocouple is going bad. Either it’s not getting heated, or something is wrong with the controller. Either way, you need to clean or replace it.

Why does my furnace thermocouple keep going bad? The most likely reason is carbon residue, mainly soot. Soot buildup can easily compromise a thermocouple’s sensitivity, especially if the thermocouple is already aging.

Summary 

Now you know why your furnace pilot won’t stay lit even after replacing the thermocouple. You also know how to test a thermocouple and re-install an incorrectly installed thermocouple. Don’t hesitate to call the experts if the problem persists.

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