Thermocouples / Flame Sensors

Thermocouples and flame sensors are integral for boiler performance and safety. These devices are cheap to replace and repair but often overlooked by homeowners. 


Many people don’t know how to tell when these appliances are beginning to fail, resulting in potential danger. These tools may be small, but they help control the gas and flame release within a boiler, making them necessary for any heating and cooling system.


Some people assume that a thermocouple is the same tool as a flame sensor. These tools are relatively similar, their only significant differences stemming from which types of devices they service. 


Below, we’ve broken down some of the significant functions of these devices and how we can help homeowners repair them.


Thermocouples might be small, but they keep an entire boiler safe. They act as a safety device for a boiler, shutting off the gas when the pilot flame burns out. This measure helps prevent a gas leak, explosion, or other deadly situation at a homeowner’s property.

Thermocouples have several primary functions, including:

  • Keeping standing pilot appliances safe
  • Shutting off the gas when the pilot frame burns out
  • Closing the gas valve and preventing potential gas leaks

What It Does

A thermocouple senses the temperature, creating a small flow of energy that helps release gas. If the temperature is hot, the thermocouple assumes the pilot flame is burning and releases gas. 


If the temperature is colder than usual, the thermocouple will not dispense gas. Instead, it will close the gas valve, preventing deadly gas from leaking. This safety measure prevents a potentially fatal gas leak or explosion.


Thermocouples only exist in standing pilot appliances, which have a standing pilot flame tube. These pilot tubes don’t have an electronic ignitor, which is where the thermocouple comes in. 

When to Replace a Thermocouple

Thermocouples should undergo service to ensure a long lifespan. However, since these parts are so cheap, many homeowners go ahead and repair the entire unit. Sometimes, it can be hard to tell if the thermocouple needs a thorough clean or a replacement. Replacing rather than cleaning ensures that the thermocouple is always in working order.

Signs that a thermocouple needs replacement:

  • The glass tube appears damaged
  • Copper wire or connecting nuts appear dirty
  • The metal gas tube seems to be warped or damaged

Thermocouples can begin to fail in around ten years, but it is best to get them repaired as soon as possible. If a homeowner suspects that their thermocouple might not be working as well, as usual, they should call an HVAC professional to remedy the problem.


Delaying a replacement could result in a boiler either not functioning or creating a dangerous household situation. Gas leaks are an extremely deadly potential consequence of not maintaining or repairing a thermocouple. To prevent such an issue, a professional should assess the situation and replace the part.

Flame Sensor

Somewhat different from a thermocouple is a flame sensor. A flame sensor doesn’t have a glass tube like a thermocouple. Instead, it has a metal probe and bracket encased by a wire fitting. Inside the sensor is a ceramic insulator, which keeps the unit warm and accurately detects temperature.


Flame sensors are also distinguishable from the thermocouple by their lack of a wire lead. While thermocouples contain a metal wire lead that helps feed the gas to the flame, flame sensors contain a different type of wire. They have a quick-disconnect wire that makes it easy for the system to turn gas on and off. 

Flame sensors have the following primary functions:

  • Intermittently igniting the flame, keeping the system hot
  • Shutting off entirely when the unit begins to fail
  • Keeping the system hot when the fire is not burning

What It Does

Flame sensors exist in boilers that use electronic ignition. Unlike the standing pilot system, these electronic ignitors only intermittently keep a flame burning. Instead, they ignite only when necessary, using the system’s heat to start a flame.

If a flame sensor is failing, it shuts off entirely as a safety measure. This measure can help keep the gas and flame apart, preventing a potentially deadly gas explosion and fire.

Signs that a flame sensor may be failing include:

  • The ceramic insulator is worn or cracked
  • The quick-disconnect wire is warped or broken
  • The unit appears dirty or worn down
  • Metal appears dull and stripped

When to Replace a Flame Sensor

A professional should replace a flame sensor about every two to three years. Since these units have a vital role in a boiler, they should be diagnosed and fixed immediately. An HVAC professional can ensure that a homeowner’s system is in working order, which can keep a home safe from potentially deadly gas leaks.

What to do if a Thermocouple or Flame Sensor Fails

If a thermocouple or flame sensor in your boiler fails, we recommend following these steps to stay safe.

1. Turn off the Gas

Ensure gas is not flowing to the system if you suspect there is a problem with a thermocouple or flame sensor.

2. Turn off the Power

After the gas has stopped flowing, turn off the power to the system. This off switch should exist on the boiler itself. You can also turn off the switch or circuit connecting your home’s power to the boiler for good measure.

3. Call Stevensons Air Control

While it may seem tempting to try to fix the issue yourself, Stevensons will ensure that your system is up and running. Fixing the issue yourself could cause potential problems down the road. Stevensons Air Control will ensure your job’s thorough and professional completion, all while providing you with a great rate.

Contact Stevensons Air Control

Waiting until the last minute to schedule an appointment or consultation with us is the worst thing you can do. This delay could result in your unit experiencing further problems. Boiler failures can be expensive if unchecked for an extended period. 

Stevensons Air Control can help remedy any problems your boiler may be experiencing. We will be able to identify and fix your thermocouple or flame sensor. For more information and to schedule a consultation, call us today at (847) 266 8765.