What You Need to Know About Your Furnace Flame Sensor

A flame sensor is a crucial component of every furnace. Essentially a safety feature, it’s designed to prevent carbon monoxide from leaking into your home. In this article, Cafco Services, one of the leading heating and air conditioning maintenance companies in Ohio, talks about important things you need to know about the flame sensor inside your furnace, such as its primary functions, signs of trouble and maintenance requirements.

The Importance of a Furnace Flame Sensor

The flame sensor ensures that there’s a burning flame inside your furnace. If the flame inside the burner assembly of a furnace goes out, unburned gas will escape from the heating system. Gas may then fill the house, which could result in carbon monoxide poisoning or even an explosion. The flame sensor, however, helps prevent this hazardous scenario by detecting the presence or absence of a flame. If the flame extinguishes, the sensor will automatically shut off the heating system in order to prevent a gas leak. Taking care of this little device is an absolute must because it helps protect your entire family from harm and your valuable possessions from damage.

How a Furnace Flame Sensor Works

The flame sensor is a short, thin metallic rod that creates a small current of electricity to confirm the presence of a flame burning within the furnace. As the gas valve opens to kickstart the combustion process, the current moves from the sensor to detect the heat from a flame. Heating continues when the sensor detects the flame and all elements are in good condition. However, if the flame sensor senses that there’s no flame present within 10 seconds of the gas valve opening, it will shut down the furnace. It will then attempt to cycle two more times before completely shutting down the control board and locking out the heating system. You might be able to reset the furnace yourself, or you may need to call on an HVAC repair technician to do it.

Types of Furnace Flame Sensors

There are two basic types of flame sensors: an optimal reader and a bi-metallic strip. Both of these sensors will send out a small-voltage current in the presence of a flame. Optimal readers are generally used on industrial furnaces and boilers, and these types of flame sensors can detect a flame and generate a larger voltage current. The voltage is in the range of two to five volts direct current. On the other hand, bi-metallic strips will dispatch a small amount of voltage in the form of a mille volt. When the tube-enclosed strips come in contact with a heat source, voltage is produced. Generally, bi-metallic strips are the type of flame sensors that are commonly used on residential furnaces.

Signs of a Failing Furnace Flame Sensor

If the flame sensor on your furnace stops functioning, it could pose a health hazard and put the safety of your family and/or residence at risk. Since gas is odorless and invisible, it’s difficult to tell if there’s a problem until it’s too late. The good thing is that a failing flame sensor is quite easy to spot. Watch out for these warning signs that your flame sensor is failing and may need to either be cleaned and repaired or replaced by an HVAC repair technician.

• The furnace turns on but shuts off after a few seconds.

• There are cracks on the porcelain protecting the flame sensor.

• There’s soot and other debris covering the end of the sensor.

• There’s corrosion on the flame sensor.

• The tip of the flame sensor is black.

While these signs may indicate a problem with your flame sensor, don’t purchase a replacement part just yet. The sensor may have gone bad, but carbon buildup is often to blame as well. It’s possible that cleaning the safety device can resolve all your problems. In other cases, a short-cycling furnace may point to electrical issues. If a flame sensor replacement is necessary, however, it’s a low-cost fix that technicians from HVAC installation companies can perform.

Lifespan of a Furnace Flame Sensor

If your furnace is newly installed or the flame sensor was recently replaced, you can expect the device to last for about five years. The service life of a furnace is 10 to 20 years on average. This means you’ll have to replace a flame sensor once, twice or three times over the course of the furnace’s life. Having a bad flame sensor is completely normal–you just need to know how to properly identify when it’s gone bad so you can replace it on time.

To extend the lifespan of a flame sensor, seasonal furnace cleaning and maintenance is necessary. Even if your heating system appears to be operating smoothly, thoroughly cleaning the sensor at least once a year can prevent you from calling for emergency heating services in the middle of winter.

Cleaning  & Maintenance of a Furnace Flame Sensor

A dirty flame sensor can’t do its job efficiently. Over time, dirt and debris can accumulate on the sensor, and the furnace will struggle to satisfactorily warm your living space. While a professional heating and air conditioning technician is the best person to perform the cleaning of a flame sensor, it is something that any handy homeowner can accomplish themselves.

HVAC installation companies recommend following these simple steps to clean the flame sensor inside your furnace:

1. First and foremost, cut the power. Before you start the cleaning process, be sure to turn off the gas and shut off the electric power to the furnace at the breaker box. If the furnace has been running, wait at least 30 minutes before attempting to do the following steps.

2. Locate the flame sensor. You’ll find the sensor toward the back of the unit. Look for a rod-shaped device in the path of the burner. Then, remove the furnace access cover.

3. Unfasten the flame sensor from the burner assembly. Loosen the screws or bolts and disconnect the wiring that leads to the sensor and control box. Then, carefully pull the device down from the bracket.

4. Clean the flame sensor. Using either emery paper or light-grit sandpaper, gently scrub soot and any other residue off of the metal rod. Make sure not to scrub too hard, as this may scratch or damage the sensor. Once finished, wipe away any soot buildup with a lint-free cloth.

5. Reattach the flame sensor to the furnace. Reconnect the wiring to the sensor and control box and tighten the screws. Make sure the sensor is secured to the burner assembly. Then, reattach the furnace access cover.

6. Test the furnace to see if cleaning the flame sensor worked. If the unit turns on and off properly, a flame sensor replacement may not be necessary.

Cleaning your flame sensor only takes a few minutes, and it may extend the service life of your heating system. Make it a regular practice to inspect it each season to make sure it’s free from soot, dirt and residue. If your system still fails to function correctly, it may be time to replace the sensor or call in a technician to service the furnace.

Contact Us for All Your HVAC Needs Today!

Let one of the premier heating and air conditioning maintenance companies in Cincinnati care for your furnace and its components. Our certified technicians at Cafco Services are committed to providing high-quality products, strong warranties and personable customer service. In addition to HVAC services, we offer comprehensive maintenance programs to care for refrigeration and hot side systems of commercial and industrial customers. Call (513) 653-7446 or fill out our contact form to schedule an appointment.