5 Furnace Safety Features You Should Meet

Your furnace is a controlled fire burning in your home.

Before you let that scare you, let’s talk about the amazing safety features your furnace has to keep you safe while it keeps you warm!

There are five safety features some furnaces have that are important to learn about: a Flame Rollout Sensor, a Flame Proving Sensor, a High Temperature Limit Control, a Flue Exhaust Pressure Sensor, and a Fan Limit Sensor.

Flame Rollout Sensor

Do you know the three ingredients of fire?

Your furnace is a controlled fire burning in your home. Fortunately, there are safety features that keep things safe for your family.

Oxygen, heat, and fuel! In order to make your home warm and cozy, your furnace starts a fire in a fire box by using these three ingredients. The gas valve releases raw gas to serve as the fuel. This gas in the fire box comes in contact with the heat source. Depending on your unit, this would either be a pilot light, a hot surface ignitor, or a spark ignitor. When the gas and heat source meet, they use any available oxygen while in the fire box to ignite a fire. This fire is supposed to stay in the fire box, where it’s safely contained and can provide your furnace with warmth. If there’s not enough oxygen for combustion, however, the flame will “roll” out of the furnace in search of the oxygen it needs! Your Flame Rollout Sensor senses when the flame leaves the furnace, and it tells your heating system to shut off the furnace.

What happens when the Flame Rollout Sensor isn’t working right? If this sensor malfunctions, the flame might catch the floor joists, nearby walls, or attic trusses on fire!

Recommended: “The Silent Killer Hiding in Your Home”

Flame Proving Sensor

Our safety experts take the time to check each safety feature during your maintenance to make sure they’re working right to protect your family!

Remember when we talked about how the gas valve releases raw gas into the fire box? Your furnace may also be equipped with a Flame Proving Sensor, which detects when there’s a flame in the fire box. If it detects a flame, it allows the gas valve to stay open and continue releasing gas to keep the fire alive. When it doesn’t detect a flame, it shuts off the gas valve.

What happens when your Flame Proving Sensor isn’t working right? If this sensor malfunctions, it continues to allow raw gas into the combustion chamber without a flame actively using it up. If this happens, even a small spark on that much gas can cause a huge explosion.

High Temperature Limit Control

Some of the safety features in your furnace will shut your furnace off when they detect a problem, and some of them require a professional to reset them. If your furnace won’t turn on, give us a call! We’ll get to the bottom of things!

In order to heat your home, your furnace has got to get really hot. Of course, there’s a limit to how hot your furnace should get though! Your furnace comes equipped with a High Temperature Limit Control, which has a set limit of how high the temperature of the furnace is allowed to get. If your furnace exceeds this limit, the High Temperature Limit Control shuts off your system.

What happens when the High Temperature Limit Control isn’t working right?

If this control malfunctions, it could burn out your heating system. This can cause one of two problems: either there will be too much heat and you’ll end up with damage to your fire box, or you won’t have enough heat in your home because your unit keeps shutting off. When this happens, you’ll need to call a professional to fix your unit.

Recommended: “Why Your Furnace Is Blowing Cold Air”

Flue Exhaust Pressure Sensor

In order for your furnace to safely ignite, it has to do a couple things. It’s gotta provide the proper amount of combustion air to the gas and flame, and it has to make sure there are no blockages in this process. Basically, it makes sure the inducer motor (it’s like a fan) is pulling smoke through the heat exchanger and pushing it through the flue so it can escape your home. If there are no blockages, the sensor will let your unit know it’s ok to turn on the gas and ignitor. If there’s a blockage, your furnace won’t turn on.

Did you know? Your inducer motor has a short lifespan. They usually only last between 8-10 years, so it’s important to get it checked every year!

What happens if the Flue Exhaust Pressure Sensor isn’t working right? If this sensor malfunctions, your unit won’t be able to turn on and you’ll need to call out a professional to fix it.

Fan Limit Sensor

In order to bring heat into your home, your furnace uses a type of fan called a blower to blow air over the heat exchanger. If the blower stops working or isn’t providing enough airflow over the heat exchanger, your Fan Limit Sensor will shut off the furnace.

What happens if the Fan Limit Sensor isn’t working right? If this sensor malfunctions, your heat exchanger will continue to get really hot and won’t have the air from a fan to push heat away from it. This means your heat exchanger would get red hot, which could allow cracks and splits in the steel. A crack in your heat exchanger is dangerous because it can allow carbon monoxide into your home!

Related: “Do You Have a Crack in Your Heat Exchanger?”

Staying Safe and Warm

When things start getting cold in the Carolinas, a furnace is a fantastic way to keep your family warm and cozy! We know all of this can make them sound a little scary, but don’t worry! These sensors and controls are meant to keep your family safe. The best thing you can do for your furnace is schedule a Safety Check to make sure all your furnace’s safety features are in great shape to last the winter ahead! Each of our technicians is a certified safety expert, and when you schedule service, your tech will check out everything super thoroughly. We don’t take shortcuts, especially when it comes to your family’s safety. You deserve to stay safe and warm this winter!

Photodiodes and Arrays

Photodiodes exemplify the simplest of the solid-state detectors, and are built upon semiconductor p-n junctions. An individual p-n junction is constructed from the union of a positively-doped material with a negatively-doped material, and contact between the two surfaces results in the local motion of major charge carriers. Electrons near the junction drift towards the p-type material, and holes drift towards the n-type material, resulting in a depletion zone that has an intrinsic electric field, which creates a potential barrier across the junction.

Figure 4: An unbiased p-n junction.

This junction is known as a diode, and will allow current to flow through it in only one direction. When the diode is subjected to an external potential, called a bias, the energies of the p- and n-type materials are shifted relative to each other, changing the magnitude of the potential barrier. When a positive voltage is applied to the p-type side and a negative voltage to the n-type side, the holes in the p-type and the electrons in the n-type sides are forced into the depletion zone. This is known as a forward bias, and when it is large enough, the depletion zone and its potential barrier will disappear and the diode will allow current to flow through it. However, if the potentials are reversed, into what is called a reverse bias, the electrons in the n-type side and the holes in the p-type side are pulled away from each other, widening the depletion zone, and increasing the potential barrier across the junction, preventing current from flowing through the diode.

Figure 5: A reverse-biased p-n junction.

In photodiode detectors, the diode is held under reverse bias, which maintains a fairly large electric field throughout the depletion zone that will sweep free electrons towards the anode and free holes towards the cathode. When a diode is illuminated with electromagnetic radiation with energy sufficient to excite an electron from the valence band to the conduction band of the semiconductor, electron-hole pairs are generated. As these electron-hole pairs are separated and accelerated in opposite directions by the electric field, a photocurrent is produced, by which the incident radiation can be quantified.

The spectral response of a photodiode can be tuned to different regions by carefully selecting the materials used to make the p-n junction. For example, silicon-based photodiodes are sensitive to radiation between 190-1100 nm, making them useful for UV, visible, and limited near-IR spectroscopies, while lead sulfide photodiodes are sensitive in the range of ~1000-3500 nm, enabling detection in both near-IR and short-wave-IR. Additionally, solid-state photodiodes provide a very nearly linear response throughout a large dynamic range. These properties, coupled with the fact that photodiodes do not require high-voltage power supplies and are physically small, relatively inexpensive to manufacture, and much more robust than photocathodes makes them an excellent choice for use in many spectroscopic instruments.

One additional advantage that stems from the small size of photodiodes is the ability to arrange multiple detectors into an array, known as a photodiode array (PDA). These arrays can take any shape and contain any number of detector elements, but are often arranged in a linear fashion, enabling the detection and encoding of spatial data as well as the energy and intensity of the incident light. As these arrays can be read out in parallel, a PDA has the same response time as an individual detector, allowing for the collection of much more data with the same response time.

However, there are certain aspects of photodiode detectors, such as overall sensitivity, response time, and noise levels, that prevent them from being completely superior to photocathode technologies. Conventionally operated photodiodes, even with quantum efficiencies around 80%, do not have any internal gain, and thus cannot compete with photomultiplier tubes in terms of the ability to detect very small numbers of photons. Furthermore, photodiodes generally have slower response times than similar photocathode detectors, on the order of microseconds, as there is a small but significant time-lag for the electron-hole pairs to be swept from the depletion zone to the electrical contacts. Finally, while shot, thermal, and dark current noise is present in all detectors, photodiodes are also subject to generation-recombination noise, though most photodiodes are limited by thermal noise and dark current noise. Many photodiode detectors must be cooled to reduce overall noise to acceptable levels.