The National Fire Protection Association (NFPA) conducted a study of fires that occurred between the years of 2013 – 2017. During this time period, fire departments responded to an average of about 354,000 home fires each year. These fires caused an average of 2,620 deaths, 11,220 civilian fire injuries, and almost $7 billion in direct property damage.
These numbers should reinforce to you the importance of having smoke detectors installed in your home. In this article, I will explain how the different types of smoke detectors work, where smoke detectors should be installed in your home, and how to maintain them. According to the NFPA the death rate due to house fires is two times greater in homes with no smoke detector than it is in a home with any type of smoke detector. The chart below shows the results of a study conducted from 2012 to 2016, and it shows the death rate due to fires based on the number and types of smoke detectors in the homes involved with the fires.
The original “smoke detectors” were not actually smoke detectors, but worked by detecting heat. They either responded to the heat in a room reaching a certain level, or to an extreme increase in temperature. In many fires, smoke is the first and most obvious indication of a fire. An extreme temperature rise normally follows the smoke, making heat-sensing smoke detectors less than ideal for use in homes.
Smoke detectors can sense the presence of smoke in a home much more rapidly than a heat detector can sense heat. You’ve heard the saying, “where there’s smoke, there’s fire.” Smoke detectors have proven very effective in alerting a home’s occupants to the presence of a fire, and they have saved thousands and thousands of lives over the years. If you already have smoke detectors in your home, make sure they have good batteries, and test them regularly. If you don’t have smoke detectors in your home, please go out and purchase them today. They could save your life or your children’s lives.
How Do Smoke Detectors Work?
You do not need to know these details to be able to choose the best smoke detector for you home, but just in case you are curious, I am going to explain to you in some detail how these two types of smoke detectors work.
There are two main types of smoke detectors: ionization and photoelectric. Let’s look at them one at a time.
How an Ionization Smoke Detector Works
An ionization smoke detector contains a small amount of radioactive material, and it is this material that is the key to the operation of an ionization type of smoke detector. Here’s how it works. There are two charged plates in the smoke detector. The smoke detector’s batteries cause one of the plates to be positively charged, and the other to be negatively charged.
Meanwhile, the radioactive material, americium-241, is emitting alpha radiation. Don’t worry. Alpha radiation is not dangerous under most circumstances, so as long as you don’t disassemble the smoke detector, it is perfectly safe. (Alpha radiation cannot even penetrate a piece of paper, so it does not take much to shield against alpha radiation.)
The alpha radiation strikes some of the air particles in the ionization chamber and ionizes them. Ionization means that some of the atoms in the air lose an electron. The loss of the negatively-charged electron causes the air particle to be positively charged. This results in positively and negatively charged particles “floating around” between the charged plates mentioned above and shown in the diagram below.
Since opposites attract, the negatively-charged electrons will move toward the positively-charged plate, while the positively-charge air particles will be attracted to the negatively-charged plate. This flow of particles to the plates creates an electrical current with the space between the plates as part of the circuit. The current flows from the battery, through the wire, to the plate, and across the space between the plates. This flow of electrons is an electrical current that can be measured. Since americium has a very long half-life, the flow of current will be essentially constant, and will raise no alarms.
When there is smoke in the home, some of the smoke enters the ionization chamber. Some of this smoke will attach itself to some of the ions. Now, with fewer ions to flow to the plate, the electric current is decreased. This decrease in current is immediately detected and sets off the alarm. (See diagram above)
How a Photoelectric Smoke Alarm Works
A photoelectric or optical smoke detector works on the principle of light being diffracted by smoke. Here’s how it works. A light beam shines at an angle to a sensor. Since they are at an angle to each other, the light normally does not reach the sensor (see diagram). When smoke enters the detector, the light is diffracted in all directions, and some of it will reach the sensor, setting off the alarm. Take a look at the simplified diagrams below.
Which is the Best Type of Smoke Detector for My Home
Both ionization and photoelectric smoke detectors are very effective in detecting and alerting a home’s occupants to the presence of smoke in the home, but there are some differences between the two. The main difference is in how they respond to fires. Ionization smoke detectors tend to respond best to the small smoke particles produced by fast-moving fires. This sensitivity to smaller particles also makes them more prone to false alarms due to burning food or steam. A photoelectric smoke detector normally responds better to the larger smoke particles produced by slow-moving, smoldering fires. You can get both of these features by purchasing a dual sensor (combination type) smoke detector which combines both the ionization and photoelectric sensor in one smoke detector.
Another thing to consider is the use of interconnected smoke detectors. Interconnected smoke detectors will all alarm if only one detector in the house detects smoke, so they provide more protection than simple, stand-alone, battery-operated units. National standards required the use of interconnected smoke detectors in all new construction. It’s not normally an easy thing to wire an older home in order to be able to interconnect the smoke detectors, but you can purchase smoke detectors that interconnect wirelessly. The most important thing is to have working smoke detectors in your home.
Personally, if I could install only one type of smoke detector in my home, I would install photoelectric smoke detectors. They are a little more expensive, but they respond quicker to the most common type of home fire, the slow smoldering fire. Ideally, you will have both types of smoke detector in your home. Having both types is easily accomplished by installing combination (dual sensor) smoke detectors. You can look on the back of your smoke detectors to determine which type you already have installed.
Where are Smoke Detectors Required in a Home?
Smoke detectors are required in all bedrooms, in hallways adjoining bedrooms and on all levels of the home. The NFPA gives the following guidance concerning smoke detectors, their use and maintenance:
Installing Smoke Detectors:
- Choose smoke alarms that have the label of a recognized testing laboratory.
- Install smoke alarms inside each bedroom, outside each sleeping area and on every level of the home, including the basement.
- On levels without bedrooms, install alarms in the living room (or den or family room) or near the stairway to the upper level, or in both locations.
- Smoke alarms installed in the basement should be installed on the ceiling at the bottom of the stairs leading to the next level.
- Smoke alarms should be installed at least 10 feet (3 meters) from a cooking appliance to minimize false alarms when cooking.
- Mount smoke alarms high on walls or ceilings (remember, smoke rises). Wall-mounted alarms should be installed not more than 12 inches away from the ceiling (to the top of the alarm).
- If you have ceilings that are pitched, install the alarm within 3 feet of the peak but not within the apex of the peak (four inches down from the peak).
- Don’t install smoke alarms near windows, doors, or ducts where drafts might interfere with their operation.
- Never paint smoke alarms. Paint, stickers, or other decorations could keep the alarms from working.
- For the best protection, interconnect all smoke alarms. When one smoke alarm sounds, they all sound. Interconnection can be done using hard-wiring or wireless technology.
- When interconnected smoke alarms are installed, it is important that all of the alarms are from the same manufacturer. If the alarms are not compatible, they may not sound.
- There are two types of smoke alarms – ionization and photoelectric. An ionization smoke alarm is generally more responsive to flaming fires, and a photoelectric smoke alarm is generally more responsive to smoldering fires. For the best protection, both types of alarms or combination ionization-photoelectric alarms, also known as dual sensor smoke alarms, are recommended.
- Keep manufacturer’s instructions for reference.
Testing Smoke Detectors
- Smoke alarms should be maintained according to manufacturer’s instructions.
- Test smoke alarms at least once a month using the test button.
- Make sure everyone in the home understands the sound of the smoke alarm and knows how to respond.
- Follow manufacturer’s instructions for cleaning to keep smoke alarms working well. The instructions are included in the package or can be found on the internet.
- Smoke alarms with non-replaceable 10-year batteries are designed to remain effective for up to 10 years. If the alarm chirps, warning that the battery is low, replace the entire smoke alarm right away.
- Smoke alarms with any other type of battery need a new battery at least once a year. If that alarm chirps, warning the battery is low, replace the battery right away.
- When replacing a battery, follow manufacturer’s list of batteries on the back of the alarm or manufacturer’s instructions. Manufacturer’s instructions are specific to the batteries (brand and model) that must be used. The smoke alarm may not work properly if a different kind of battery is used.
What to Do About Smoke Detector False Alarms
Fortunately, smoke detector false alarms are more common than alarms that alert us to the presence of real fire and smoke. In a common occurrence repeated in hundreds of homes across America every day, smoke detectors give false and annoying alarms. When a smoke detector alarm goes off, homeowners and renters start looking for a way to stop the sound. Opening doors and fanning the air around sometimes helps. Taking the detector down and permanently removing the battery is also an oft-used remedy, but certainly not a wise one. Let’s talk about some of the things that cause these false alarms, and how to properly address them.
Here is a List of Things to Look at if Your Smoke Detector is Alarming
- Make sure that there is no actual fire or smoke.
- Is the unit within ten feet of a cooking appliance? If so, move it further away from the appliance.
- Is the smoke detector near an open window? Small particles entering through an open window can cause a false alarm. These particles can include things such as smoke, heavy pollen, insects, or dust.
- How old is the smoke detector? It is recommended that smoke detectors be replaced at least every ten years. Also, dust, insects, cobwebs and other things can clog the detector over time and cause it to falsely alarm. They can often be cleaned with a shop vac.
- Is the smoke detector installed near a bathroom or other steamy area? Although, steam is not smoke, it can cause nuisance alarms.
It is critical that you have working smoke detectors in your home. Smoke detectors have saved many lives over the years. Studies have shown that fatalities due to house fires are much greater in homes with no smoke detectors present. There are two main types of smoke detectors, ionization and photoelectric, which each respond to different types of fires in the home. Smoke detectors should be installed in every bedroom, in hallways outside of each bedroom, and on every level of the home. Interconnected smoke detectors provide a greater level of safety than stand-alone smoke detectors. So, protect your family and make sure that you have enough working smoke detectors in your home.
© 2020 Mike Morgan
This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at firstname.lastname@example.org. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.