Time to Replace my Smoke Alarms

from Universal Security Instruments website

 It is recommended that smoke alarms should be replaced after about 10 years. There have been very few studies to determine the actual failure rate though it is widely believed to be 3% per year regardless of age based on an almost 40 year old small Canadian study when smoke alarms where still a new invention. Smoke alarms have come a long way since 1980’s, and in theory, the electronic components in a smoke detector should last at least 30 years. But a smoke detector could fail at any time and fire safety officials recommend changing them every 10 years because that provides a reasonable margin of safety and after that time their sensors can begin to lose sensitivity.

The test button you have been dutifully pressing each year only confirms that the battery, electronics, and alert system are working; it doesn’t mean that the smoke sensor is working. To really test the sensor, you need to use an aerosol can of smoke alarm test spray that simulates smoke. Fire-safety officials have long believed that the leading cause of smoke-detector failure is a power-source problem, primarily dead or missing batteries since most detectors are battery powered. The result has been the campaigns to get consumers to change their batteries twice a year when they reset their clocks. But many of those experts are increasingly concerned that some detectors may fail to work because they are simply too old. According to the Fire-protection association smoke detectors’ sensitivity to smoke tends to change over time. Sometimes becoming more sensitive and causing more nuisance alarms, sometimes becoming less sensitive and not alarming when there is a fire.

The U.S. Fire Administration for Homeland Security, the National Fire Protection Association (NFPA), the National Electrical Manufacturers Association (NEMA) and the Red Cross agree after working for 87,000 hours or 10 years in normal environmental conditions in the home it is time to replace your smoke alarms. I replace my smoke alarms at the recommended interval.

Every home should have smoke alarms, and all homes with oil, natural gas or propane burning appliances such as a furnace, water heater, stove, cooktop or fireplace should have a carbon monoxide monitor. If you have an all-electric home you do not really need a carbon monoxide alarm unless you operate a generator during power outages. If you are replacing your smoke alarms, it is a good time to consider your options.

There are two basic types of residential smoke detectors, ionization and photoelectric. Ionization models are excellent at detecting the small particles typical of fast, flaming fires, but tend to be poor at detecting smoky, smoldering fires. Ionization units are generally prone to false alarms from burnt food and steam-classic causes of annoying false alarms. Photoelectric smoke alarms are excellent at detecting the large particles typical of smoky, smoldering fires, but all were poor at detecting fast, flaming fires. Photoelectric units are less prone to false alarms from burnt food and steam, so you can mount them closer to kitchens and bathrooms.

By far, most residential smoke alarms are ionization sensor models; though I’m not sure that is the best choice. These types of smoke detectors contain a very tiny amount of radioactive material, americium-241 embedded in a thin gold foil in an ionization chamber. An ionization chamber is very simple. It is basically two metal plates a small distance apart. One of the plates carries a positive charge, the other a negative charge. The radioactive material is contained within a laminated material thick enough to completely retain the radioactive material, but thin enough to allow the alpha particles to pass. Small particles from fires and smoke interfere with the movement of the alpha particles and the circuit is broken the smoke detector alarms.

Photoelectric smoke alarms use a T-shaped chamber fitted with a light-emitting diode (LED) and a photocell. The LED sends a beam of light across the horizontal bar of the chamber. The photo cell will generate a current, when exposed to light. Smoke will interfere with the circuit, but they can be insensitive to small particulates.

Combination smoke and carbon monoxide alarms are recommended those using propane or natural gas appliances or fireplaces in their homes. These alarms can detect smoke as well as carbon monoxide. Typically, these are ionization and the carbon monoxide monitor uses an electrochemical sensor that had a predicted life of 7 years initially, but has improved over time and is now calibrated at 10 years. If you buy a combination ionization and carbon monoxide alarm, you might want to also get a separate photoelectric unit to be fully protected of use a dual-sensor smoke alarm.

Dual-sensor smoke alarms. These combine ionization and photoelectric technology to save you the hassle of installing two separate smoke detectors. Fire protection authorities recommend that both ionization and photoelectric smoke alarms be used together to help ensure maximum detection of the various types of fires that can occur within the home. Ionization sensing alarms detect invisible fire particles (associated with fast flaming fires) sooner than photoelectric alarms. Photoelectric sensing alarms detect visible fire particles (associated with slow smoldering fires) sooner than ionization alarms.

Each time I replace my smoke alarms, I check to see if they have come out with a combo ionization, photoelectric and carbon monoxide alarm. This time I found one, the Universal Security Instruments AMIC1510SC 3-in-1 Sensing Plus® Hardwired Smoke, Fire & Carbon Monoxide Alarm is one of few combination detectors that has both photoelectric and ionization sensors for smoldering and flaming fires in addition to a carbon monoxide sensor in a hardwired unit. It is pictured above.  This is the deluxe model (there is a cheaper hardwired model) with several bells and whistles I wanted including a 10 year battery.