Monday, April 20, 2015

Is it Time to Replace Your Smoke Alarms?

Currently, it is recommended that smoke alarms should be replaced after about 10 years. 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.

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.

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 a 30 year old small Canadian study when smoke alarms where still a new invention. Thus 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.

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.

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 grill 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.

Combination smoke and carbon monoxide alarms. These alarms can detect smoke as well as carbon monoxide. Typically these are ionization and the carbon monoxide monitor uses an electrochemical sensor that has a predicted life of 7 years. If you buy a combination ionization and carbon monoxide alarm, it is recommended that you also get a separate photoelectric unit to be fully protected.

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 insure 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. With this combo alarm you'll still need separate carbon monoxide units if you have any oil, natural gas or propane burning appliances such as a furnace, water heater, stove, cooktop or grill or use a generator adjacent to the home.

So what am I doing? I replaced my old hard wired smoke alarms with combination smoke and carbon monoxide alarms and then added three separate photoelectric smoke alarms.


  1. Do you think disposable smoke alarms are more convenient than hardwired models? You do not have to worry about replacing the batteries and simply have to throw them away after 10 years.

  2. Good point, though the hard wired are tied together and alarm the whole house so that a smoke event cannot be missed.