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Smoke Alarms

Admin Electrical, Smoke Alarms

Smoke Alarms

Smoke Alarms

A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial security devices issue a signal to a fire alarm control panel as part of a fire alarm system, while residential smoke detectors, also known as smoke alarms, generally issue a local audible or visual alarm from the detector itself.

Smoke can be detected either optically (photoelectric) or by physical process (ionization). Detectors may use either, or both, methods. Three different types of smoke alarms are commercially available: (a) ionization smoke alarms (b) photoelectric smoke alarms and (c) dual-sensor smoke alarms.

  • Ionization smoke alarms have a small amount of radioactive material, typically americium-241, between two electrically charged plates, which ionizes the air and causes current to flow between the plates. When smoke enters the chamber, it disrupts the flow of ions, thus reducing the flow of current and activating the alarm.
  •  Photoelectric smoke alarms aim a light source into a sensing chamber at an angle away from the sensor. Smoke enters the chamber, reflecting light onto the light sensor; triggering the alarm.

Which one is better?

Sensitivity of smoke alarms varies depending on the type of fires. Ionization smoke alarms are more sensitive to the flaming fires, while photoelectric smoke alarms are better at detecting smoky (smoldering) fires which are common types of house fires. Ionization smoke alarms are usually cheaper but more prone to false alarms triggered by non-hazardous events than photoelectric smoke alarms.

The tests showed that ionization smoke alarms are much slower to respond to typical house fires. Note that most residential houses are equipped with ionization smoke alarms which failed sometimes. It is better to replace them with photoelectric smoke alarms or dual-sensor smoke alarms.

Smoke Alarm Required Locations

Smoke alarms shall be installed in the following locations:

  • In sleeping room
  • Outside each separate sleeping area in the immediate vicinity of the bedrooms
  • On each additional story of the dwelling, including basements and habitable attics but not including crawl spaces and uninhabitable attics.

When more than one smoke alarm is required to be installed within an individual dwelling unit the alarm devices shall be interconnected.

Locations to Avoid
  • In the garage, unfinished attics, or crawl spaces
  • In very humid areas (greater than 85% RH)
  • In the kitchens (Normal cooking may cause nuisance alarms. If a kitchen alarm is desired, it should be an alarm silence feature or be a photoelectric type.)
  • Near fluorescent lights (Electric noise may cause nuisance alarms)
Installation

Install smoke alarms according to manufacturer's instruction. The best place for installing them is on the ceiling in the middle of the room. The typical instructions are as follows.

  • When mounting an alarm on the ceiling, locate it at a minimum of 4" from the side wall.
  • When mounting the alarm on the wall, use an inside wall with the top edge of alarm at a minimum 4" and a maximum of 12" below the ceiling.
Carbon Monoxide (CO) Alarm Requirements

Carbon monoxide is a colorless, odorless and tasteless poison gas that can be fatal when inhaled. CO inhibits the blood’s capacity to carry oxygen. CO is completely undetectable to your senses. That’s why it is important to your safety that you have a CO alarm. Every home with at least one fuel-burning appliance/heater, attached garage or fireplace should have a carbon monoxide alarm. If the home has only one carbon monoxide alarm, it should be installed in the main bedroom or in the hallway outside of the sleeping area.

More information can be found at the National Fire Protection Association website.

FAQ