Smoke Detectors

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Smoke alarms save lives and every household should have at least one.

Legal Requirements

Whilst there is no legal requirement to have smoke alarms in older houses, houses built after 1992 were required to have grade D or E systems installed. Houses having some types of building work carried out (eg loft conversion, 2 storey extensions) are also required to have grade D or E systems. New builds are now required to have grade D systems installed.

Basic detection

What Sort of System should I have?

Ungraded systems

An ungraded system consists of a number of battery powered non interlinked smoke alarms. Although classed as ungraded, this is the most common setup seen in the UK. The location of the smoke alarms should follow the guidance given in this article.

The 3 standard types of system used in dwellings that are 3 stories or less, have a floor area of less than 200m2 and that are NOT Houses of Multiple Occupation are

Grade F

Battery powered interlinked detectors. Grade F is no longer allowed if the installation is subject to building regulations.

Pros - Cheapest to fit.

Cons - The batteries are often removed and not replaced, either due to false alarms or low battery power.

Grade E

Mains powered interlinked detectors with no battery backup/ backup. Grade E is no longer allowed on new builds

Pros - There are no batteries to replace

Cons - They do not work in a power cut, occupiers can turn them off by turning their power supply off. Cost of installation

Grade D

Mains powered interlinked detectors with battery/standby backup. This is the standard that applies to new builds.

Pros - Works in a powercut.

Cons - Batteries will still need to be replaced (unless the unit uses an integrated rechargable backup), cost of installation.

Grades A, B and C

Grade A, B and C systems are fire detection systems that have a central contol panel. Whilst they are in principal no more difficult to wire up than normal interlinked smoke alarms they are usually used in offices, HMO, schools etc and so are not covered in this article.

Where to Locate Detectors

The following recommendations are made in BS5839 and form a good guide if you are intending to install smoke detectors
  1. At least one smoke detector should be sited in each hallway or corridor and on each main landing of every staircase. In hallways or corridors exceeding 7.5 m in length, no point within the hallway or corridor should exceed 7.5 m from the nearest detector.
  2. Where smoke alarms are installed, no bedroom door should be further than approximately three metres from the nearest smoke alarm.
  3. At least one smoke detector should be located between every bedroom and every other room in the dwelling, other than a toilet, bathroom or shower room. In a single-storey dwelling protected by a single detector, the detector should be as close as possible to living accommodation. However, where such rooms are located on both sides of any bedroom, a smoke detector should be sited mid-way between the doors to these rooms.
  4. Subject to compliance with b), in a multi-storey house, at least one smoke detector should be located on the ground floor between each staircase and every room, other than a toilet, bathroom or shower room. However, where such rooms are located on both sides of a staircase, a smoke detector should be sited mid-way between the doors to these rooms.
  5. In open plan accommodation where a stair is open to a living/dining area (or any other room in which fire might start), the living/dining area (or other room) should be treated as a circulation area and be protected by a smoke detector.
BS5839 also gives the following recommendations for new builds
  1. In addition, heat or smoke detectors (as appropriate) should be sited in all rooms in which protection is deemed necessary. Alternatively, subject to (soon to be added), carbon monoxide fire detectors may be installed in these rooms.
  2. Heat or smoke detectors (as appropriate) should be sited in all rooms and other areas of the dwelling, including lofts. Alternatively, subject to ), carbon monoxide fire detectors may be installed in these rooms and areas.
  3. Detectors within rooms should be sited such that no point is further than 7.5 m from the nearest smoke detector or, in rooms protected by heat detectors, no further than 5.3 m from the nearest heat detector.
  4. Detectors should preferably be mounted on ceilings and should be located at least 300 mm horizontally from any wall or light fitting unless, in the case of light fittings, there is test evidence to prove that the proximity of the light fitting will not adversely affect the efficiency of the detector.
  5. Ceiling-mounted detectors should be located such that their sensitive elements are between 25 mm and 150 mm below the ceiling in the case of heat detectors, or between 25 mm and 600 mm below the ceiling
In the case of smoke detectors
  1. If ceiling mounting is impracticable, in rooms and hallways not exceeding 10 m in both length and breadth, and not exceeding 50 m2 in area, detectors may, alternatively, be mounted on a wall provided that:
    1. the top of the detection element is between 150 mm and 300 mm below the ceiling; and
    2. the bottom of the detection element is above the level of any door opening; and
    3. the manufacturer’s instructions state that the detector is suitable for wall mounting.
  2. Detectors should not be mounted adjacent to, or directly above, heaters or air-conditioning vents.
  3. Detectors should be mounted in positions that are reasonably accessible for maintenance. NOTE This is particularly important in the case of smoke alarms that incorporate a battery or that have a routine testing or alarm silence facility. Such detectors should not, for example, be mounted above a stairwell.
  4. Where the use of carbon monoxide fire detectors is appropriate, location and siting of these detectors should comply with the recommendations for the location and siting of smoke detectors

Types of detector

The type of detector used is a compromise between the ability to detect the type of fire that may occur and to eliminate false alarms. Smoke detectors are the main life savers and are used in habital rooms & circulation spaces however a fire alarm system may be supplemented with heat and CO detectors in kitchens and garages.

Types of Smoke Detector

Smoke detectors may be used in any room or area of a dwelling, other than kitchens, bathrooms and shower rooms.


Ionisation smoke detectors are the cheapest available detectors. They are sensitive to cooking fumes and should not be placed in rooms next to kitchens.

Ionisation detectors are better at detecting fast flaming fires and are less sensitive to smouldering fires or fires some distance away.


Optical smoke detectors detect light being scattered by smoke particles. They may give a false alarm if exposed to steam and should not be located close to poorly ventilated bathroooms.

They are more sensitive to smouldering fires and are recommended for use in the circulation spaces (BS5839-6 2004).

Dual sensors

Ionisation and optical are sensitive to different types of fires, so combining both in one alarm gives the best possible detection speed.

Types of Heat Detector

Heat detectors are used in places that are unsuitable for smoke detectors such as kitchens, boiler rooms and garages. Heat detectors take longer to respond to a fire than smoke detectors and are a compromise to reduce false alarms.

Fixed Temperature

Fixed temperature heat detectors operate when they reach a preselected temperature (often 58degC). These are the most common type of heat detector in use and would be used in areas where rapid temperature fluctuations may be experienced (eg a kitchen when cooking.) The downside is that it takes a blaze to reach 58C.

Rate of Rise of Temperature

Rate of rise heat detectors operate when the temperature rises at an abnormally fast rate (about 7 deg C a minute), rather than just at a fixed temperature and can operate at a lower temperature than a fixed temperature detector. They may however not respond to slow burning fire and often have a built in fixed temperature sensor as well.

These detectors are no good for kitchens where opening an oven door will cause a rapid change in temperature.

Carbon Monoxide Detectors

CO detectors conforming to BS EN 50291:2001 may be used as part of a fire detection system. These detectors operate at a much lower CO detection rate than the CO detectors used to detect a faulty gas appliance. CO detectors are most sensitive to smouldering fires and fires in which the rate of burning is controlled by the supply of oxygen. Although not often used inside dwellings as a means of fire detection thay may be suitable for certain store rooms and boiler rooms as they are unaffected by steam and dust. CO detectors are a supplement to smoke detectors and not a replacement for them. Combined smoke and CO detectors are available.

Electrical Installation Guidelines

Grade E systems require the detectors to be powered from their own MCB/RCBO at the main CU. Grade D systems may be run from a local lighting circuit but should incorcorate a means of isolation to allow maintainance on the detectors and leave the lighting circuit powered.

Neither type of system should share an RCD that is used for sockets outlets.

The interconnecting cable is usually 3 core and earth cable, however T&E plus a single is sometimes used. When installed in conduit then single cables would be used. RF interlinked detectors are available that do away with the interconnecting cable


Keeping spare batteries makes replacement instant. Batteries usually need replacing every 1 to 2 years. 10 year lithium batteries are available that makes this unessesary. Mains powered detectors are available that use capacators or integrated rechargable batteries and so need no battery replacement

Chirping Alarms

When the battery is reaching the end of it's life then the alarm will chirp every few minutes to warn you. Annoyingly this most often happens at night when the temperature drops reducing the voltage of the battery. Since usually all the detectors sound the same, if you don't get up and find it, it will only annoy a few more nights. Just get it done.

Maintenance and Replacement

Test regularly using the test button and not with a flame. Detectors should be cleaned with a vacuum cleaner every 6 months to keep down the buildup of dust. Some detectors have a lifespan of about 10 years and usually have a replacement date on them.