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Smoke Alarm Buying Guide

Which smoke alarms for which rooms?

When choosing smoke alarms (also called smoke detectors) it can be hard to find the right type of alarm for a particular room in your house, tenanted property or workplace. Manufacturers have over the years developed a range of different smoke detector sensors - and for very good reasons! Fitting the right type of detector in each room will not only reduce false alarms but also identify real fire hazards quicker. Below you will find a list of rooms together with the type of sensor type you should fit and a brief explanation as to why.

Ionisation smoke alarms (stairwells, landings, offices):

Ionisation smoke detectors have traditionally been used throughout properties for many years. Designed to react quickly to fast flaming fires, ionisation smoke alarms are most sensitive to small particles. When fires produce little or no smoke but the fuel is subject to rapid combustion, the ionisation smoke detector is the quickest to sense its presence. These fires tend to originate from materials such as paper and clothing.

If you are unsure what type of smoke alarm you have installed, see if you can find a radioactive symbol or compartment inside the alarm (if this is accessible). Unlike optical smoke detectors, ionisation smoke alarms feature a tiny radioactive source in the sensor chamber that enables the ionising technology to work. It's quick reaction to small particles can make the ionisation smoke alarm prone to false alarms, particularly when located near kitchens. Because of this, it is recommended for ionisation alarms to be installed in stairwells, upstairs landings and offices. This should avoid the occurrence of nuisance alarms when cooking or burning toast!

Optical smoke alarms (also called photo-electric smoke alarm) (bedrooms, lounges, hallways, also: offices and landings):

Less prone to false alarms than ionisation, optical smoke detectors are slightly quicker at detecting slow smouldering fires that tend to produce a lot of smoke. These fires tend to originate from upholstery type materials or over heated wiring. Also known as photo-electric alarms, this quick response time to smouldering fires is down to the optical smoke alarm's high sensitivity to large particles in the air. The optical sensing chamber effectively "sees" when smoke is present, as the large particles block and cause an infrared light to scatter.

Optical smoke detectors are suitable for installing in bedrooms and living rooms where soft furnishings and cables are common place. They can also be installed in downstairs hallways as they are significantly less likely than ionisation alarms to sound falsely when cooking fumes are present from the kitchen. Optical alarms are, however, not suitable for areas open to the elements and very dusty environments.

Heat alarms (kitchens, garages):

Responsive to heat rather than smoke, heat alarms are ideal for installing in kitchens where they are designed to detect a rapid change in temperature. Featuring a fixed temperature thermistor, they are triggered when the temperature inside the room reaches approximately 58°C. This specific heat sensing technology avoids false alarms altogether, as the unit does not react to cooking fumes and steam. Please always refer to the manufacturer's instructions for the optimum ceiling positioning - typically about 1m from the cooker. Heat alarms are also suitable for installing in garages. Wherever vehicle fumes are present, a normal smoke alarm would become a nuisance with constant false alarms.

Multi-sensor alarms (stairwells, landings, offices, bedrooms, lounges and hallways):

The combined optical smoke and heat alarms contain both optical smoke as well as heat sensing technologies, providing the alarm with the best features from both sensors. Working together, the two sensors intelligently monitor the air for any signs of fire, therefore providing an earlier response and a reduced risk of false alarms in comparison to single sensor detectors. Their wide response envelope makes the combined alarms suitable for almost all areas of a property including landings, hallways, living rooms and bedrooms. Please note, although the combined alarms are containing a heat alarm, they are not recommended for use in kitchen areas. For kitchens, always install a single heat sensor heat alarm.

A word about mains powered smoke alarms

Conforming to BS 5839 Pt 6 grade D, mains powered smoke detectors and heat alarms contain a back-up battery. In the event of a mains failure, the back-up battery ensures that the alarm will continue to power until the mains feed is restored. Safelincs offers a range of mains powered alarms, where customers can choose from standard alkaline, non-rechargeable lithium upgrade or sealed in lithium back-up battery. Mains smoke and heat alarms with an alkaline battery back-up provide the cheapest (up front cost) solution. Depending on how often the alarms are tested, the back up battery will last approximately 12-18 months before replacement is required. Manufacturers recommend for the battery to be replaced every 12 months to ensure a low running battery is not forgotten. Mains powered smoke alarms have the option to upgrade to non-rechargeable lithium back-up battery. Although not sealed in the alarm, the battery will last 10 years before replacement; with the exception of when used with radio interlinked alarms (2-5 years lifespan). Finally, mains powered alarms are available with a sealed in rechargeable lithium back-up battery. This battery is designed to last the life of the alarm (10 years) and can not be tampered with during its lifespan. As well as saving costs from replacement, the sealed lithium battery option is ideal for rented properties where the removal of batteries or leaving them to run flat could be an issue.

To obtain a mains feed, smoke detectors and heat alarms can be wired to the nearest lighting circuit or through a dedicated circuit from the distribution board. Power supply from the light fitting is generally preferred as it is more cost-effective and less likely to be disabled to stop the "nuisance" of beeping alarms should the back-up battery run low.

Whilst mains smoke detectors give you peace of mind for many years without false alarms or defects, it is dangerous to assume that they work forever. Smoke and heat alarms should be replaced after ten years, as they then start to become unreliable.

You will typically find the following smoke alarm requirements:

  • Domestic, owner-occupied housing: BS5839 part 6 Grade F
  • Extensions of domestic, owner-occupied housing: BS5839 part 6 Grade D
  • Small businesses, many let properties: BS5839 part 6 Grade D with additional manual break points in circulation spaces and sometimes additional Grade F smoke or heat alarms within let flats.

What do the different Grades mean?

Grade D

Mains-powered, interlinked smoke detectors and heat alarms. These are usually powered with a cable from the nearest light fitting. To create an interlink (all alarms go off at the same time in the case of a fire) they either require cable connection between the units or a radio-interlink connection. In businesses and larger let properties, these are often combined with manual break points.

With the appearance of ten year sealed smoke alarm units with radio-interlink, these are becoming more acceptable as an alternative to mains-powered, interlinked smoke alarms. Radio-interlinked manual break points are also available. However, we still recommend you check with your Building Inspector (in case of extensions), Local Council (rented accommodation) to ensure these alternative alarms are acceptable.

Grade F

Battery powered, free standing smoke detectors and heat alarms. On their own, they are mainly designed for domestic, owner-occupied housing. However, F Grade alarms with radio-interlink (especially when provided with 10 year sealed lithium batteries) can sometimes be applied as if they were Grade D. Again we recommend that you check first with the relevant authority.

Recommended reading

  1. LACoRS guide: Housing: Fire Safety Guidance on fire safety provisions for certain types of existing housing
  2. Fire Risk Assessment Guides
  3. A guide to BS 5839 Part 6

Reviewed: 04/09/2017 (doc:36 V1.0). Our articles are reviewed regularly. However, any changes made to standards or legislation following the review date will not have been considered. Please note that we provide abridged, easy-to-understand guidance. To make detailed decisions about your fire safety provisions, you might require further advice or need to consult the full standards and legislation.