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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 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. Their quick reaction to small particles can make the ionisation smoke alarm prone to false alarms, particularly when located near kitchens or open windows. 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 known as photoelectric detectors, are less prone to false alarms than ionisation models and 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. 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, responsive to the ambient temperature rather than smoke, are ideal for installing in kitchens. Featuring a fixed temperature thermistor, they are triggered when the temperature inside the room reaches approximately 55°C. This specific heat sensing technology avoids false alarms altogether, as the unit does not react to cooking fumes and steam. Some models are also designed to detect a rapid change in temperature, known as rate of rise technology, so that they can sound an alarm for fast-flaming fires even before the static temperature level has been reached. 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.
Combined alarms with multiple sensors are available with several different combinations that could be useful due to the varied nature of our homes. Mixing smoke detectors with a rate of rise heat sensor allows for improved detection of fast-flaming fires and a reduced risk of false alarms, while adding a carbon monoxide sensor to a smoke or heat alarm will help protect you from this deadly gas which human senses cannot detect.
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 that although some combined alarms contain a heat sensor, these models are not recommended for use in kitchen areas.
Conforming to BS 5839 Pt 6 grade D1 and D2, mains powered smoke detectors and heat alarms contain a back-up battery and interlink with each other so that all alarms will sound when one detects a fire. 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.
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.
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 and how many times they activate (both real and nuisance alarms) 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.
Some mains powered smoke alarms have the option to upgrade to non-rechargeable lithium back-up battery, but you should confirm in the product manual as lithium batteries behave slightly differently and not all alarms can work with both. Although not sealed in the alarm, a lithium back-up battery will last 5 to 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, but care must be taken to ensure the light switch does not also cut power to the alarms.
You will typically find the following smoke alarm requirements:
Below are short summaries of alarm Grades D and F as well as the 3 Categories of system for domestic properties. Please see our guide to BS 5839 Part 6 for more details about domestic alarms, or our summary of BS 5839 Part 1 for non-domestic premises.
Grades D and F were both split into two parts in the 2019 update to BS 5839-6. A number 1 is for sealed, tamper-proof batteries and a number 2 is for user-replaceable batteries.
Mains-powered, interlinked smoke detectors and heat alarms. These are usually powered with a cable from the nearest light fitting as long as the light switch does not also cut power to the alarms. 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, separate from the power circuit, or a radio-interlink connection.
Some building control officers or local councils may accept Grade F1 alarms with radio-interlink in place of mains-powered alarms, but you will need to confirm this before starting work to ensure such alternative alarms are acceptable.
Battery powered smoke detectors and heat alarms which can be easily installed with minimal disruption and no need for an electrician. Most are standalone products but some may have wireless interlink capability via radio frequency, and a select few are available with hardwired interlink.
Reviewed: 04/02/2021 (doc:36 V2.0). 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.
