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RF Controls – The Way Forward
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Grasslin:Tower has been supplying quality heating controls for more than 30 years to worldwide manufacturers in both the electrical and heating industries. In recent years they have been seeing rapidly increasing sales through the merchants direct to installers and contractors, and through specifiers for housebuilders, local authorities and housing associations too, who recognise that these controls do exactly what they want – at a competitive price.
The range of products available includes central heating programmers, thermostats – including specially designed versions for underfloor heating systems – and timeswitches, and include both timeclock type controls and the latest RF (radio frequency) wireless controls – and everything in between. In addition they supply a range of twilight and staircase switches and a range of plug-in timeswitches too.
There has been a massive increase in the use of radio frequency controls to provide ‘wireless’ installation in the past couple of years in particular. Building Regulations Part P now requires certification of much of the electrical work carried out in a home – RF systems can be very attractive in this respect, and can minimise or obviate electrical work in a property by pretty much eliminating ‘hard wiring’ requirements.
There has been dramatic growth in the use of RF types of controls in the last two years. This is largely due to the combined effect of the implementation of Building Regulations Part L and Part P. There are two main groupings of RF controls on the market: those which operate at 433 MHz and those which operate at 868 MHz.
433 MHz has been licensed for low-power domestic controls for many years, and consequently this is a fairly busy RF band. Numerous devices, as well as heating controls, operate at this frequency, including garage door opening systems, baby alarms and door bells. Protection against cross-talk – when one system inadvertently controls a different system – is by coding, so that devices on a common frequency only recognise their legitimate ‘partner’, but there are quite a few 433 MHz RF systems which only have a limited number of codes – perhaps just 4 to 16 combinations.
The 868 MHz frequency was more recently released for low power RF controls, and consequently, there is less RF ‘traffic’ at that frequency because there are, for the time being, fewer producers of 868 MHz systems. Also the licensing protocol limits the transmission duty cycle to 1%. This means that for any given period, 99% of the time is transmission free – allowing any other systems to operate without any conflict. Code combinations of the more sophisticated controls also run into thousands or even tens of thousands. The combination of a less intensively used frequency, with 1% duty cycle, and a more secure coding technique, means that 868 MHz systems have a significantly higher immunity to interference or cross-talk from other systems.
There are factors that need consideration when specifying or locating RF controls within buildings. Conventional domestic structures – including most homes – present little or no problem to RF signals for these types of heating control systems. It is not unusual to have two, three or even four dividing walls or floors between transmitter and receiver without any problem. Radio waves move, or ‘propagate’ freely through open air, and will penetrate solid materials, but as they go through things, the signal strength will weaken. The signal strength cannot be increased because regulations define the maximum strength permissible, and with which such systems must conform. Also – depending on the material – radio waves can be deflected or simply stopped. In the same way that visible light is reflected by a mirror, so radio waves will also be reflected by the same mirror.
Consequently installers specifying and fitting RF controls should take into account the presence within buildings of steel re-inforcing materials within walls or floors, foil-backed plasterboard or loft insulation, metallised decorative wallcoverings, mirrors and large appliances with steel casings. These materials should have an impact on the siting of RF transmitters (Tx) and receivers (Rx). For instance, a solid wall surfaced in foil backed plasterboard between Tx and Rx is likely to prevent signals from getting through. Even if there are no metallic barriers, large volumes of construction material can still absorb some of the signal strength. This is particularly true if there is any natural stone in the structure – natural stone frequently exhibits ‘paramagnetic’ properties which increase the ‘opacity’ of such materials to RF. To their advantage however, radio waves at 868 or 433 MHz only require a small gap or opening of 10 – 20 cm respectively in order to get through. Alternatively the radio wave can reach its target by reflection, so that the direct line-of-sight route may be blocked, but the wave can bounce off a surface and thereby reach its target. Generally, through appropriate location of Tx and Rx, and reduction or removal of metallic and construction material barriers, the opportunity for the signal to be received can be maximised.
So RF controls are a great step forward in easy-to-fit, user-friendly controls for heating systems. Whilst there is still a tendancy for specifiers to plump for timeclock type heating controls – we Brits still find change and progress difficult at times – the eventual move towards RF controls seems inevitable and unstoppable!
