Something in the air

BMS, controls, Trend Controls Systems, wireless
Explaining wireless systems — Chris Monson.

In the first of a series of articles examining issues within the building-controls sector, Chris Monson answers readers’ questions about the application of wireless technology.

How do the costs associated with a wireless controls system compare to a wired one?

The major variable when it comes to installing a building-controls system is the cost of labour.

While the price of the constituent parts of a wireless system may be more expensive than wired products, the cost of installing a wireless system is much lower. For example, the installed cost of just four wireless sensors and one receiver is around 30% less than the average price for fitting four conventional sensors and wiring them back to a controller. If there are 12 sensors the cost is approximately halved, while for 32 the saving works out at over 60%.

With a wireless system there is only one fixed overhead — the receiver module. Numerous sensors can operate alongside this module, so in terms of scaled costs wireless becomes more cost effective on larger projects. Also, additional items such as cable and containment obviously are not required for a wireless installation.

What are the technical benefits of wireless?

Although a wired system is more resilient, a correctly specified, configured and installed wireless system is its technical equal. The main, and most obvious, benefit of wireless is that there are no wires to contend with, and most systems are effectively plug and play.

However, there is a certain amount of risk associated with wireless, and signals are subject to possible disruption. However, some building energy management systems (BEMS) can be programmed to provide an alert if the signal isn’t working properly.

Maintenance also needs to be considered as many of the sensors are powered by batteries that need to be periodically replaced. This should be scheduled into an overall maintenance regime.

How do wireless temperature sensors actually work?

The sensor unit comprises of a thermistor sensing element and a transmitter, which are both housed in a standard wall mounting or plant type enclosures. Potentially up to 50 sensors can communicate with a single receiver unit, and these can be sited up to 100 m away — thereby offering a highly scalable and flexible solution.

In some leading systems the receiver is simply plugged into a RS232 port on the building-services controller. In contrast, the receivers used on some other wireless systems have an individual output for every sensor, which means that the maximum number of sensors per receiver is far fewer, making connection to the controller much more awkward.

BMS, controls, Trend Controls Systems, wireless
Wireless devices in a building-management system can reduce installed costs by over 60% in larger systems.

I’ve heard a lot about ZigBee. What is it?

The first ZigBee style networks arrived in 1998, when many installers realised that both Wi-Fi and Bluetooth would be unsuitable for many applications. ZigBee was originally developed as a standards-based wireless technology to address the unique needs of low cost, low-power wireless sensor and control networks.

It uses the 2.4 GHz radio frequency and takes the form of a wireless mesh network. This means that separate devices communicate with one another via radio nodes that can find one another. This configuration means that entire buildings can be covered. If one node can no longer operate, the remaining nodes can still communicate with each other, directly or through one or more intermediate nodes.

Although ZigBee has become reasonably popular, some manufacturers are still weighing up its benefits, and it could be some time before we see mass adoption.

Are there any types of building where a wireless system is particularly suitable?

All buildings can deploy this technology. However, the main areas of application for wireless sensors for controlling heating, ventilation and air conditioning (HVAC) have been in heritage refurbishments that have been upgraded with modern air-conditioning systems. These types of buildings often require sensors to be installed without disturbing the fabric of the building.

Other successful applications have taken place in buildings where demountable partitioning is deployed and where sensors need to be relocated without involving specialists every time. Also, buildings where innovative environmental solutions are deployed requiring empirical tuning of the environmental measurements — particularly where structures do not lend themselves to the hardwired locating of sensors — are good candidates for this technology.

What is the best way to find out if a building can use a wireless system?

I would always recommend carrying out an audit to make sure that the building structure and how it is used is appropriate for this technology.

The audit can be carried out by locating a receiver and siting a number of sensors where they are likely to be fixed, and then checking that the signal can be picked up. Put simply, if the receiver can get the signal it’s highly likely that the building is a suitable candidate for wireless.

With so much wireless technology in the modern building, sensors are having to become highly sophisticated in order to define what signals are relevant and which ones aren’t. An audit should also identify how many other devices are working in the same frequency band and examine the likelihood of signal clashes occurring.

Chris Monson is strategic marketing manager with Trend Controls Systems.

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