Pacific Northwest
National Laboratory
Operated by Battelle for the
U.S. Department of Energy

Wireless Sensors and 
Controls for Buildings

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Cost Comparison Between Wired and Wireless Designs for In-building Temperature Sensors

Cost Comparison Between Wired and Wireless Designs for Packaged Rooftop Systems

Cost-effectiveness Discussion for Retrofit and New Construction

Representative Wireless Data Acquisition System Costs

The table immediately below provides representative costs and characteristics for commercially available wireless data acquisition equipment in Spring 2002.  This information is based on an  informal preliminary survey of appropriate wireless technologies.  It is not an exhaustive survey.  For the serial communication products, maximum communication distance is a significant cost variable.  Products with lower communication ranges are significantly less expensive than those that transmit over several miles (rows 3 and 4). The Bluetooth product listed represents currently available products.  The cost target for a Bluetooth radio chip is significantly lower and assumes mass-production for consumer products.

[1]:  FHSS: frequency hopping spread spectrum
[2]:  DSSS: direct sequence spread spectrum techniques

In-Building Systems

For the cost comparison estimation for in-building temperature sensors, we considered a wired system design with in-plenum wiring.  The cumulative wiring distance for all temperature sensors is about 3000 feet with the majority loose in-plenum wiring.  Eighteen AWG cable is assumed for sensor connections at an approximate cost of $0.07/ft. and a labor cost of $1.53 per linear foot of wiring (RS Means 2001).  The cost comparison is shown in the table below.  The cost for the wireless system includes an assumed installer mark-up of 50%.  For the radio frequency (RF) surveying and RF installation we estimated the labor rate of $100 per hour for an engineer.  Omitted in the cost comparison is the cost for the sensor configuration in the Johnson Controls Metasys network, which is assumed similar, if not equal, for both the wired and the wireless designs.  For simplicity, the labor cost for battery change-out, expected to occur every 5 years, is not included.  This activity can be estimated at about $300, assuming a battery cost of $3 per battery and 2 hours of labor for replacing 30 batteries.

Rooftop Systems

The table below shows the system costs for a wired base case and wireless systems configured from commercially-available components.  The key cost differences between the wired system and the wireless systems are attributable to the communication components.  For the wired case, cable and conduit must be installed to each HVAC unit.  For the wireless case, the cable and conduit are replaced with RF transmitters and receivers.  Three HVAC units, located as shown in the diagram below, are monitored.

The results show that low-cost wireless data collection has advantages over wired data collection; however, the high-cost wireless solution is not competitive.  Greater numbers of HVAC units will generally increase the cost-effectiveness of wireless data acquisition because distances to the units will decrease on average.  In addition, the cost of the system receiver is allocated over more sensor points.  Regardless, the low-cost wireless solution still maintains a cost advantage.

Retrofit and New Construction

The cost-effectiveness of wireless sensor systems in buildings compared to wired systems depends on many factors.  We define cost effectiveness as the ratio of capital cost for a wireless system to the capital cost of a wired system (Cost_wireless /Cost_wired).  A ratio of less than unity indicates that wireless technology is more cost effective.  For this comparison, only the cost associated with the transport of a signal over a given distance is used.  We do not consider the cost of other components (e.g., sensors, controllers, and actuators) that are common to both wired and wireless systems. 

The cost of the wired system depends primarily on two key factors:  1) the difficulty routing wires and the need for shielding and wire support to meet code requirements and 2) the distance.  For simplicity, we neglect the effect of different wire material.  In general, the installation of wiring in new construction is less difficult because of the relatively easy accessibility to routing channels.  As a consequence, we assume the wiring cost to be lower for new construction than for retrofit installations.

The key drivers for the cost of wireless systems are the signal attenuation and signal-to-noise ratio.  In general, we find that the higher the attenuation in a building, the more repeaters that are required.  The cost model for the wireless system used in this analysis corresponds to the serial frequency hopping spread spectrum (FHSS) technology shown in the table of system costs at the top of this page.  In addition we estimated cost for the integration into a wired system  (e.g., a BAS) at $500.

The cost effectiveness ratio  (Cost_wireless /Cost_wired) is then a function of distance, installation type (retrofit versus new construction), and number of repeaters.  The figure below shows this relation. 

Click on image for large view.

Consider the points A, B, C, and D in the figure representing different cost ratios for a constant length of 3000 ft for the wiring.  For the retrofit example, we establish a wiring cost of $6,600, assuming a cost per linear foot of $2.20 including wires.  For new construction, we assume a reduced wiring cost (because of easier access) in the amount of $2,010 for a cost of $0.67 per linear foot.  We assumed that wiring conduits already exist and thus, the wiring cost excludes the cost associated with installing conduits.  Point A (cost ratio=0.3) represents the cost competitiveness of a wireless system in a retrofit case with no repeater necessary.  Point B (cost ratio=0.9) represents the cost for a building with high attenuation characteristics, requiring 10 repeaters.  Corresponding points for new construction are C (cost ratio=1.0) and D (cost ratio=2.9).

While this cost-effectiveness analysis is simplified, it illustrates the sensitivity of the key drivers for wireless technologies in HVAC applications.  It indicates that early adopters of this technology will implement wireless devices most likely where they are cost effective, in existing buildings that do not pose difficulty in transmission of the RF signal.  Likely applications are rooftop connectivity with line-of-sight transmission and applications in light construction that do not require repeater devices.  Commercially-available wireless data acquisition in new construction is not yet cost competitive with wired systems.  Today’s wireless technologies are still expensive for universal use for data communication in building operation.  With lower costs for wireless technology and increased availability for products to integrate wireless networks with building automation systems, wireless technologies should become an attractive solution for HVAC control networks, coexisting and augmenting wired systems.