The HVAC Factor: Double Duty

By Tormod Larsen
From the June 2014 issue of Today’s Facility Manager

The proliferation of smart phones and tablets, coupled with the general acceptance of Bring Your Own Device (BYOD) in offices, has created a new challenge for facility managers (fms) and property owners alike. Users have come to expect mobile connectivity everywhere, especially where they work and live. It’s no longer enough to provide cell service coverage in a building. Now, fms face the bigger expectation for high-bandwidth, seamless data connectivity throughout a facility. Delivering mobile connectivity in high-rise, mixed-use buildings is no small task for fms. In large part, this is a data tsunami that fms have to gear up for in order to satisfy occupants.

Conventional approaches to implementing indoor wireless systems involve installing antennas throughout the interior of a building, which can be costly, unsightly, and time consuming. Practical impediments, such as lack of agreement among owners, managers, and occupants/tenants, can further slow the process and hinder performance. What if that part of the process could be bypassed by taking advantage of infrastructure already in place?

HVAC diagram
The existing HVAC system may be the path to ubiquitous indoor mobile connectivity. As the diagram illustrates, existing infrastructure—ducts and diffusers—serve as RF waveguides, the basis for a distributed antenna system. (Diagram: ExteNet Systems.)

In many cases, the existing HVAC system in a facility may hold the key to delivering the outstanding mobile connectivity users demand. HVAC duct systems are designed to distribute air evenly throughout a facility. In similar fashion, they can serve as a giant waveguide to distribute radio frequency (RF) signals and enable a wireless distributed antenna system (DAS).

HVAC systems work well in DAS applications for several reasons. First, the antennas do not affect building aesthetics as they are located within the HVAC ducts. And the metal in HVAC ducts enables them to perform well as waveguides. Lastly, HVAC systems are typically divided into zones based on the occupancy density, which is also a consideration for RF signals. HVAC systems are already designed to provide ubiquitous service to an entire building.

In order to determine if an HVAC based DAS is a suitable solution for a facility, there are a variety of factors to consider, and these include duct size, type, and system configuration.

Primarily, this approach boils down to physics: The lower frequencies used for mobile connectivity require larger ducts. The science behind this concept is Maxwell’s general equation for electromagnetic waves. Therefore the size and shape of the ducts are evaluated using specific calculations to see if they are suitable for DAS. Other duct characteristics can also affect RF signal propagation, including duct branching, duct bends, and changes in duct size.

Next, the architecture of the HVAC system is evaluated to see if it supports network design requirements. Designers need to consider the layout of the ducts, placement and type of air handling units (AHUs), and zone coverage. For example, centralized AHUs may provide more flexibility for probe placement while distributed AHUs may provide stronger signal levels. This helps to determine the ideal probe and antenna placement to propagate RF signals efficiently and effectively throughout the system.

The configuration of the HVAC zones is also important to planning a DAS. The way the zones are laid out will drive how designers will identify sectors and design nodes. This will help match coverage to current capacity needs and plan ahead for additional nodes and increased capacity over time.

Taking all of these considerations into account, a DAS can be designed to suit a variety of needs. There is flexible equipment for specific situations. For example, different types of probes and antennas provide different frequency ranges, directivity, physical dimensions, connectors, and performance. By mixing and matching equipment according to the specifics of a facility, designers can minimize variability and path loss and increase coverage. As with any technology, testing is key.

If the HVAC approach is determined to be a good fit for a facility, it can deliver solid benefits. By leveraging existing infrastructure, it is less intrusive and creates less disruption overall. That translates to cost savings, which can typically be between 30% and 50%. In addition, it can be implemented quickly, in roughly half the time of a conventional DAS. And DAS is carrier neutral; a variety of carriers, including AT&T, Verizon, T-Mobile, and others, can provide service in the building.


For buildings that have employed an HVAC based DAS, the cost and time savings make it feasible to provide mobile users the ubiquitous connectivity, robust reliability, and uniform performance they have come to expect.

Larsen is vice president and chief technology officer for ExteNet Systems. The company designs, builds, owns, and operates distributed networks in key strategic markets for use by wireless carriers and venue owners.