Control Variables

CONSULTING-SPECIFYING ENGINEER: Oftentimes, only a small percentage of a project's cost is allocated for building system controls. Does this pose challenges for controls design?

STANKE: A low budget for controls may or may not make it difficult to do what the design requires. One has to consider the type of project. For example, a simple control system design for a simple building with simple automation may be executed quite well at low cost. In fact, small buildings with rooftop units are prime candidates for variable-air-volume (VAV) systems with pre-engineered, factory-installed controls.

On the other hand, a more sophisticated control design for a more complicated building is less likely to be executed successfully if, for instance, control capabilities are diminished or deleted during the value engineering process.

Packaged equipment with pre-engineered controls, designed to integrate with pre-engineered BAS, may offer the most cost-effective way to provide the benefits of VAV. As more and more building designs opt for unitary products, use of pre-engineered rooftop/VAV systems continues to increase.

MUMMA: I agree that low budgets make many building projects difficult to execute well, and controls systems are no exception. However, it doesn't mean that the difficulties cannot be overcome. When the engineering community makes a good case for energy savings and improved indoor air quality through better design and control, experienced investors budget more for controls.

STOWELL: The percentage of the allocation isn't critical; owner education is. The best way to address how to balance the budget with controls system costs is to show the owner how to leverage what they've already got.

LIZARDOS: A project that is driven by first cost almost always means packaged equipment with factory-mounted controls. With packaged equipment, it's important to specify open-protocol, stand-alone direct digital controllers so that in the future, a communication bus link can be installed to an energy management head-end. This will allow for centralized and off-site monitoring and alarms, as well as adding energy savings control features. This head-end enhancement feature can also be treated as an add-on or deduct from the project costs.

STANKE: For larger buildings, perhaps standardized, detailed and proven control specifications, which clearly present control system requirements, would help designers and control contractors fend off attacks during the cost-cutting phase of the project. And, of course, whenever controls can be made simpler to specify, install and apply, the design becomes more cost-effective.

CSE: So, what are the main roadblocks to implementation of more effective building control systems?

MUMMA: Financial.

LIZARDOS: Money's the easy answer. But more specifically, it's the cost of software. If we could reduce software expenses in the same way we have reduced hardware costs, we could implement more effective building control systems.

STANKE: Time is another roadblock. Due to market pressures, or perhaps to custom control design complexities, designers are often unable to include sufficient time in their project cost estimates for effective control-system design. It takes time to design a custom control system for a building, but in general, the building design market either cannot—or will not—support the design fees required to execute well-thought-out, cost-effective building control systems.

CSE: What role does VAV play in BAS strategies? Have VAV technology and its implementation shown any recent improvement?

LIZARDOS: One change is the use of adjustable-frequency drives in lieu of inlet vanes. ASHRAE 90.1, which either limits the length of unzoned fin-tube radiation at the perimeters or provides heating coils at each perimeter or top-floor VAV terminal, is another improvement.

STANKE: A VAV system undoubtedly adds sophistication and complexity to building controls, when compared to a building with multiple, constant-volume rooftop units or a building with water-source heat pumps in each zone and a central dedicated outdoor air handler. VAV requires system-level coordination between the central air handler and the VAV terminal units, as well as fan capacity control, part-load ventilation control and building-pressure control, especially during economizer operation.

It was mentioned earlier that VAV control technology is available and often pre-packaged in the BAS. Controls can be used to reset fan capacity based on box position; calculate ventilation requirements and reset intake airflow based on current CO2 levels or system ventilation efficiency; and modulate relief airflow to maintain appropriate building pressure in all operating modes.

STOWELL: In the North American market, VAV systems continue to play an important role in building strategies because of their ability to manage these four issues: ventilation, building code compliance, tenant comfort and energy savings. Recent improvements by engineers in VAV box equipment allow individual speed control of fan-powered terminal boxes for an entirely new level of building control.

MUMMA: On the other hand, VAV has a number of inherent problems that have been with us for more than 30 years.

• Poor air distribution.
• Poor humidity control.
• Poor acoustical properties.
• Poor use of plenum and mechanical shaft space.
• Serious control problems, particularly with tracking return fan systems.
• Poor performance as an energy transport medium.
• Poor resistance to the threat of biological and chemical terrorism; and
• Poor and unpredictable ventilation performance.

Relatively recent application of CO2-based demand-controlled ventilation has helped overcome some of the problems of poor ventilation performance. However, dedicated outdoor air systems (DOAS), with appropriate parallel systems to remove the balance of the sensible load not borne by the DOAS, are capable of overcoming VAV's inherent problems. Consequently, the industry is beginning to respond to the DOAS approach. (See "HVAC: The Next Generation," CSE 01/03 p.38)

CSE: In your assessment, how "open" are open protocols? In other words, when it comes down to practical applications, how easy is it to integrate multi-vendor packages?

LIZARDOS: With the advent of BacNet and LonMark, open protocol systems are a reality. However, until the front-end control vendor invests in creating the software interface to read the competitors' control package, the process is hindered.

STANKE: While multi-vendor system integration has become easier with open protocols, it's unrealistic to expect the plug-and-play simplicity of personal computers. It's just not that easy in the world of custom-built BAS. So instead of hoping for an all-inclusive protocol solution, designers and building owners should focus on finding a supplier of controls, HVAC equipment, or both, with a broad range of products, systems and capabilities to flexibly meet control system needs using a range of protocols. Consequently, owners purchase VAV systems, not merely a set of VAV components. Very few owners want VAV box controls from several different suppliers because they do not want to order and maintain several sets of service parts, set-up tools and so on.

The appeal of open systems is the expectation that they will deal with legacy building automation systems and controls, including non-HVAC sub-systems such as security and lighting, and that they will provide insurance against being "locked in" to one control system vendor by providing the ability to select alternate suppliers in the future. However, these are still largely expectations only. For now, owners find that the reality of single-source supply, including parts, support and training, outweighs the promises and appeal of truly open systems.

CSE: How can engineers and manufacturers better appeal to building owners to invest more in controls and VAV?

LIZARDOS: Explain to them how better control strategies yield better comfort conditions and reduce operating costs.

MUMMA: It's important to demonstrate that the operating costs and worker productivity can be improved with better controls. As for VAV, the designer must explain how the inherent problems that I enumerated earlier can be solved.

STANKE: Additional investment in controls may raise productivity as a result of better comfort and ventilation. It may also lower operating costs by reducing fan horsepower, reheat requirements or intake airflow. In addition, it may lower maintenance costs by centralizing fans, cooling coils, drain pans, filters and so on. If these benefits can be estimated accurately, payback periods can be calculated for the additional investment in VAV systems and controls. Sufficiently short payback periods appeal to owners.

Good economic analysis models, such as DOE 2 or TRACE, can help designers to accurately estimate the value of system configurations and control approaches. They can help to show building owners when a small additional investment into controls can quickly pay for itself and continue to result in reasonable financial return-on-investment.