HVAC systems have been rapidly growing more complex, thanks in part to new factors to the equation. Today's systems are expected to meet changing heating and air conditioning needs, while performing at higher levels of energy efficiency than systems installed as recently as 10 years ago. At the same time, they must meet new standards for indoor air quality and communicate with other building systems.

The equation has become even more complex with the recognition of the role HVAC systems play in protecting the health and safety of the building occupants. Aware of system vulnerabilities, facility executives are striving to protect facilities and occupants from the threat of chemical and biological threats, whether accidental or intentional.

The challenge to system designers and facility executives is achieving goals for improved system performance, efficiency and security without making systems unaffordable or unmanageable. At first glance, the challenge may seem tough enough in new system design, and overwhelming in retrofit applications. Fortunately there are options that allow facility executives to reach many of these goals, and to do so without breaking the bank or creating systems that cannot be maintained.

ENERGY EFFICIENCY

The energy efficiency of HVAC systems and equipment has been improving steadily for many years.

New-generation chillers offer peak operating efficiencies that are 25 percent higher than those of only 10 or 15 years ago. Even higher annual operating efficiencies can be achieved by adding variable frequency drives to both older and new-generation chillers.

Similar improvements in operating efficiency can be achieved with boiler systems. New condensing boilers offer an operating efficiency of about 92 percent, as opposed to 75 to 85 percent for older units. On an annual basis, this improvement in operating efficiency reduces energy use by 20 to 25 percent.

Further improvements in boiler operating efficiency can be achieved by adding such features as oxygen trim controls and economizers. Conventional boiler controls are set to provide 10 to 20 percent excess combustion air to eliminate the generation of smoke under a wide range of boiler loads. Oxygen trim controls allow clean operation of the boilers with as little as 5 percent excess air, significantly increasing the boiler's operating efficiency.

One way to improve boiler efficiency is to use a modulating control. Conventional boiler controls shut down the boiler for a short period of time when the load is satisfied. As pressure and temperature fall in the system, the boiler fires. This cycling of the boiler reduces overall operating efficiency. In contrast, modulating boilers automatically vary boiler firing to match the load. This typically results in an increase in annual operating efficiency of approximately 10 percent over conventional boilers.

Economizers can be used to recover heat from the flue gas and transfer it to the boiler feed water, raising the overall operating efficiency. As a rule of thumb, for every 10-degree Fahrenheit increase in feed water temperature, boiler efficiency is improved by 1 percent.

Energy efficiency can also be improved by careful selection of electric motors used in pump and fan systems. Motor efficiency has steadily been improving. While high-efficiency motors are more costly than standard efficiency ones, the additional first costs can typically be recovered in one to two years.

MATCHING THE LOAD

One of the most significant changes in HVAC system design practice today is right sizing. In the past, it was accepted practice to calculate the loads that the system would have to meet, then add anywhere from 20 to 50 percent additional capacity. Several factors combined to make this the long-accepted practice.

One factor was that load estimates used in sizing equipment were just that - estimates. Without today's modeling tools, it was too time-consuming to calculate loads more accurately. Designers would use rules of thumb, then round the numbers upward.

Another factor was the belief that additional capacity provided flexibility. Oversizing systems was intended to offer spare capacity in the event of changes to operations within the building. In practice, however, changes in operations either had little impact on total building loads, or they were so significant that additional or specialized systems had to be installed anyway. The result was that the spare capacity was rarely needed.

Designers also liked to build in a safety factor for system deterioration over time. As dirt accumulated on fan blades or within ductwork, it reduced the possible airflow through the system. Similarly, scaling and corrosion within boilers, chillers and heat exchangers reduced the efficiency and capacity of the equipment. To compensate for these losses, designers simply increased the size of the components.

TARGETING PEAK EFFICIENCY

While having some additional capacity is a necessity, 20 to 50 percent spare capacity is rarely advisable. The additional capacity increases installation and maintenance costs without providing any benefit. Even worse, oversizing in almost all applications decreases operating efficiency. Practically all HVAC equipment operates at its peak efficiency at or near its full-load rating. Operating under part-load conditions can significantly reduce this efficiency.