The Control Zone

Whether its control over profit earnings or over use of the copy machine, maintaining control over systems, operations and equipment is a struggle for corporations around the world. While engineers may not be able to solve these larger "issues" of control, they certainly can address issues of HVAC control.

Consequently, when it comes to getting a better handle on HVAC—both from a comfort and energy-savings perspective—zone control is an effective way to achieve greater efficiency.

The key technologies that allow zone schemes to work most effectively—both in large areas and small—are variable-air-volume (VAV) and direct-digital-control (DDC) systems.

Variable-air systems allow for variable airflow to be delivered to individual zones, with a capability of delivering air in proportion to a zone's current sensible cooling or heating load.

The advantages of using a VAV system are twofold. The first advantage is that office space temperatures can be controlled within acceptable limits over widely varying internal and external heat gains caused by factors like weather conditions or exposure to the sun. The second advantage is reduced energy consumption.

The former benefit contributes to occupant comfort and productivity, whereas the latter is attractive to owners and building operators. In sum, properly designed VAV systems show favorable payback periods, increased levels of space temperature control and the ability to perform most maintenance activities outside the actual office space.

Designers traditionally select zones by grouping adjacent rooms that have similar cooling load profiles. If a zone contains private offices, it is possible that up to eight offices can be served by one VAV box. The exact location of the box's controlling temperature sensor is selected to reside in one of the spaces, often based on the status of the office occupant.

A drawback to this particular strategy is that while the office with the sensor will maintain a temperature close to the setpoint, those offices without the sensor may not. Consequently, the temperature or flow rate of air to the other spaces depends on the balancing dampers serving each space.

This problem can be resolved by increasing the number of VAV boxes and local controls. In the example above, two VAV boxes, each serving four spaces, could be installed to provide a smaller control zone and overall improved comfort level. But, adding equipment can result in a higher cost. Fortunately, modern controls technology offers even more effective alternatives. Over the past few years, significant improvements have been made in DDC and its accompanying software. These advances, combined with the greater acceptance of BACnet, and the increasing trend toward industry standardization, have resulted in more powerful building-automation systems.

Today, it's expected that a BAS will come equipped with the capacity to manage programmed control sequences devised to improve office environments, reduce energy consumption and monitor a broad range of office setpoints—temperature, airflow rate, CO₂ levels, lighting and sound masking, for example. It should also be able to troubleshoot setpoint variances and help monitor preventative maintenance programs.

Take something as simple as the installation of an occupancy sensor in each room as another example. Not only will this device save energy by switching off lights when the space is unoccupied, but the sensor can also signal the HVAC control system. In other words, if temperature sensors are installed in each office, the DDC software can use the occupancy and temperature information to reset the VAV box setpoint to the average temperature of the occupied offices. Consequently, the control system is democratic in that it only allows occupied offices to vote for a preferred setpoint temperature (see figure on p. 38).

Properly zoned, open-plan areas usually have the advantage of room air mixing, which averages the cooling or heating over the entire space. However, problems can arise in open areas that are densely populated, either by people or equipment. Providing smaller zones can usually control these hot or cool spots.

This concept of smaller zones can be carried all the way down to the diffusers. A design variation used successfully for many years is VAV diffusers. These units are modulated by either self-powered thermal-sensing elements or electric motors, and can be controlled by the DDC system. Static pressure in branch ducts serving multiple diffusers must be held reasonably constant to achieve good control. Keep in mind that the VAV diffuser system effectively provides local temperature control, but may tend to be noisy from an occupant's perspective.

Correspondingly, a group of rooms that have varying occupancies, space cooling profiles and heating loads or seasonal changes should not be grouped together in the same zone. Good practice requires that spaces such as conference rooms—or rooms with unusual exposures like corner offices—have individual zones.

Smaller zoned systems such as water-source heat pump and VAV systems are generally the most prevalent and are fairly easy to modify for renovations. A technique for bringing supply air at the floor level to directly serve a desired zone is to use an underfloor air-distribution system. In this scheme, an air outlet specifically designed for the application is located close to the occupant. In addition, less costly systems use a low-pressure constant airflow rate, with manually adjustable devices in the outlet that the occupant can manipulate to control the volume of air to suit individual preferences.

Underfloor VAV applications are relatively new on the market and employ an automatically regulated terminal mounted in a floor tile with controls and actuators that respond to the zone space temperature. Reheat coils can be used to provide comfort for zones on the building exterior. The system is flexible because additional outlets—installed in floor tiles—can easily be used to boost local cooling or heating, and can be relocated when requested by the occupant. At the same time, underfloor VAV systems tend to have higher first costs. Thus, the system's cost savings are only found with clients who have high reconfiguration or "churn" rates because they are easy to renovate.

A variation of the underfloor system that introduces air at the desktop has also been developed. With at least one proprietary system on the market and others in development, these microclimate HVAC systems are designed to provide workers with individual control integrated into their workstations (see "A Prototype of Comfort," on p. 37).

The system is comprised of a local control unit in each workstation combined with an occupancy sensor, allowing the occupant to manipulate his or her environment; a fan unit, connected to two adjustable diffusers located on the work surface; and an optional electric radiant panel, mounted below the work surface.

Conditioned air is introduced to the fan unit from a raised floor or from ductwork in the ceiling space of the floor below. Using the control unit, the worker can adjust his or her office temperature, task-lighting levels and background-masking, white-noise level. The integrated occupancy sensor can contribute to energy savings by shutting down the workstation if it is unoccupied beyond a programmable period.

The workstation control units can also be connected to a BAS for additional benefits such as morning warm-up and cool-down; workstation shutdown for extended absences; lighting control; and monitoring local space temperatures—which in turn adjust the main system discharge air temperature.

Traditionally, the ventilation air for office environments was provided at a constant flow rate to the spaces based on the maximum occupancy, whether the occupants were there or not. In addition to controlling the airflow rate and temperature, the institution of reasonably priced, stable and accurate CO₂ sensors allows effective monitoring in individual zones. CO₂ sensors, when coupled with occupancy sensors, can be utilized to maintain the required ventilation rates in environments with fluctuating populations, such as conference rooms, classrooms or training rooms.

A relatively new design concept known as "demand control ventilation," maintains the CO₂ level—and thus ventilation rates—within the ASHRAE 62-2001 standard control range. In other words, only when controls sense the room occupancy level has increased—based on the CO₂ level—will the outdoor-air ventilation be increased. If the occupancy level declines or the space is unoccupied, outdoor-air ventilation will be reduced or stopped. In this arrangement, the cost of monitoring is compensated by the greater potential for saving energy.

Sensors for controlling, monitoring, troubleshooting and trending have a common goal: to minimize energy use while maintaining environmental comfort within an acceptable range, conducive to good productivity, worker contentment and health.

There are many factors that come into play when choosing an HVAC system and its controls: first cost, life-cycle cost, energy savings, personal preference, company standards, ease of maintenance and user satisfaction.

The cost of electronic-based zone controls and sensors has steadily dropped, while the number of variables that can be effectively monitored—such as occupancy, CO₂, light, humidity, temperature and sound—has increased.

The programming capabilities can now allow many advanced functions to be included at a reasonable cost, thus providing a system that can be more responsive to the occupant and building needs.

While control issues will inevitably exist in the business world, the evolving design of HVAC systems will, at the very least, allow for the pursuit of business in a pleasant office environment.