Variable-air-volume (VAV) systems with air terminal units have been used extensively in commercial and institutional buildings in the United States for decades. Unfortunately, the optimized design of a VAV system with terminal heat is difficult at best because of limitations inherent in VAV and complications posed by design standards and regulations. One new approach involves the pairing of a dedicated outdoor-air system (DOAS) with a variable-refrigerant-flow (VRF) system. By separating the goal of achieving ventilation rates from the goal of maximizing thermal comfort, we can avoid situations in which the two goals are in conflict and efforts suffer from the resulting compromises. What’s more, we can simplify the design process and find system efficiencies that go far beyond those commonly achieved with VAV systems with terminal heating.
In the simplest VAV system, incoming outside air and return air are mixed in a central air-handling unit (AHU) and then pre-heated or pre-cooled. The tempered air is sent by a supply-air fan to various occupiable zones at a temperature generally suitable for cooling. In each zone, a terminal unit adjusts airflow based on cooling demand. When a zone requires heating, supply-air flow usually is reduced to a minimum setting and heated, typically via a terminal-unit heating coil.
In its simplest form, a DOAS is an AHU dedicated to ventilation, not sized to provide cooling air. DOAS often are supply-only systems with relief to outdoors; however, they also can include exhaust heat recovery. Generally, they are not sized to provide 100 percent air economization (cooling using outside air in lieu of mechanical cooling).
VRF systems use individual high-efficiency fan coils in interior spaces in combination with high-efficiency condensing units that can serve multiple zones. VRF systems can be arranged to provide energy recovery, moving heat from zones requiring constant cooling to zones that sometimes require heating.
ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, provides minimum outdoor-air-flow requirements for design conditions. ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Buildings Except for Low-Rise Residential Buildings, meanwhile, requires some systems to be operated so ventilation capacity modulates to match ventilation load (i.e., demand). Designing systems to meet both of these requirements is complex.
To help match capacity to load, ANSI/ASHRAE Standard 62.1 allows “dynamic reset,” but leaves the details to the designer. A common approach combines system-level outside-air-damper reset with zone-level demand-control-ventilation (DCV) strategies. For VAV-system energy efficiency to be maximized, the system-level intake/exhaust/relief-damper-reset sequence for space-temperature control and building pressure must be coordinated with an air-economization-reset sequence. Such controls can be quite complex, particularly when pressurization between zones and interaction with exhaust systems also requiring variable-volume control are considered. Weather conditions also can have a major impact on building pressurization and space ventilation.
Other common control sequences for large VAV systems include occupied-hours control, optimal start/stop, fan-pressure reset (“critical terminal unit” control), “ventilation optimization”1 (DCV), ventilation space-temperature setback, supply-air-temperature reset, dynamic space-pressure control, economizer, energy recovery, natural ventilation, and interfaces with building lighting controls, smoke detection, fire alarms, and even security systems. The interplay between these control schemes vastly complicates efforts to optimize building energy use.
Further complicating VAV-system design is the need to follow the ANSI/ASHRAE Standard 62.1 ventilation-rate procedure, which asks designers to use the multiple-space equation (MSE) to calculate the “critical” zone, the space driving the overall (system-level) outside-air fraction. Zone-level flows then are changed to meet zone-level requirements. Once the critical zone has been chosen, heating-turndown requirements push designers to over-ventilate some zones by increasing heating minimum settings. This typically changes the critical zone and reduces the overall outside-air fraction at the central-system level, substantially affecting AHU components.
To respond to changes in zone population, dynamic reset of VAV systems, combining zone-level DCV with system-level ventilation reset, often is applied. Ventilation reset is a control scheme by which the MSE is solved dynamically to change system outside-air setpoint. This often is applied with airflow-measuring stations, along with electronics and software to control dampers based on relative airflow, at the central system.
Central-system design usually is based on the static-condition (design) critical zone, while in the real world, zone population varies dynamically along with building HVAC load (internal and weather-related) over the course of a day. Maximizing energy efficiency under these conditions pushes the envelope in terms of building system design and building-operation hardware and software. Some commercial software packages can aid ventilation reset, calculating critical ventilation zone per ANSI/ASHRAE Standard 62.1. Calculating the minimum system outside air, however, usually requires the overriding of some terminal-unit turndowns, which changes the critical zone. This is allowed by ANSI/ASHRAE/IES Standard 90.1 because only a few zone overrides can change a system outside-air setting by many percentage points, saving considerable pre-treatment energy and reducing central-system size. Meanwhile, inputs to these calculations vary with system load, which also can change the critical zone. At the same time, other overrides, such as of supply airflow for makeup to areas with exhaust flows exceeding ASHRAE minimum rates, are necessary. These overrides and simultaneous variations result in the need for iterative calculations involving all environmental variables; such calculations currently are beyond the scope of commercially available software.
Zoning is another limitation. History shows building-construction economics can drive VAV-system designers to combine up to several rooms on a single ventilation and temperature-control zone. Because heating and cooling loads can vary widely between rooms on a single zone, this often leads to discomfort in certain rooms when other rooms on the same zone are not at the design load.
In summary, designing a VAV multi-zone HVAC system can be challenging to say the least. Overlaying the various requirements, exceptions, and system functions results in iterative design simulations that must be re-run whenever a room’s size changes. Complying with all codes while addressing the competing interests of ventilation and energy optimization is theoretically possible,2 and technical solutions partly exist and are being developed. But in practice, designing and redesigning systems with complex, iterative calculations is not very practical, and manual overrides generally do not fully optimize system designs. Meanwhile, designers often estimate “block” (net) load based on building-envelope heating and cooling, considering neither variations in internal and ventilation heating/cooling loads nor airflows needed for ventilation and space pressure control, significantly undersizing or oversizing systems as a result.
Fortunately, a design paradigm is emerging to compete with the old VAV model. This new model can reward us with simplified system design while letting us achieve the increased system efficiencies that energy costs are demanding.