Innovative Energy Engineering

Variable Air Volume - VAV

Variable Air Volume systems are an advancement of less efficient Constant Volume (CV) systems. CV systems cool a constant airflow and re-heated the flow in each zone to meet the demand. Fan energy use is high and re-heat is inherently wasteful. Imagine driving a car full throttle and regulate the speed by applying the brakes. VAV systems modulate the airflow in each zone by means of actuated dampers and air-flow measurement stations (in a VAV box or air valve). The fan speed is adjusted to meet a pressure point. This pressure setpoint is re-set based on actual demand for flow (i.e. damper positions). VAV reduces fan, refrigeration and re-heat energy compared to CV systems. Under part load noise will be reduced and with load diversity the fan, AHU and refrigeration plant can be downsized. If designed and controlled properly, VAV systems can be more efficient than most of other system types. VAV systems can be easily combined with geothermal systems.

Relief fan (or just relief damper) systems typically are more favorable for VAV systems due to often lower capital and operating cost. Return fans may be necessary in high return duct pressure drop systems.

Heating coils in AHU and at Terminal Units?

Heating performance of VAV systems is limited since most diffusers are optimized for cooling and supply from above. Buoyancy of warm air prevents proper heating and ventilation with warm air. In fact increasing DAT can reduce heating performance. If DAT is 10°C (15°F) above room temperature, the Outside air Rate needs to be increased according to ASHRAE 62.1. In general, heating only should be performed by VAV systems in moderate climates. Perimeter or in-floor radiant heat is required if the wall load is above 250 Watt per linear meter (250 Btu/h per linear foot) or when ceilings are high.

In heating season the air supplied by the AHU is a mixture of warm Return Air (RA), and preheated (by ERV) Outside Air (OA). Unlike older systems without ERV, this keeps temperatures high enough to not require heating. As long as the space heater i.e. (perimeter heat) is sized to account for the ventilation load, there is no need to add heating coils in AHU or terminal devices. This reduces pressuredrop and chance of freezing. In systems with water Cooling-Coil (CC) , a freeze-stat needs to be located downstream of the CC or the coil drained during heating season.

Plenum or Duct Return?

Plenum return typically uses less space, less fan energy, and is cheaper to install as it requires less ductwork. For relief in economizer operation often a relief damper suffices instead of a relief fan. Relief dampers can be located in many locations in the building, which increases flexibility. A case for ducted return can be made if the return plenum is likely to experience a lot of infiltration or if the it consists of combustible material (i.e. wood trusses) and contains non-plenum-rated equipment and wiring.

Dedicated Ventilation?

Dedicated ventilation, if not reheated, can cause discomfort due to over-cooling. Unless one equips the dedicated ventilation system with VAV terminal devices, sophisticated ventilation reset cannot be performed. Overall energy consumption and first-cost will be higher. Using a DOAS to pre-treat and dehumidify Outside air (OA) and feed into the AHU is a better option.

With the outside air already de-humidified in the DOAS, the AHU can employ more aggressive Discharge Air Temperature (DAT) reset to minimize reheat. Outside Air Temperature (OAT) determines a temperature band. Based on zones calling for cooling or heating, DAT setpoint increases or decreases within this band. This will require some adjustment as humidity is a concern if DAT is too high. If an AHU is connected to zones that require heating and cooling simultaneously (i.e. core and perimeter zones), DAT likely will be cold enough to require re-heat. Therefore it is recommended to separate AHU for zones that most likely require cooling or heating at the same time.

Air Economizers and Building Pressure

Airside economizers use colder or less humid OA to pre-cool the air to eliminate or minimize the cooling coil load. The economizer can be enabled based on dry-bulb temperature when OAT is 8°C-12°C (15°F-20°F) colder than Return air (RA). This occasionally can lead to slight energy waste when OA is more humid as the dry-bulb economizer does not take humidity into account. In theory enthalpy economizers solve this problem as they compare enthalpies of OA and RA and therefore always would save energy. But in reality enthalpy sensors are not accurate enough, seldom get calibrated or replaced as required and end up using more energy. Therefore drybulb economizers are recommended.

Because economizers bring in air, it pressurizes the building to the point of problems with door operation. This air needs to be relieved to control building pressure to acceptable levels. Many theories exist on what building pressure to maintain. In practice all theories fail due to bad sensor location. In theory one needed 4 outdoor sensors, to average wind direction, and multiple indoor sensor since spaces themselves are pressurized or under-pressurized. In reality thee will be one outdoor sensor and one indoor sensor, often in a pressurized or under pressurized area. It is best to balance all ventilation and exhaust flows around the ERV and to control the relief fan and damper only during economizer operation.

Building pressure control would be required to maintain a pressure between spaces with different contaminations. Measuring flow into the space and measuring the flow out is a good (if expensive) way to control critical spaces, such as labs. In less critical spaces where an exact pressure difference is not required (i.e. keep garage negative to office), some added exhaust flow is sufficient to accomplish the goal reliably.

Relief in VAV systems should be provided by relief dampers or relief fans in ducted return systems. Use of return fans is is appropriate in constant volume systems, but in VAV systems those are less efficient.

Fan-powered VAV boxes

Sometimes fan-powered VAV boxes are used to improve ventilation and heating performance. They have many disadvantages and should be avoided. Modern motors and controls reduced, but not eliminated some of their inherent problems. Fan-powered boxes add significant cost due to the fan, motor, added controls and electrical supply. They add maintenance (filter, repair) in occupied spaces. Noise levels and energy consumption increases due to the constant volume nature and the less efficient smaller fan and motor.

Zone Control

In zones with highly varying occupancy (i.e. conference rooms) minimum flow-rate should be reset based on space CO2 level. Spaces with operable windows should disable the zone when a window is open (provide heating only when temperature is near freezing). Motion sensors should put the zone in a "standby" mode when no motion is detected. This should reduce minimum flow rate to "0" (as read on flow-station, which still allows 5-10% flow) and widen the deadband by 1-2°F.

Minimum flowrates for both cooling and heating season should be properly calculated. Many designers just assume a 30% minimum flow, which isn't necessarily correct.

Supply Temperature

Traditionally cooling supply is at around 13°C (55°F). this rule of thumb may not be optimal for all situations and based on latent load and sensible loads design temperatures could be 10° - 14°C (50°F - 58°F) or even outside this band. The space comfort temperatures can also be adjusted to optimize airflows (e.g. higher space temperature at lower humidity feels as comfortable as lower space temperature at higher humidity). In addition supply temperature should be re-set based on actual cooling demand. Based on number of zones calling for cooling the supply temperature will be increased or decreased within a band. Upper and lower limits of this band depend on ambient temperature. In order to be able to cool interior zones in cold weather, those space air flow rates could be designed for a higher supply temperature.

Airflow Measurement

the cheapest VAV terminal devices are Pitot-tube style VAV boxes. Disadvantages include need for field calibration (with all inaccuracies based on unskilled trades), low accuracy at low flow conditions and bad inlet conditions, and susceptibility to dirt and tube leaks. Once these VAV boxes fail or become inaccurate (which is only a question of time), the VAV system loses its ability to measure airflow and malfunctions. This can be unnoticed for a while because the system could work as a very inefficient constant volume system. A better solution are air valves with Vortek flowstations. At a slightly higher price the air valve does not require field-calibration, can be used in dirty air flows, measures very low flows and is less sensitive to inlet conditions.

only search this site

Information about: