VAV systems have a reputation as energy hogs, but they can be highly efficient systems. Local codes require control strategies such as fan pressure optimization, ventilation optimization, and optimal start/stop. However, these systems are rarely installed properly, leading to subpar performance. So, what does this mean, why is it important, and how do you go beyond the par performance to have a highly efficient system?
Let’s look at fan pressure optimization first. To start, the building automation system determines the critical zone, or the zone which needs the most pressure. Then, it continually resets the static-pressure setpoint to the value where the damper in that zone is almost fully open. This reduces the work of the fan, saving energy while also reducing noise levels, increasing reliability, and avoiding surges. Energy code also requires that the BAS determine rogue zones – zones where the damper is open more than it should be. Identification of this zone enables building managers to find the cause of the open damper and fix any mechanical issues, saving more energy consumption and cost.
Ventilation optimization or demand control ventilation requires that the amount of outside air delivered by the VAV air handler vary in response to the people expected in the space. This strategy frequently consists of installing CO2 sensors, basing the amount of air needed on CO2 levels, an indirect measurement of the number of people in a space. This is just one method and it’s not always the best. CO2 sensors are expensive and require routine calibration. They are best deployed in areas where the population is very diverse – like cafeterias or gymnasiums. They don’t work as well in offices or classrooms where, for the most part, the population stays consistent day to day. For these environments, a schedule in the BAS or occupancy sensors are simple but underutilized methods for telling the BAS when to vary outside air. Outside air is the most expensive air to condition, which is why energy code requires that VAV systems continuously analyze data. It determines the requirements of the critical zone and the ventilation required by each zone. This data, as well as the expected or measured population, is used in an equation to determine the amount of outside air the AHU must condition and distribute.
Optimal start/stop minimizes the system run time. The building automation system intelligently determines how long it takes to get to the occupied setpoint temperature. It uses the current outdoor air temperature and previous measured times to schedule the HVAC start time as late as possible. This gets each zone to the right temperature just in time for occupancy. The stop point is determined using the time it takes for the temperature to drift from the occupancy setpoint to the unoccupied setpoint. The system is turned off just long enough before the end of scheduled occupancy that the temperature only changes a few degrees to avoid impacting occupant comfort.
VAV systems are often the best choice for optimizing large multiple zone systems when these code-required BAS sequences are implemented properly. Energy recovery wheels and code required economizers combined with properly implemented controls strategies would probably be enough for VAV systems to shed their poor reputation. However, we can take it a step further. Unlike many other systems, traditional chilled and hot water systems can use natural gas as a low-cost fuel source and leverage thermal storage during the cooling season. This significantly reduces electric demand charges and provides evidence that VAV systems are clearly not getting the attention they deserve for their low operational costs.
When combined with Trane’s Earthwise strategies of low temperature air, high delta T water systems, discharge air temperature reset controls strategies, and optimized duct design, VAV systems will outperform most other systems on the market today.