K-12 schools in New England are becoming multifunctional facilities with uses beyond academic. Year round operation is now the norm. Indoor thermal comfort requirements have reached a point where cooling is being provided to the entire building.
A critical item to consider when selecting a HVAC system with cooling is ventilation. Ventilation is the amount of outside air required by the building code to ensure good air quality. If more ventilation improves indoor air quality, why don’t we provide 100% outside air and never recirculate air? The
answer is outside air needs to be either cooled or heated more than return air to achieve 55F supply air. It requires more energy annually to condition than return air. The major driver for high ventilation requirements is population. Schools have high concentrations of people.
The system must always provide the minimum amount of required ventilation to the space during occupied hours unless demand control ventilation (DCV) is included as a control strategy. A DCV system monitors the amount of CO2 air in the room and allows lower than code required ventilation air so long as acceptable CO2 levels are maintained. This system saves energy but requires additional first cost for more controls. The controls are more complex which may lead to problems if not designed and installed correctly.
High ventilation requirements encourage air systems versus hydronic solutions because the system needs to provide a high volume of air by code. There are a number of HVAC options to provide air conditioning to classrooms. Two (2) all air options are summarized below. Both are centralized systems (larger air handling units providing air to many spaces).
Overhead Variable Air Volume (VAV)
An overhead VAV system is a traditional system used to condition buildings for many years. A large air handing unit supplies 55F air throughout the building. The VAV system saves energy due to the VAV box. There are several types. In its simplest form, it is a modulating damper which opens or closes maintaining temperature setpoint. Reducing the amount of air to a space saves energy. The air enters the room from the ceiling and mixes the entire room to 76F. By adding a hot water heating coil to the end of the VAV box, it is possible to heat the entire classroom from the VAV system and eliminate perimeter finned tube radiation. The design of the exterior wall and location of supply diffusers in the ceiling is critical.
Side Wall Displacement
Side wall displacement is similar to VAV with a few differences making it more appealing regarding energy savings. A large air handling unit also provides air throughout the building but is delivered at 65F which saves energy. The tradeoff is humidity control. 65F air can cause humidity problems. By providing an air handling unit (AHU) which can cool the air to 55F (only when required) and reheat to 65F humidity can be controlled to acceptable levels. The design and control of the AHU is critical.
The air enters the classroom from the wall down low versus in the ceiling for VAV. The air is at a lower velocity reducing noise and fan power requirements saving energy. Also the air displaces the room air (versus mixing for an overhead VAV system) so cleaner air is provided to the breathing zone and the dirtier air rises and exits the room from the ceiling improving the air quality. An important issue to consider in sidewall displacement is room temperature control. The optimal solution is to provide a VAV box serving each room. It functions in the same way as the overhead VAV system and modulates air supply to maintain room temperature. Another option is to provide the system without VAV boxes but thermal comfort will be compromised as the load is continuously changing between spaces.
Both these systems are valid air distribution systems to meet thermal comfort requirements, but not all designs are guaranteed to provide the required thermal comfort. The devil is in the details. Make sure to ask the appropriate questions to the design team to make sure the system design can maximize energy savings while meeting the thermal comfort requirements and be within the project budget.
Scott Robbins is an Associate Principal at R.G. Vanderweil Engineers in Boston.