Various studies have shown that noise exposure—even modest levels of ambient noise—can negatively affect learning, especially for young children. To create cleaner, healthier indoor environments that lower student and staff absentee rates and improve teacher retention, advanced, energy-efficient heating and cooling systems can help. According to the American Society for Heating, Refrigeration and Air Conditioning Engineers, regularly scheduled maintenance is an important part of keeping the HVAC system in optimum working condition.

ASHRAE’s “Advanced Energy Design Guide for K-12 School Buildings” says that neglecting preventive maintenance practices can quickly negate any energy savings expected from HVAC system design. It says that filters should be replaced when pressure drop exceeds manufacturer recommendations for replacement, or when visual inspection indicates the need for replacement.

Energy recovery ventilators need to be cleaned periodically to maintain performance. Dampers, valves, louvers, and sensors must all be periodically inspected and calibrated to ensure proper operation.

This is especially important for outdoor air dampers and CO2 sensors. Inaccurate CO2 sensors can cause excessive energy use or poor IAQ, so they need to be calibrated as recommended by the manufacturer.

A building automation system can be used to notify operations and maintenance staff when preventive maintenance procedures should be performed. This notification can be triggered by calendar dates, run-time hours, the number of times a piece of equipment has started, or sensors installed in the system (such as a pressure switch that indicates when an air filter is too dirty and needs to be replaced).

Control strategies can be designed to help reduce energy. Having a setback temperature for unoccupied periods during the heating season or a setup temperature during the cooling season can help to save energy by avoiding the need to operate heating, cooling, and ventilation equipment.

Programmable thermostats allow each zone to vary the temperature set point based on time of day and day of the week. But they also allow occupants to override these set points or ignore the schedule altogether (by using the “hold” feature), which thwarts any potential for energy savings.

A more sustainable approach is to equip each zone with a zone temperature sensor and then use a system-level controller that coordinates the operation of all components of the system.

This system-level controller contains time-of-day schedules that define when different areas of the building are expected to be unoccupied. During these times, the system is shut off and the temperature is allowed to drift away from the occupied set point.

A pre-occupancy ventilation period can help purge the building of contaminants that build up overnight from the off-gassing of products and packaging materials. When it is cool at night, it can also help pre-cool the building. In humid climates, however, care should be taken to avoid bringing in humid outdoor air during unoccupied periods.

Optimal start uses a system-level controller to determine the length of time required to bring each zone from the current temperature to the occupied set point temperature. Then, the controller waits as long as possible before starting the system, so that the temperature in each zone reaches occupied set point just in time for occupancy.

This strategy reduces the number of hours that the system needs to operate, and saves energy by avoiding the need to maintain the indoor temperature at occupied occupied set point even though the building is unoccupied.

Air Handlers

For Multiple-Zone, VAV Air Handlers, each variable air volume air handler should have an outdoor air (OA) intake through which OA is introduced and mixes with the recirculated air, prior to being delivered to the zones. Alternatively, a dedicated OA system could be used to deliver OA directly to each zone, to individual dual-duct VAV terminals that serve each zone, or to the OA intake of one or more VAV air handlers.

Note that the occupant load, or exit population, used for egress design to comply with the fire code is typically much higher than the zone population used for ventilation system design. Using occupant load, rather than zone population, to calculate ventilation requirements can result in significant over-ventilation, oversized HVAC equipment, and excess energy use.

Buildings with multiple-zone, recirculating ventilation systems (MZS) can be designed to account for recirculated OA, as well as system population diversity, using the equations found in ASHRAE Standard 62.1.

In effect, the MZS design approach allows ventilation air to be calculated on the basis of how many people are in the building (system population at design) rather than the sum of how many people are in each space (sum-of-peak zone population at design).

This can reduce the energy required to condition ventilation air in K-12 schools. ❑

 Source: ASHRAE Advanced Energy Design Guide for K-12 School Buildings.