Engineering in K-12 schools: HVAC systems

03/17/2014


CSE: What indoor air quality (IAQ) challenges have you recently overcome? Describe the project, and how you solved the problem.

Hedman: The International Building Code allows the use of operable windows as the source for ventilation air for classrooms. However, while fresh air can be introduced via operable windows, the air is not filtered. Filtering the air reduces the quantity of pollen, which is a major catalyst to allergies. Furthermore, noise and air pollution is not filtered when operable windows are used. In designing a new private school in New York City, all windows were fixed and fresh air intakes serving mechanical units were located at the upper levels.

Najafi: This was encountered in the design for the renovation of an existing school, in which the desire to replace existing systems included an upgrade of ventilation requirements to current code standards. The desire to maintain limited equipment space use, yet design for increased amounts of outside air, yielded a humidity concern that was best addressed by employing demand ventilation techniques.

CSE: In your experience, have alternative HVAC systems become more relevant? These may include displacement ventilation, chilled beams, etc.

Jefferson: Nontraditional systems have certainly been more relevant in K-12 design because the fundamental challenges that districts have with operational/energy budgets remain in place. I think school districts will continue to be willing to explore new ideas and concepts that decrease their energy costs over the life of the building. But beyond just energy, the systems have to be responsive to educational needs, so that means great thermal comfort and air quality too. While certainly not qualifying as a new technology, I’m really excited about how many of our projects are using natural ventilation strategies. Early in my career, getting a school district to consider operable windows or fresh air louvers was an uphill battle.

Oathout: The new “big thing” in HVAC system design for K-12 facilities is variable refrigerant flow (VRF) systems. The VRF systems offer a compelling combination of energy efficiency and affordability. There is still work to be done by the manufacturers to make these systems more occupant- and operator-friendly.

Najafi: We are very impressed with the energy performance approach of distributed VRF with ground coupled (geothermal) thermal sinks. This employs a three-tier energy recovery approach, with localized direct exchange of heat and low transport energy (as with refrigerant flow only), a building level exchange via water based transport, and a seasonal exchange via the ground coupled heat sink. Also, we are seeing displacement ventilation play a large role in energy savings when applied appropriately.

We are currently using displacement ventilation in our design of the Duke Ellington School of the Arts. The large 850-person theater at the school provides an excellent opportunity for displacement ventilation and cooling by capitalizing on the natural stratification that occurs as warm air rises because of the high ceilings and the high occupancy of the space. Cooling and ventilation will be supplied at very low velocities along the floor level and will be returned higher in the space. As a result, contaminants are forced away from occupants and more efficiently removed from the space as opposed to being mixed throughout the space. This approach also reduces the amount of energy required to cool the space as the supply air is provided first to the occupied zone prior to mixing with the entire volume of air. The displacement approach reduces energy, and provides high thermal comfort and better indoor air quality, all of which contribute to an enhanced design.

Hedman: We see chilled beams becoming more relevant in school projects. With the noise criteria for classrooms set forth by ANSI 12.60, it is becoming difficult to design classrooms with unit ventilators and fan coil units similar to how classrooms were designed in the past. Chilled beams generate minimal noise in the space, allow for smaller duct runs for ventilation air only, and require little to no maintenance.



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