Your questions answered: K-12 schools and colleges/universities
Providing cooling/heating comfort and energy efficiency in school buildings is an ongoing challenge for mechanical engineers. Emphasis on building performance and adoption of energy-efficient building codes is increasing. Designs are based on a host of variables beyond space type and understood loads including operating costs, first cost, lifecycle cost, and measurement and verification (M&V) requirements. Demand for historically typical HVAC technologies, such as variable-air-volume (VAV) and water-cooled chillers, is giving way to a variety of systems, such as displacement ventilation, combined heat and power (CHP), geo-exchange, simultaneous heating/cooling systems, variable refrigerant flow (VRF), passive design, chilled beams, dedicated outdoor air systems, and evaporative cooling.
Presenters Art Lantagne, MEP Associates LLC, Norman, Okla.; and Rodney Oathout, PE, CEM, LEED AP, DLR Group, Overland Park, Kan. respond to questions not answered during the live HVAC: Schools (K-12 and colleges/universities) on March 29, 2018.
Question: Talk about handling the changing ventilation demands of classrooms and other large assembly areas that are typically used for short duration (less than 2 hours).
Art Lantagne: Efficiency is improved when ventilation air is limited. Untreated ventilation air is costlier to heat and cool compared to recirculated air. Classroom ventilation can be handled a few different ways:
- Fixed ventilation: A fixed outside air (OA) damper is the simplest, yet least efficient method for ventilation, as it admits OA whenever the HVAC system is in operation regardless of whether the space is occupied.
- Scheduled ventilation: This method uses a motorized damper connected to a programmable space thermostat (or automation system). The OA damper opens during periods of scheduled occupancy. Occupied periods are set by the building user. This is more efficient than fixed OA, as ventilation air is only introduced into the space during scheduled periods of occupancy.
- Demand ventilation: This method uses a motorized OA damper typically connected to a CO2 sensor, although we have seen occupant counters and occupancy sensors in use. This is the most energy efficient method of ventilation, as OA is only introduced into the space when occupants are detected or when air quality measurements, such as CO2 concentration determine that ventilation air is required.
For large spaces such as gymnasiums, you can take advantage of room volume and/or unit sizing to maximize ventilation efficiency for short-occupancy events.
- Using room volume, you can reduce the amount of ventilation by using demand ventilation (see above) and circulating the air in the space to dilute the air in the breathing zone using the air handling equipment or high volume low speed (HVLS) fans. The substantial air volume of a gymnasium (or auditorium) can serve as a buffer for short-duration events.
- With schools using DX equipment, we have had success using multiple units to serve large spaces. A gymnasium typically has three different loads: unoccupied, low occupancy (such as gym class or practice), and maximum occupancy for a game or event. We typically size one unit to serve the heating/cooling/ventilation load of the gym for unoccupied and low-occupied periods and then size one or two additional units to provide the heating/cooling/ventilation load for periods of maximum occupancy. Using this method, we can prevent inefficient overventilation, which can be an issue for wood flooring in certain climates and provide energy savings by not running large equipment to serve a small load.
Q: Do you see an opportunity to use thermal energy storage on your campus projects to avoid on-peak charges?
Rodney Oathout: Yes. This is an excellent application for thermal energy storage. We have found that by addressing the on-peak charges, the size of the thermal energy system can be smaller, thus more affordable and have a better return on investment.
Q: What do you see for economizer cooling in variable refrigerant flow (VRF) systems?
Lantagne: It has been our experience to install airside economizers on the evaporators on only the evaporators whose capacity exceeds the threshold requirement for economizers for a given climate. Depending on how the ventilation dedicated outside air system (DOAS) unit is ducted for a given VRF application, the DOAS unit could be set up to function in free cooling mode.
Q: How do you accommodate the varying load requirement within a building that has a library, multipurpose, auditorium, and classroom spaces?
Oathout: The answer depends on the HVAC system planned to serve these spaces. In the context of this webcast, let’s assume the HVAC system is chilled water, hot water, and indoor air handling units (AHUs). All of the spaces listed can have large swings in load depending on occupancy. We prefer to use VAV systems to serve these spaces. In cooling, the sequence requires constant SA temperature to manage humidity with verifying air flow. We use CO2 control to manage the outside air flow. It is important to avoid using CO2 control with energy recovery and verify AHU performance at low airflows. Multiple smaller AHUs are normally better than one large AHU.
Q: When using VRF systems, are there concerns with ASHRAE 15 compliance regarding potential refrigerant leak?
Lantagne: Yes. The design engineer will need to perform a preliminary layout to determine the capacity of the system, quantity of system components, as well as size and length of refrigerant piping. After a layout has been performed, the amount of refrigerant can be determined. After the quantity of refrigerant has been calculated, it can be verified whether the system meets the RCL/minimum room volume requirements of Standards 15 and 34.
Design engineers need to familiarize themselves with ASHRAE Standard 15-2016: Safety Standard for Refrigeration Systems and ASHRAE Standard 34: Designation and Safety Classification of Refrigerants. Standard 15 establishes system definitions and safety requirements for refrigeration systems. Standard 34 establishes safety classifications and refrigeration concentration limits for refrigerants.
Q: Can we use a variable frequency drive (VFD) for the pump and a VFD for the AHU together in chilled water system?
Oathout: Yes. This is common practice in modern HVAC system design. The VFDs will modulate water flow and air flow as necessary to achieve varying requirements of the system.
Q: On the energy use intensity (EUI) slide for elementary schools, is it site or sourced energy?
Lantagne: The EUI values shown on the school comparison table were site EUI values.
Q: Neutral means reheat. Did you consider a cooler air supply temperature for DOAS in reference to the first VRF example?
Oathout: We have implemented a sequence in the DOAS that adjusts the SA temperature based on OA temperature to reduce the requirement for reheat and controlled humidity. A couple of cautions: DOAS needs to be zoned so cool air is not being introduced to a zone that requires heating. And, don’t be too aggressive with the SA setback in the DOAS. The cool air from the DOAS can force the primary system into heating unnecessarily.
Q: What is the MBH cooling unit? Please define.
Lantagne: MBH = 1,000 Btu/h 36 MBH = 36,000 Btu/h.
Q: Please identify long-term (beyond 20 years) maintenance items and costs for geothermal ground-source water loops?
Oathout: Most geothermal pipe has a 50-year warranty with plenty of very reliable projects to justify this warranty. The maintenance and quality of the water mixture is an important factor. The biggest issue with older ground source loops is maintaining the temperature in a desirable range. Over a period of time, the ground simply stores the energy. In the winter, heat is pulled out of the ground and in the summer, heat is rejected into the ground. In a building with a balanced heat/cool load, this exchange can go on indefinitely. Unfortunately, there are very few buildings that have balanced loads. Climate changes also are causing buildings that were thought to have a balanced load to not have one. The cost pressure on projects also are forcing designers to be less conservative in there well field design resulting less robustness and more susceptibility to large temperatures in the loop temperature. I would not design a ground source system without valved connections so a cooling tower (or boiler) could be added to the system at a later date to help manage the loop temperature.
Q: I believe it was said that a boiler and cooling tower system would be used in conjunction with a geothermal system. Why is this?
Lantagne: No, the boiler and cooling tower at Plaza Towers were used instead of a geothermal ground heat exchanger (GHEX). The challenge was to keep the as much of the South Lake Elementary HVAC system design intact without using GHEX to meet the compressed construction timeline and the insurer’s building replacement budget.
Q: For a VRF split air conditioning system, how do you make sure the refrigerant volume meets the ASHRAE 15 requirements?
Lantagne: The design engineer will need to perform a preliminary layout to determine the capacity of the system, quantity of system components, as well as size and length of refrigerant piping. After a layout has been performed, the amount of refrigerant can be determined. After the quantity of refrigerant has been calculated, it can be verified whether the system meets the RCL/minimum room volume requirements of Standards 15 and 34.
Design engineers need to familiarize themselves with ASHRAE Standard 15: Safety Standard for Refrigeration Systems and ASHRAE Standard 34: Designation and Safety Classification of Refrigerants. Standard 15 establishes system definitions and safety requirements for refrigeration systems. Standard 34 establishes safety classifications and Refrigeration Concentration Limits for refrigerants.