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Case Study: Ductless HVAC at Brentwood Middle School
Jeff Owens, Project Manager, describes the major project at Brentwood Middle School that installed a green, ductless, HVAC system expected to save 25%-30% of the energy previously required to heat and cool a large school building.
Brentwood Middle School opened in August 1972. It's a large school with 1,250 students in grades 6 through 8. In 2014, it was named by thebestschools.org as one of the top 30 schools in theO USA. Learning can be hampered by environments where students are distracted by noise, or in temperatures that are too hot or too cold. In 2013, it was decided that the 41-year-old school should replace its antiquated HVAC system. They chose a system that would need no ducts for conditioned hot and cool air, would keep classrooms quiet, would let teachers control their individual classroom's temperature, and would use no floor space for equipment. The project required removing all the ceiling tiles, all lighting, all the old ductwork, all floor-mounted air handlers, and all roof-mounted equipment. The work was done during two summer breaks so there was no need to close any classrooms.
The system uses a Mitsubishi VRF (Variable Refrigerant Flow) system. In each of the classrooms we have a ceiling cassette that takes care of this particular room and it's thermostatically controlled by a white thermostat on the wall. The teacher can control the temperature in each room. Before there was one central thermostat controlling all the rooms.
This is called a ductless system and you have no ductwork above the ceiling and you have four directional blowers with air flowing in four different directions in the classroom. There are approximately 74 units throughout the building. Another good feature for these Mitsubishis is that they're quiet - only 30-35 decibels. You can't even hear the thing running in the classroom. It is very quiet unlike a conventional system. Each cassette is connected to a condensing unit on the roof. There are eight condensing units on the roof and each feeds 15-20 air handlers. There is a flow of refrigerant and as the system requires heating or cooling it will flow the refrigerant needed to heat or cool a particular room. This is a two-ton capacity unit so it can use as much as 24,000 BTUs of refrigerant.
Of course there has to be a way to bring fresh outside air into each classroom and that does require ducts and vents. Reznor make-up units on the roof filter the air, and blowers pump the air to the vents. There are no return air ducts. The fresh air pumped in by the make-up units result in the rooms having a positive air pressure, which then exits through leaks around windows and doors. There are five makeup air units on the roof that condition the outside air. 100% outside air goes in and is heated or cooled as required and then is directed into each classroom.
As the sun comes up in the morning, one side of the building requires cooling while the other side does not. An energy management system determines which of the eight condensing units to run. This saves energy. The energy used is being measured by the energy management system and is expected to be 25 to 30% less than the old system. Data from the old system has been retained and in a year we should be able to report exact figures.