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Heat Pumps Upgrade School HVAC Efficiency
Margarita Middle School, in the Temecula, CA, school district, used vertical water-source heat pumps for heating and cooling. When a planned renovation was designed in late 2015, the California Clean Energy Jobs Act (Prop 39) funded a retrofit to include all-new heat pumps, a central boiler plant, ventilation fans, and a DDC control system. School administrators jumped at the chance to upgrade. To select a new heat-pump brand, the installation team evaluated multiple manufacturers and models based on two qualifications. First, physical size, because the existing mechanical closets limited dimensions of the heat pumps. Second was an index of installed cost/ton for groupings of Seasonal Energy Efficiency Ratio (SEER).
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“The major challenge was constrained space of the existing utility closets,” said Darren Smith, P.E., and LEED-accredited professional with Enovity, Irvine, CA, the project facility’s engineering firm and the prime engineering consultant, which supervised building energy analysis and system commissioning. “We had to find heat pumps to fit the closets that provided sufficient energy efficiency to meet Prop 39 payback requirements.”
INSTALLATION The installation team replaced 37 water-source heat pumps totaling 209 nominal tons. FHP, Fort Lauderdale, FL; sales representative Tony Lobito of KSR Associates, Los Angeles; and FHP regional sales manager Nic Jones, worked with IDS Group Mechanical Engineers, Irvine, CA, and NKS Mechanical Contracting, Anaheim, CA, to meet performance requirements and ensure that the heat pumps fit in the restricted space in the existing mechanical closets.
Twenty-nine FHP EP and eight EC heat pumps are installed in exterior closets, with access from the classrooms. Model EP single-compressor water-source heat pumps have EC motors and use three motor taps to stage airflow with the compressors to reduce energy demand and improve dehumidification at low stage. The EC mo
Inset. Heat pumps needed to fi t in the restricted space in the existing mechanical closets.
tors were field programmed to match the motors to the job conditions. Model EC units are two-compressor water-source heat pumps with variable-frequency drives for the supply fans that stage airflow to match compressor stages. Insulated bulkheads in vertical EC models separate the compressor and blower sections.
The closets are a return plenum and operate under a high negative pressure due to a restricted return-air system. To lower the negative pressures in each closet, the airflow/ton was reduced and acoustically treated, and transfer grilles were installed above the closet doors. To compensate for the reduced pressure and variable-supply airflow, each heat-pump closet has a variable-flow EC motor outdoor-air fan. Outdoor-air fan speed is reset based on carbon dioxide levels in the classrooms. Each water-source heat pump has a pressure-independent two-position water valve that closes when the compressor is off.
Units are designed to be serviced from the front. The project included high-performance window tints, which allowed some heat pumps to be reduced in size.
The new central plant is variable water flow. Individual heat-pump design flows were reduced from the original 4 gpm/ton to 2 1/2 gpm/ton, and the condenser water pump motor size reduced from 40 HP to 25 HP.
The replacement cooling tower is counterflow, with three variable-speed 5-HP propeller fans, and a variable-speed 7 1/2-HP basin pump that replaced a 40-HP centrifugal fan blow-through tower with a 5-HP basin pump. When additional heating is required, two modulating boilers inject heat into the water loop. The DDC control system integrates central plant variable-frequency drives, boilers, and chemical treatment controllers.
SYSTEM BENEFITS Construction began in late 2015, and installation of the facility’s heating/cooling system took place over two months of the 2016 summer shutdown. The system was successfully started in time for the fall semester. After startup, the engineers completed air balancing, control programming, graphical-interface modifi cations, and commissioning of the system.
Edgton Wright from Bosch Thermotechnology, Ft. Lauderdale, FL, performed training as a key component of the commissioning service. Having a manufacturer perform start-up consulting has a number of benefits, including greater potential for lower energy and operational costs, facility staff training for improved technical support, end-user confidence in product functionality and performance, documentation of start-up activities and guidance for warranty support, and an ongoing personal touch through technical support for the duration of the start-up program.
“Training for the facilities staff was very productive,” said Matt Waller, P.E., senior engineer with IDS, the design firm that worked as a sub-consultant to Enovity that developed the system design and prepared construction documents. “It incorporated a comprehensive presentation, plus hands-on training in front of an installed heat pump unit inside a vertical utility closet.”
Darren Smith agreed. “It’s not often you receive positive comments about training, so we were pleased to hear that [they found the training productive],” he said.
The Bosch consulting service offers support across the entire Bosch FHP line of products, from the beginning product-selection phase by the user to post installation and start up. Each consulting package includes a visit from an expert field technician to provide start-up consulting services, including on-site supervised start-up assistance, hands-on personnel training, and a review of the job.
After the project was completed, Smith said that the newer pumps were a worthwhile investment and proved to be quieter and more efficient than the former units. CA
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