When a school such as the 85-year-old West View Elementary needed an air conditioning retrofit, many engineers would have taken the common path of specifying conventional products.
However, the Pittsburgh-based North Hills School District hired BDA Engineering Inc., West Homestead, Pa., to think way outside the box and take the specification path less traveled by many engineers in a quest for more energy efficiency, lower job costs, installation expediency, and great indoor air comfort.
Gary Albert, P.E., LEED AP, a BDA associate and project leader, specified 290 tons of cooling via 28 rooftop variable refrigerant flow (VRF) heat pumps ranging from 6 to 24 tons. VRF technology helped eliminate ductwork, roof penetrations, and curbs and provided optimum indoor air comfort control to individual classrooms, while causing minimum disruption to the school's daily schedule. Albert estimated the VRF system saves the school 30 to 50 percent or more in yearly energy savings. Additionally, upwards of $50,000 more was saved on the $2.3 million project due to Albert's rooftop equipment mount specification.
Design team members included construction manager Thomas & Williamson, Pittsburgh, and HVAC wholesaler, Comfort Supply Inc., Pittsburgh, which provided technical design configuration and installation assistance with the prefabricated rooftop mounting systems by Big Foot Systems represented by RectorSeal, Houston. Comfort Supply also provided factory-certified design and start-up assistance with City Multi® VRF equipment manufactured by Mitsubishi Electric.
EQUIPMENT MOUNT SAVINGSEquipment mounts were one of the most significant value-engineered specifications. Instead of two 4-foot-long rails with roof-piercing flashing for each of the 28 heat pumps, BDA specified more than 180 linear feet of Big Foot Systems equipment mounts. The strategy eliminated the need for a roofer and general contractor, thus saving tens of thousands of dollars in labor costs. The project was the largest VRF project to date for Comfort Supply, which has been selling ductless equipment for more than 25 years.
"Using VRF and combining it with this type of equipment mounting is a big trend the last few years due to the quick installation, energy efficiency, and LEED [Leadership in Environmental and Energy Design] aspect," said Justin Kern, commercial sales manager, Comfort Supply.
Besides roofing labor savings, the project's mechanical contractor, R&B Mechanical, Pittsburgh, was able to bolt the individually identified equipment mounting components together in considerably less time than setting conventional rails, according to Thomas Recker, vice president, R&B Mechanical. "Prefabricated modular systems are the way to go today, and they definitely saved the school district significant project costs versus conventional rails and flashing," said Recker, who has used Big Foot Systems on several past projects requiring conventional HVAC rooftop equipment.
The mounting system, which consists of 1/6-square-inch, corrosion-resistant, hot-dipped galvanized tube modules, sits 18 inches above the roof surface using 1-square-foot, glass-filled nylon footings with anti-vibration mat bases. Matching footings specifically designed for utility piping were also used for electrical, refrigerant, and control wiring piping runs across the roof.
Not making dozens of roof cuts for equipment rails also eliminated concerns about potential leaks to an older-style roof built 85 years ago. Future roof resurfacings won't require heat pump and piping disconnections because there are no roof penetrations, therefore one leg at a time can be removed and temporarily supported to provide access to the roof membrane below.
BDA considered many options before arriving at the VRF. The project used 18 Mitsubishi CMFR-Series, 100 percent outdoor air VRF heat pumps and air handlers combined with the school's existing room ventilator exhaust fans to pressure balance the system. Additionally, nine Y-Series heat pumps supply mixing boxes that distribute cooling to classroom wall-mounted or ceiling cassette fan coils.
"No classroom or space receives recirculated air from another space," said Albert. "This will improve indoor air quality, which is proven to have a direct effect on reducing student absenteeism due to illnesses."
BDA's design also optimizes energy savings during the fall and spring by using the 100 percent outdoor air system as an economizer strategy and/or heat pumps for heating dictated with a BACnet(r)-based building automation system (BAS) by Andover Controls, div. of TAC, and installed by Combustion & Service Equipment Co. The VRF system monitors and controls itself, but also feeds data to the BAS for a total HVAC overview. The energy savings climbs significantly during the off-season when heating and cooling might be needed simultaneously throughout the school in various spaces, according to Albert. "From my standpoint, the potential for such a high level of individual cooling (and heating during seasonal transition) control per space can't be accomplished with other concepts," said Albert.
Unlike common process piping, VRF systems require special engineering considerations based on piping run lengths, pipe diameters, the number of elbows, and other resistance factors. Therefore BDA computer modeled Albert's design in-house with confirmation from Comfort Supply's Mitsubishi-trained sales/engineers.
While some engineering firms might have resorted to more a conventional HVAC solution, West View's project manager, HVAC engineer, and wholesale supplier took innovative steps outside the box to arrive at a cutting-edge air conditioning solution such as VRF that ultimately saves the district money and energy.