School Receives One of the US' Largest VRF A/C Retrofits

VRF A/C installation with cost-saving rooftop equipment mounts saves school more than 30% vs. standard A/C.

When a school such as the 85-year-old West View Elementary needed an air-conditioning retrofit, some engineers would have taken the path of specifying conventional rooftop package units, curb and rail mounts, and metal ductwork.

However, the Pittsburgh, PA-based North Hills School District hired BDA Engineering Inc. to think outside the box and take the specification path less traveled by considering energy efficiency, job costs, installation expediency and indoor air comfort.

Gary Albert, P.E., LEED AP, a BDA associate and project leader, specified 290 tons of cooling via 28 rooftop VRF heat pumps ranging from six to 24 tons—one of the U.S.'s largest VRF projects. 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%–50% or more in yearly energy savings vs. a chiller system and a variety of "ducted non-VRF" systems, respectively. Additionally, $50,000 or more was saved on the $2.3 million project due to the rooftop equipment mount specification.

Design team members included construction manager Thomas & Williamson and HVAC wholesaler Comfort Supply Inc., which provided technical design configuration and installation assistance with the prefabricated rooftop mounting systems by Big Foot Systems represented in the U.S. by RectorSeal Corp. Comfort Supply also provided factory-certified design and start-up assistance with City Multi VRF equipment manufactured by Mitsubishi Electric Cooling & Heating.

Equipment mount savings

Equipment mounts were one of the most significant value-engineered specifications. Instead of two 4-ft-long rails with roof-piercing flashing for each of the 28 heat pumps, BDA specified more than 180 linear ft of Big Foot Systems equipment mounts. The strategy eliminated the need for a roofer and general contractor, saving tens of thousands of dollars in labor costs. While Big Foot Systems has been used worldwide for large projects such as the British Airport Authority's Heathrow Airport terminal rooftop A/C equipment, the West View project is one of the largest U.S. Big Foot Systems projects to date, according to RectorSeal spokespeople. The project was also the largest VRF project for Comfort Supply.

"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 aspect," said Justin Kern, Commercial Sales Manager, Comfort Supply.

In addition to roofing labor savings, the project's mechanical contractor, R&B Mechanical, 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.

The mounting system, which consists of 1/6-sq-in., corrosion-resistant, hot-dipped galvanized tube modules, sits 18 in. above the roof surface using 1-sq-ft, 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 will not 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.

HVAC equipment selection

BDA considered many options before arriving at the VRF. Solutions such as chillers to supply the existing unit ventilator system in every classroom, or DX conventional rooftop systems with ductwork, were considered too costly, not as efficient and disruptive for the school. Instead, the project used Mitsubishi Electric CMFR-Series, 100% outdoor air VRF heat pumps and air handlers combined with the school's existing room ventilator exhaust fans to pressure balance the system. Additionally, Y-Series heat pumps supply mixing boxes that distribute cooling to classroom wall-mounted or ceiling-cassette fan coils.

"No classroom or space receives re-circulated 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% outdoor air system as an economizer strategy and/or heat pumps for heating with a BACnet-based building automation system by Andover Controls—division 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.

When a roof can't handle rails

The prefabricated equipment mounting systems also made certain placements on less fortified roof areas of the West View Elementary school possible.

"Rails would have spanned across structurally weaker parts of the roof whereas the footings were positioned only on the strongest parts of the roof steel," said Albert.

The Big Foot Systems supporting several heat pumps atop the less-fortified gymnasium roof were instrumental in space framing and weight distribution. Because only certain far-spaced steel roof joists could support the weight of heat pumps, the equipment mounting was designed and assembled to position and span the footings out to the strongest joist points, regardless of the equipment's small footprint.

For more information, visit www.rectorseal.com.