West / Regional Reports

Cutting Costs With Innovative HVAC Design

April 2, 2004
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MURIETTA, Calif. - Rooftop units and conventional metal ductwork are construction norms. However, an architect and engineer creatively used an alternative HVAC system that not only cut installation and construction costs, but also reduced future maintenance expenditures.

Proof is in the new 25,000-square-foot, slab/concrete tilt-up style building for the Calvary Chapel Bible College, located here. It uses seven 74-foot-long runs of fabric duct, each connected to its own respective 12.5-ton, 6,000-cfm cooling/heating rooftop unit in the adjoining gymnasium/chapel.

In this case, consulting engineer Thomas Krusic, P.E., principal, Krusic & Associates, Irvine, Calif., and architect Richard Dayton, principal at Dayton Associates-Architects, Newport Beach, Calif., worked together to create this system, which incorporates polyester blended fabric duct rather than round metal duct.

The fabric duct was specified black to blend into the intended black ceiling structure. Linear diffusers that run the entire length of each duct are designed to distribute a gentle more even airflow than the more traditional concept of metal duct with registers every 10 feet. (All photos courtesy of DuctSox.)

Believes In Fabric Duct

With 39 buildings on the 47-acre campus, Karl Bentz, director of facilities, was an advocate of Krusic's fabric duct proposal because the college had used fabric duct in other, previous Calvary Chapel building projects to save time and materials.

More costs in ductwork material were saved because of rooftop placement. Instead of putting rooftop units on the chapel roof, which would necessitate plenums, elbows, airflow streamlining, and other accessories, equipment was placed on an adjoining portion of the building. This 12,000-square-foot, L-shaped area of the building has a lower roof that allowed the rooftop units to use horizontal discharge straight through the wall of the chapel at the roof level and into the seven straight runs of fabric.

This close collaboration between engineer and architect not only saved HVAC installation time and labor, but also more than $100,000 in building structure costs. By utilizing the lower and stronger roof of the accompanying building area to support the rooftop units, Dayton was able to incorporate a lighter and less expensive roof design for the chapel. Subsequently, the lighter roof design resulted in a more aesthetic looking chapel inside because it didn't require obstructive column supports.

Additionally, Dayton was able to design a higher chapel roof because the parapet, which would have needed a four-foot height to hide rooftop equipment, was designed for the minimum 12-inch height. Lowering the roof to a code complying 48 inches below the parapet cap, combined with local building height restrictions, would have resulted in a lower ceiling that would have affected the chapel's usage capabilities and interior aesthetics.

Since the chapel and its accompanying L-shaped building required a total of 135 tons of air conditioning, a chiller and chilled water circuit was one of Krusic's possible choices of cooling equipment. But because Calvary Chapel's maintenance staff is trained on direct expansion rooftop units, Krusic specified rooftop units over a chiller system.

The chapel/auditorium is bordered on two sides by a two-story office/classroom building that serves as a support structure for the seven rooftop units. Because the taller chapel roof (barely visible beyond the two-story facade) doesn’t have to support several tons of equipment and heavy metal ductwork, the architect was able to specify a lighter and less expensive ceiling structure.

Manufacturer Provides Assistance

Krusic's choice of quantity and capacity of the rooftops depended upon the weight-bearing capacity of Dayton's roof design for the chapel's accompanying building, plus the cooling load mandated by the 15 cfm-per-person outside air compliance with California's Title 24 "Energy Efficiency Standards for Non-Residential Buildings." To provide the total 87.5 tons of refrigeration required, Krusic specified seven 12.5-ton 48HJ Series DX cooling/gas-fired heating rooftop units by Carrier Corp. (Syracuse, N.Y.). Krusic had considered a lesser quantity of larger units, but the next step up to 15-ton units in Carrier's 48HJ Series represented a substantial weight load difference per unit that would have necessitated additional roofing support costs.

For mechanical contractor Air Control Systems, Placentia, Calif., the connection of the fabric duct units, manufactured by DuctSox (Dubuque, Iowa), was simple as a result of Krusic's design. Each unit's horizontal discharge goes directly through the chapel wall, where it uses a rectangular metal to round metal adapter. Then another metal-to-fabric adapter is used to make the transition to the fabric duct runs.

While designing a room with traditional round metal duct and registers would pose no problem, Krusic did get assistance on designing the DuctSox diffuser size and placement as well as pressure loss considerations. Salesman Victor Petring of manufacturer's representative, Toro Aire, Dominguez Hills, Calif., and DuctSox's factory engineers' calculations recommended that each duct run have two of DuctSox's new Sonic Vent linear diffuser systems factory installed at 4 o'clock and 8 o'clock positions on the duct.

"One of the things that led us to DuctSox is the fact it could be put up in the truss space while still providing good air distribution," said Krusic.

Because the adjacent building was built purposely shorter, Calvary Chapel also reaped a significant savings in the chapel’s simple through-the-wall duct connection to the seven duct runs. Metal duct is used to supply the air from the rooftop air conditioning unit through the wall and connects to the fabric duct with a simple collar connection.

Avoiding Dented Ducts

Since the multi-purpose room will also be used for sports activities, another fabric duct feature Krusic found attractive is durability. Instead of being dented or damaged by errant sports balls, fabric duct bounces back to its original shape.

"One thing Rick (Dayton) and I knew from past experiences is that combining kids and basketballs can result in damage when the ductwork is metal," said Krusic, who had teamed with Dayton on many previous projects.

Krusic also found fabric duct to be more sound attenuating than metal duct, which is important for chapel activities. Whereas metal duct surfaces amplify or at least transmit HVAC equipment and airflow noise, the fabric duct's softness actually absorbs it.

"It appeared to me that the sound attenuating characteristics of fabric duct were more appropriate when the room functioned as a chapel," Krusic said.

Added Dayton, "I think a hidden benefit fabric duct has is the fact some of the air blows through the material and lessens accumulations of dust on the duct, which with white spiral metal duct, for example, is a real concern after several years of use. Although this was our first use of fabric duct, I can see its additional benefits in open architecture buildings such as adding color, plus it has a cleaner, more streamlined appearance than metal ductwork."

The space could have been more colorful with coordinated wall and ceiling colors complemented by a nearly infinite possibility of standard or custom color dyes for the fabric duct. However, Bentz and his building committee felt a more subdued ceiling of black fabric ducts and painted black ceiling trusses would give chapel room activities prevalence over the building interiors.

Approve Results

The absence of a campus-wide building automation system prompted Krusic to control the units through the use of thermostatic and time clock controlling, based on predetermined 78 degrees F (cooling), 72 degrees F (heating) setpoints and chapel occupancy. The rooftops also have economizers, which allow for a minimum setting that's equal to the outside air requirement.

Besides the chapel, Krusic also specified 12 additional smaller rooftops and traditional recessed metal ductwork to accommodate various offices and smaller rooms surrounding the chapel room. Additional rooftop units include 10 3-ton units and two 6-ton units, all by Carrier Corp.

Because of the efficiency, ease of maintenance, and the indoor air quality achieved in the chapel system, Bentz said future campus building construction and retrofits with open ceiling architecture designs might take a similar approach to the new chapel's design.

Publication date: 04/05/2004

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