Integrated design produces big savings for small HVAC
Small packaged heating, ventilation, and air-conditioning systems, up to 10 tons per unit, are among the most common HVAC systems found in small commercial buildings, yet they often use far more energy than necessary to accomplish their space-conditioning goals.
According to a study funded by the Public Interest Energy Research program of the California Energy Commission, the problems arise because designers do not understand the implications of poor systems integration, do not have proper guidelines for total integration of all building elements for minimum energy consumption , and often do not have the necessary financial and market incentives to implement total integration. The study looked at 215 HVAC units on 75 buildings.
Lessons from that effort have been summarized in the Small HVAC System Design Guide, available from the California Energy Commission. The starting point is the application of integrated design principles, through which the energy consumption and the costs to buildings with small HVAC systems can be reduced by 25% to 35%.
The integrated design strategy is built on the fact that HVAC systems do not function in isolation, but are part of an interactive system. The major elements of integrated design include a set of load reduction strategies that call for reducing power consumed by lighting, the use of high performance glass and skylights, the use of cool roofs, and the use of increased roof insulation. These measures can pay for themselves by reducing the size and therefore first costs of the HVAC equipment and the associated distribution system.
Reducing lighting power
Energy efficient lighting technologies not only generate more lumens of light per watt, but in cooling dominated buildings, they can reduce the size of the HVAC system required to remove the heat generated by the lights. Technologies to consider include new generation light sources such as super T-8s, T-5s and advanced metal halide lamps, as well as task/ambient lighting design, lighting controls, and daylighting.
High efficiency fluorescents . The latest generation of T-8 lamps uses improved phosphors that provide better color rendering and improved efficacy. Unofficially dubbed “super T-8s,” these lamps offer color rendering indices in the 80s and provide 34% more light output per watt of input power than standard T-8 lamps. That improvement makes it cost-effective to do a lighting upgrade even if you’ve already upgraded from T-12s to standard T-8s. The light output over the life of the lamp is also improved.
Another product, the T-5 high output lamp, is not generally applicable to lighting upgrades because of its metric length, but offers improved optical control and luminaire efficiency because of its thin profile. T-5s are 37.5% smaller in diameter than equivalent T-8s, which also reduces the size of the luminaire required. These lamps are best used in linear indirect/direct luminaires or high ceiling applications to minimize glare.
Advanced Metal Halides . Metal halide lamps are now available in reflector lamp configurations to replace incandescent PAR lamps in downlighting, track lighting, and retail display lighting applications requiring high color rendering. Ceramic metal halide lamps use a ceramic arc tube to improve color quality and reduce color shift over the lamp life. Compact metal halide lamps offer efficacy improvements of 100% and far longer lamp life than incandescent lamps.
Pulse-start metal halide lamps offer improved lumen maintenance, color stability, and lamp life over standard metal halide lamps. Initial lamp efficacy remains about the same, but the improved lamp lumen depreciation allows designers to specify about 25% lower wattage when considering maintained lumens. Adding electronic ballasts, now available for metal halide lamps in a wide range of sizes, boosts efficiency and decreases lumen depreciation even further.
Use high performance windows
High-performance windows feature assemblies of high performance glazing, frames, and spacers. High-performance glazings are “spectrally-selective,” reducing solar heat gains while transmitting a greater proportion of visible light than non-selective glazings. This selectivity is accomplished through a combination of tints (pigments added to molten glass) and coatings (low-e and/or reflective surface treatments). Tinted, low-e glazing systems cut solar heat gain and thermal conduction, which helps to reduce the size of the air conditioning system required.
These window systems also have benefits beyond improving HVAC. They improve occupant comfort by reducing “hot spots” from direct solar gain. Well-designed buildings using high-performance windows can reduce glare through proper selection of visible light transmittance values and/or incorporating architectural features such as light shelves or louvers. Similarly, high-performance skylights are available that reduce solar heat gain and heat conductance, while maintaining sufficient visible light transmission for daylighting applications.
Use cool roofing materials
Cool roofs cut cooling loads through the use of reflective materials that limit solar heat gain, which reduces the temperature of the plenum space between the drop ceiling and the roof. That space houses the majority of the ductwork in small commercial buildings. Duct heat gains and air leakage losses (especially on the return side) can increase HVAC loads on the order of 30%, so a cool plenum can reduce energy consumption and improve occupant comfort.
Just how much a cool roof will save depends on the location and R-value of the roof insulation. Well-insulated roofing systems with the insulation applied to the roof deck or the interior of the roof surface are less affected by cool roofs than poorly insulated buildings or buildings with lay-in insulation.
Cool roofs are typically white with a smooth texture, and fall into two categories: single-ply and liquid applied. White single-ply products are made from synthetic materials such as EPDM, PVC, and Hypalon. Liquid-applied products made from elastomeric, polyurethane and acrylic bases may be used to coat a variety of substrates. The accompanying table summarizes cool roof product properties.
Improve roof insulation
Roof insulation can be installed directly on the roof deck, while ceiling insulation is generally applied on top of the drop ceiling (called “lay-in” insulation). Applying the insulation to the roof minimizes the impacts of duct conductive losses and duct leakage because the plenum space between the roof and the ceiling is located within the thermal envelope of the building.
Lay-in insulation is generally less effective because of lighting fixtures, HVAC diffusers, fire sprinklers, and other devices that interfere with insulation installation. Insulation installed on ceiling tiles inevitably gets displaced as ceiling tiles are moved to gain access to the plenum space for data and telecom wiring, reconfiguring the HVAC diffuser layout, and other maintenance activities (see photo).
HVAC unit placement
The placement of an HVAC unit on the roof plays a big role in its operating efficiency, reliability and serviceability. Choose a site that minimizes duct runs while staying within architectural requirements for hiding the unit from view. Be wary of high parapet walls or unit enclosures, which can inhibit air flow around the unit and increase local air temperature. High rooftop temperatures can reduce unit cooling capacity and efficiency. In addition, it is important to provide service access to all units that allows for access panels and doors to be removed without interference. Locate units away from exhaust air outlets to improve indoor air quality.
Putting it all together
To show the interactions between the load avoidance strategies discussed above and HVAC system size and costs, researchers ran a series of computer simulations on a simple “box” model of a small commercial office building. The load avoidance measures in this example reduced the size of the HVAC system and provided annual energy savings, as shown in the accompanying graph. Improving the roof insulation system had the largest effect on HVAC system size, due to reduced roof loads and duct leakage interactions.
In addition to integrated design concepts, the Small HVAC Design Guide also covers equipment sizing and selection, ducting, ventilation, controls, commissioning, and operation and maintenance. Go to www.plantengineering.com , and click on this story for the link to the full report. Article edited by Bob Vavra, 630-288-8779, BVavra@cfemedia.com .
Cool roof products
|Adapted from “Inclusion of Cool Roofs in Nonresidential Title 24 Prescriptive Requirements, Code
Change Proposal for 2005 Revision to Title 24.” San Francisco, CA. Pacific Gas and Electric Company.
|Reflective coatings||Kool seal elastomeric over asphalt shingle||0.71||0.91|
|Aged elastomeric on plywood||0.73||0.86|
|Flex-tec elastomeric on shingle||0.65||0.89|
|Insultec on metal swatch||0.78||0.90|
|Enerchon on metal swatch||0.77||0.91|
|Single-ply roof membrane||White EPDM||0.69||0.87|
The full report of Small HVAC Design Guide can be obtained by clicking on this link