Packaged Rooftop Solution Improves Energy Efficiency, Occupant Comfort

Located in Columbia, Pa., Park Elementary is a kindergarten-through-sixth-grade public school with approximately 500 students and 50 teachers. Recently, the school's aging HVAC system was in need of replacement. Each classroom was served by hot-water heating-only vertical unit ventilators and cooling-only constant-air-volume (CAV) packaged direct-expansion (DX) rooftop units. The gymnasium, cafeteria, and administrative office were served by CAV packaged DX rooftop units, each with a hot-water heating coil.

“I wanted the replacement units to be energy-efficient, provide a more comfortable environment for the students and teachers, and improve the school's indoor-air quality (IAQ), including meeting the current ventilation-code requirements,” Gary Trostle, the specifying engineer for the Columbia Borough School District, said.

OPTIONS

One proposed solution was standard variable-air-volume (VAV) packaged DX units with hot-water-reheat VAV terminal units for the classrooms and administrative office and standard CAV packaged DX rooftop units with hot-water heating coils for the gymnasium and cafeteria. It would be a low-initial-cost solution that would be easy to install and able to meet ventilation-code requirements. However, it would not improve occupant comfort because the units still would have on/off compressor staging and, thus, would not be able to tightly control the temperature of the air supplied to the VAV terminal units, resulting in classroom temperature swings. Also, the units would be only moderately more energy-efficient.

Another proposed solution was hydronic cooling-and-heating VAV and CAV rooftop air handlers. With the installation of a high-efficiency chiller plant, the system would be more energy-efficient. Also, with the air handlers able to match the building's temperature and humidity loads with their modulating water valves, it would improve occupant comfort and IAQ, as well as be able to meet ventilation-code requirements. However, the cost of the air handlers, chiller plant, installation, and system maintenance would be significant, making the solution nearly infeasible.

SOLUTION

Ultimately, Park Elementary chose AAON RM Series packaged DX units with AAONAIRE total-energy-recovery wheels and Digital Scroll compressors, which effectively met all of Trostle's replacement goals.

“Including factory-installed energy-recovery wheels and Digital Scroll compressors on the units substantially improved the units' energy efficiency and ability to provide occupant comfort,” Troy Schreffler, an AAON sales representative with HC Nye Co., said.

For heating, the CAV units and VAV terminal units include hot-water heating coils.

Because RM Series units can be specified with up to 100-percent outside air, meeting ventilation requirements is not a problem. Also, the units include additional IAQ-improvement features, such as double-wall construction, which provides a cleanable interior surface, reduces resonated noise, and prevents insulation from being directly in the air stream, and 4-in. MERV 7 pleated filters, which remove contaminants down to about 3 microns. Lastly, the RM Series solution required no more installation and requires no more maintenance than the proposed standard-packaged-DX-rooftop-unit solution.

FEATURES

Also attractive to the school were several standard features of RM Series units:

• The exterior paint, which exceeds a 2,500-hr salt-spray test, per ASTM B117-95, Standard Practice for Operating Salt Spray (Fog) Apparatus.

• The slide-out backward-curved supply-blower assembly, which provides increased static-pressure capabilities and is easy to service.

• The full-length stainless-steel piano hinges and zinc-cast handles on the access doors and the unit-specific color-coded wiring diagrams in both point-to-point and ladder form, which make the units easy to service.

The gymnasium and cafeteria units include two options the VAV classroom and administrative-office units do not:

• Hot-water heating coils to match up with the school's boiler system, allowing the units to provide energy-efficient load-matching heating, along with Digital-Scroll-compressor load-matching cooling.

• Return-air carbon-dioxide (CO₂) override sensors, which open the outside-air dampers more than the standard occupied amount to provide more fresh air and better IAQ when indoor CO₂ levels exceed a field-adjustable set point.

DIGITAL SCROLL COMPRESSORS

The Digital Scroll compressors vary the amount of refrigerant flowing through the cooling system. Cooling capacity can be modulated from 10 to 100 percent, eliminating the supply-air-temperature swings common with standard DX VAV units. The compressors run for longer periods of time, dehumidifying the air more and cycling on and off less, which reduces wear.

Trostle said he specified the compressors “because the VAV units needed something that would give us good temperature control as the volume of air supplied is decreased.”

The Digital Scroll compressors also allow the CAV units to match space load consistently, improving those units' energy efficiency as well.

TOTAL-ENERGY-RECOVERY WHEELS

The optional AAONAIRE total-energy-recovery wheels included on the school's rooftop units help to reduce energy consumption by preheating, precooling, humidifying, and dehumidifying ventilation outside air (depending on the conditions). Mounted in the outside-air-intake and exhaust-air streams of each unit, the wheels transfer heating (or cooling) capacity from the exhaust-air stream to the entering outside-air stream. Depending on the air conditions, an AAONAIRE energy-recovery wheel preconditioning outside air can increase a unit's tonnage by as much as 30 percent and double its energy-efficiency ratio.

OCCUPANT COMFORT

Park Elementary School is extremely satisfied with the RM Series packaged DX units.

“The AAON units provide the students and teachers with a comfortable learning environment and save the school district money because they are energy-efficient and easy to service and maintain,” Mike Miller, Columbia Borough School District's operations-and-maintenance manager, said.