High performance ducted air systems (HPAS) use the best of what ducted systems can offer, and compared to VRF, provide better comfort, meet ventilation codes, have a lower initial cost, consume less energy, carry no refrigerant risk, require less maintenance and performance is certified by AMCA, AHRI.
HPAS consists of advanced digital controls, low-leak economizers for free-cooling, spiral/oval static-regain ducting, variable air volume technology, well-placed diffusers, high efficiency fans, superior filtration systems, air-to-air energy recovery, and variable flow compressors.
Comfort, flexibility, and efficiency remain the key objectives of office HVAC. With high performance air systems it begins with multiple zones using temperature control which can be as small as one individual office. VRF zone size is limited by the necessity for rooms large enough to disperse the refrigerant charge of the entire system per ANSI/ASHRAE Stds. 15 and 34 and local codes.
High performance air system offers better filtration with the option for MERV-13 or better filters, or photocatalytic air cleaning compared to the residential type filters found in VRF systems. Noise can also be an issue with non-ducted systems because of the small fans in the room or above the ceiling. Regarding ventilation, ASHRAE 62 ventilation codes must be met at all times during occupied hours. VRF systems must use a separate ducted system to provide ventilation air to each and every zone. This coupled with the need to run refrigerant lines throughout the building drive up the first cost significantly beyond a HPAS.
From a flexibility standpoint, it is easier to move diffusers and duct take-off branches, vs. DX fan coils and refrigerant lines.
What about cost? Typical installed costs* for various systems are summarized here
|VRF||$20 to $26/sq.ft.|
|Chilled beam||$30 to $45/sq.ft|
|VAV Rooftop/ High Performance Air System||$15 to $20/sq.ft.|
|Chiller and High Performance Air system||$17 to $24/sq.|
- Source: AMCA
Several energy-reducing features distinguish high performance air systems from basic ducted systems. For lower fan energy consumption, system designers achieve the best airflow performance by selecting the fan with the lowest power (not always the lowest cost, or the smallest fan). All AHU (air handler unit), rooftop, and fan manufacturer’s selection programs provide a variety of fan selections to meet the airflow and system pressure requirements Further optimization comes by avoiding oversizing design loads, lowering design supply air temperature, and specifying low leak spiral/oval ducting. Next is design of a lower pressure drop air systems using large coils, large filter banks, static-regain duct design, and aerodynamic ducts (large radius elbows and fewer transitions and joints), low pressure drop terminals and plenum returns. Complete the fan power equation by selecting efficient motors and drives, or efficient variable speed motors and drives for part load energy savings. A fan running at one-half the design airflow operates with one-eighth the power consumption.
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