Today’s tight building envelops with high occupant density and internal loads require year-round cooling in interior zones. High performance air systems offer free cooling from airside economizers at lower outside temperatures, while VRF systems must run the compressors at all outdoor air [latent too]conditions to satisfy cooling demand. VRF proponents often talk about energy-recovery. However, that requires simultaneous compressor-heating and compressor-cooling operation (compression cycle). Contrast this to a modern airside economizer used with HPAS. Additionally, HPAS offers further part load energy saving measures using advanced control technologies which include ventilation optimization based on fresh air demand by zone, comparative enthalpy economizers with remote indication, supply air temperature reset tied to measured system demand and optimum start/stop.
As with VRF, HPAS are also available with variable compressors and fans. This combined with the refrigeration/heating energy saving options not available with VRF save even more energy. These options include the ability to provide lower cost, and efficient gas heating systems, trending and automated reporting of total system diagnostics, user-specific building automation control strategies, measurement and control of airflow, thermal energy storage in the building structure (pre-cooling with economizer-flush at night) can reduce peak demand and shift the cooling load to times of cooler OA.
The most publicized comparison of energy use of between a ducted air system and a VRF system is the ASHRAE Headquarters Building in Atlanta, GA. The building uses three separate HVAC systems: a variable refrigerant flow (VRF) system for spaces on the first floor, a ducted ground source heat pump (GSHP) system, primarily for spaces on the second floor, and a dedicated outdoor air system (DOAS), which supplies outside air to both floors. The VRF system consumed 61% more energy than the ducted GSHP.
2012 Energy usage for GSHPs – 24,430.89 kWh (Jan – Dec)
Heating / cooling area for GSHPs – 15,558 sq. ft.
GSHP = 1.57 kWh / sq. ft.
Energy usage for VRF – 46,066.54 kWh (Jan – Dec)
Heating / cooling area for VRF – 18,226 sq. ft.
VRF = 2.53 kWh / sq. ft.
VRF consumed 61% higher annual kWh/sq. ft. in 2012
(2.53-1.57)/1.57* 100 = 61%
Risk of refrigerant leakage in occupied spaces is a safety issue for VRF but not for high performance air systems where refrigerant is located in an equipment room, rooftop unit, or minimal line-runs. Users must be cognizant of ASHRAE 15 requirements. The space served by each zone must be large enough to disperse the entire system refrigerant charge per ANSI / ASHRAE Stds. 15 and 34 and local codes. A maximum of 25 lbs of R-410A per 1000 cuft of room volume is allowed for non-institutional spaces. Transfer ducts between rooms may be required. ASHRAE 15 refrigerating machinery room requirements also require venting of fusible plugs. When any system contains more than 110 lbs. of refrigerant these fusible plugs must be discharged “to the atmosphere at a location not less than 15 feet above adjoining ground level and not less than 20 feet from any window, ventilation opening, or exit”
High performance air systems require no scheduled maintenance in occupied spaces. For VRF indoor unit filter replacement, coil cleaning, and visual inspection of condensate drain must be done for every indoor fan coil on a scheduled basis. Both maintenance above the ceiling, condensate pan overflow risk and refrigerant leaks can result in ceiling damage and inconvenience to occupants.
Equipment performance is certified by ASHRAE and AHRI for high performance air systems and assures owners that performance meets specified levels. ASHRAE certified air performance covers fan efficiency grades, sound levels and ducted air system leakage. AHRI certified thermal performance covers refrigeration system efficiency.
High performance ducted air systems overcome many of the disadvantages of non-optimized ducted systems. And compared to VRF, HPAS systems provide better comfort, meet ventilation codes, have a lower initial cost, consume less energy, carry no refrigerant risk, require less maintenance and provide performance certified by AMCA, AHRI.
VRF and other non-ducted systems certainly have their application as described herein. Additionally, they make sense for many mixed-use applications. However, when system designers have a choice, it is easy to see why ducted systems – especially HPAS ducted systems – are North America’s number one choice for comfort cooling.
For more information about Ruskin’s complete product line, application and design support, and our state-of-the-art manufacturing capabilities, contact your local Ruskin representative nearest you or Contact Ruskin directly at (816) 761-7476.