This study was designed to help those involved with development and implementation of California’s Title 24, Part 6, Building Energy Efficiency Standards (“Title 24”) better understand real-world performance of dedicated outdoor air systems (DOAS). DOAS ventilation, which separates outdoor ventilation air delivery from space cooling and heating for thermal comfort, is a mature technology used extensively in Asia and in parts of Europe, but with relatively little market share in the US. DOAS, when coupled with Variable Refrigerant Flow (VRF) or hydronic heat pump space conditioning systems, can provide an efficient, cost-effective, all-electric alternative to the conventional mixed-air systems more commonly found in California nonresidential buildings. However, how each DOAS HVAC system is specified, configured, and operated will determine the performance, energy savings, and emissions reduction potential for this technology.

Major Findings

Field monitoring of the four DOAS field sites with VRF revealed HVAC annual energy cost reductions of 41% to 66%¹compared to the California benchmark for building energy end uses for HVAC² (CEUS 2006).
The use of heat recovery ventilation or energy recovery ventilation in a DOAS unit reduced HVAC annual energy costs by 4% to 14%³.
The use of ventilation heat recovery reduced building electrical peak demand in summer months by 0.1 W/sf and 0.3 W/sf in winter months in mild climates (CZ03, CZ04). In the more extreme climate zone, CZ12, ventilation heat recovery was estimated to have reduced the summer peak by 0.5 W/sf and winter peak by 0.7 W/sf.
DOAS supply and relief fans in office and classroom sites used between 0.16 W/cfm and 0.57 W/cfm. Since most of the fans were smaller than the smallest fans regulated by the energy standard, the power per unit airflow was compared with the smallest fan power allowances the energy standard does regulate. On average, the ventilation fan system used 67% less power than the T24 2019 Part 6 requirements and 47% less power than fan power assumptions used in the 2022 Codes and Standards Enhancement (CASE) proposal for non-residential HVAC Controls (California IOUs, 2021).
Across the four sites using a VRF heat pump system, the average operational power of all terminal unit fans (fan coils or fan cassettes) per site was 12%, 17%, 29%, and 69% of the installed maximum fan power at full airflow for each respective site. These findings suggest a significant potential for demand and energy savings from turn down of distributed fan terminal units with multi-speed controls.
DOAS configurations carry a first-cost premium compared with single zone RTUs and mixed-air VAV systems. Market research suggests that as VRF and other systems are adopted more widely, the overall cost of a DOAS configuration will become competitive.
Analysis of VRF heat pump energy use data indicates that sites used between 16% and 34% more cooling energy during airside economizing weather conditions than a comparable building with full economizing capabilities would have⁴ these findings suggest that variable speed heat pump systems such as VRF are more efficient than the energy standard recognizes today, which may provide opportunities for further code enhancements.

Keyword Search:

Dedicated Outdoor Air System, Decoupled HVAC, DOAS, Variable Refrigerant Flow, VRF, Ventilation, Heat Recovery Ventilation, Energy Recovery Ventilation, HRV, ERV, building electrification, energy efficiency, retrofit, office, non-residential, commercial, dormitory, demand control ventilation, peak demand reduction, RTU, VAV, heat pumps, building controls, economizing

[1] Energy use data for HVAC gas and electric components normalized based on commercial energy rates of $0.20/kWh and $1.00/therm.

[2] CEUS benchmark data from 2006 for specific building types and locations was used to create two average benchmarks, one for a classroom/office building in Sacramento region, and one for small offices in the greater bay area.

[3] Calculations include evaluations of sites with HRV/ERV and sites without (DX-DOAS sites) by combining field data measured with additional assumptions for increased pressure drop and site-observed temperature conditions. The same commercial energy rates were applied to estimate the impact in energy costs.

[4] For the purposes of this comparison, the authors assumed that the economizer on a system with full economizing capabilities would provide 85% of the cooling load when outdoor temperatures are between 56F and 65F and 15% of the cooling load when outdoor temperatures are between 66F and 75F.

Code Readiness: Dedicated Outdoor Air System Field Assessment Results

Major Findings