Cost-Responsive Supply Air Temperature Reset Strategy

Innovative control sequence for building automation systems widely applicable today.

Status: Current

Funding Sources: NYSERDA CalNEXT CEC PIER program CBE match funding

Categories: HVAC Systems | Variable Air Volume Systems | Controls

Project Objective

The goal of this project is the development and demonstration of CORE (Cost Optimized REset), a new cost-responsive supply air temperature reset strategy for multi-zone variable air volume systems, which is simple enough to implement within a typical building automation system (BAS). CORE dynamically estimates the cost of fan, heating, and cooling energy at three different SATs (current, higher, lower) and chooses one with lowest cost as the setpoint.

Project Results

The initial study was a randomized six-month pilot that demonstrated approximately 17% reduction in HVAC energy costs with CORE. In three commercial medical office buildings in California, CORE led to 5–22% savings in combined heating, cooling and fan energy use, and 1–20% reductions in total HVAC energy cost relative to baseline control strategies. Another demonstration in New York City (ASHRAE Climate Zone 4A) is evaluating CORE performance in cold, humid conditions, extending validation to winter-dominant and dehumidification-limited systems.

In parallel, simulation studies showed that the CORE strategy could achieve up to ~30 % reductions in HVAC energy use and cost for representative multi-zone VAV systems operating across multiple California and New York climates. Results showed that the CORE algorithm consistently yielded higher energy cost savings than other control algorithms, despite variations in climate, energy tariff structure, and building design and operation. Compared to the best industry practice ASHRAE Guideline 36, the new CORE algorithm reduced energy costs by a mean (first – third quartiles) of ∼4 % (0.8–6.9 %) across all simulated cases. Moreover, the climatic conditions had a significant impact on the control performance. In milder climates, the new CORE algorithm achieved higher energy cost savings due to considerable economizer hours, e.g., with ∼7% (6.3–7.1%) savings compared to G36 and ∼31% (26.8–36.0%) compared to the worst-performing fixed SAT strategy for Oakland. Conversely, in more extreme climates with fewer economizer hours and dehumidification constraints, energy cost savings of the CORE algorithm were diminished, e.g., with 0.6% (0.2–0.7%) savings relative to G36 and 5.4% (4.3–6.2%) relative to the least effective Warmest SAT strategy for New York City.

Overall, the findings from both field and simulation studies confirm that CORE delivers robust performance across diverse operating and climatic contexts, with realized savings influenced by baseline control logic, sensor availability, and airflow variability. Lessons from implementation, spanning BAS integration, data quality, and commissioning, will inform ongoing efforts to simplify and scale adoption.

A statewide field study in three commercial medical office buildings in California showed 5–22% savings in combined heating, cooling and fan energy use, and 1–20% reductions in total HVAC energy costs compared to baseline control strategies

Significance to Industry

Variable air volume (VAV) HVAC systems dominate the commercial building market but have widely varying performance. Two studies by Lawrence Berkeley National Lab (LBNL) and the California Energy Commission (CEC) reported a wide variation in energy performance for various typical supply air temperature (SAT) reset strategies, with a 4-15% variation in HVAC energy use. Existing SAT reset strategies have three inherent deficiencies that explain the sub-optimal energy performance: (1) they include simplifications and assumptions about the relationship between SAT and total heating, ventilation and air conditioning (HVAC) energy cost; (2) they require tuning of key parameters whose optimal values differ for every building and vary over the life of the building; and (3) there is no easy way to determine what those optimal settings are and whether tuning is improving savings or not.

Research Approach

Building on an original field study at a UC campus building, the research team refined and extended the CORE sequence by developing a refined version with progressively reduced dependency on zone-level data. The research included field demonstrations and parametric simulation studies to evaluate CORE performance across a range of building types, climates, and control configurations.

Since the completion of this initial study, we have started additional demonstrations, in collaboration with TRC, Kaiser Permanente and Fordham University, with funding from NYSERDA and CalNEXT. Field implementations in California included three existing medical-office buildings selected for their diverse climates (California Climate Zones 8, 9, and 10), BAS infrastructure, and time-of-use rate exposure. Each site underwent baseline and CORE operation phases, and performance was evaluated using IPMVP-aligned measurement and verification protocols. The study also examined improved algorithm configurations suitable for retrofits with limited sensing, quantified commissioning effort, and evaluated packaging strategies for BAS integration. In addition, another demonstration project in New York City (Climate Zone 4A) is extending this research to colder, more humid climates. The NYC deployment is testing the algorithm’s adaptability to VAV systems under winter-dominant load conditions and integration with local utility structures. Findings from this work can inform national scalability and climate-specific tuning of cost-responsive SAT reset strategies.

Meanwhile, a parametric building energy simulation study using EnergyPlus was carried out using a typical office floor model served by a multi-zone VAV AHU, varying climate (three cities in California and another three in New York state), internal loads, etc. CORE was benchmarked against other commonly adopted SAT reset strategies, including fixed SATs, ASHRAE Guideline 36, and warmest‐zone reset approaches. These analyses provided insights into the algorithm’s performance under diverse operating conditions and informed climate-specific tuning and national scalability of CORE SAT reset strategies.

Publications and Reports

Presentations