Project Title:

Control Strategies that Reduce Minimum Airflow to Save Energy and Maintain Comfort

Low-cost control programming shows the potential to reduce HVAC energy use up to 20%.

Status (updated 8/28/2014): Complete

Funding sources: ASHRAE, California Energy Commission PIER, Industry In-Kind Support, and CBE Industry Consortium

Project Objective

The study evaluated the thermal comfort and indoor air quality of buildings in which the variable air volume (VAV) box minimum airflow rates have been reduced to conserve energy.

Significance to Industry

Variable air volume box minimum airflow rates have tremendous energy implications. The lower the flowrate, the greater the energy savings. Simulations by Taylor Engineering and CBE show that reducing zone minimums in a typical office building from 30% to 20% can reduce HVAC energy use by 10% - 20%. Savings can be achieved in both new and existing buildings with minimal financial investments through low-cost control system re-programming.

Anecdotal evidence shows VAV minimum airflow has been used successfully in many buildings, but the strategy has not yet penetrated general design practice. Now that ASHRAE, PG&E, and PIER research has addressed the VAV box controllability questions, the thermal comfort and air quality study by this project will remove the final barriers to proposing changes in ASHRAE Standards 90.1, 62.1 and 55.

Research Approach

We conducted both field and laboratory studies for this project. The field study was conducted at a the Yahoo! campus in Sunnyvale, California. The campus contains seven buildings, 980,000 square feet, and over 3,850 occupants. A second field study was conducted at a county government building in Martinez, California.

The field study included three related activities: (1) study building control data, and survey the environment satisfaction of occupants, in approximately 10 buildings that are "toggled" between standard operation and low minimum flow operation; (2) conduct detailed ‘right now’ thermal-comfort and perceived-air-quality surveys in two of these buildngs; and (3) measure the air temperature and airflow distributions for these two buildings.

Laboratory tests, conducted with Price Industries, quantify the ventilation effectiveness and distributions of air temperature and velocity occurring under several diffuser types, ranging from 10% to 30% VAV supply volumes.

Findings show that reduction of VAV airflow minima can significantly reduce energy use, with no increase in dissatisfaction. The field study buildings showed average cooling energy savings of 13.5% and 28.8% at two sites, and reduced gas use by 6.1% and 12.2%. In winter thermal comfort satisfaction was generally unchanged, howeve in summer with the low flow operation, dissatisfaction was reduced by 32% and 62% at the field study sites. The report was approved by ASHRAE in 2014 and a journal paper summarizing results was published the following year.


Arens, E. et al. 2015. Effects of diffuser airflow minima on occupant comfort, air mixing, and building energy use (RP-1515). Science and Technology for the Built Environment, July (39 pp)

Arens, E. et at., 2012. Thermal and air quality acceptability in buildings that reduce energy by reducing minimun airflow from overhead diffusers. Final report. November (243 pp)


Arens and Paliaga, 2013. Evaluation of Low‐Flow Operation for Energy
Savings and Comfort
. Presentation at CBE Industry Advisory Board Conference, April.


Layout of Yahoo! campus in Sunnyvale, CA.


Field study using the building commissioning cart.