Status (updated 1/17/2008): Continuing
Funding sources: CBE Industry Consortium, Research Grants
Project Objective
Develop a set of design recommendations for integrating operable windows with HVAC systems based on an initial field study. Later, develop a web-based library of mixed-mode building case studies, covering a range of climates, design approaches, and control strategies.
Significance to Industry
Mixed-mode refers to combining natural ventilation with air conditioning in the same building (e.g. operable windows in an air-conditioned office space) Mixed-mode strategies have the potential to offer “the best of all worlds,” by using natural ventilation to provide occupant control, high ventilation rates, and reduced HVAC energy, while using air-conditioning to maintain comfort when necessary during temperature extremes.
Designing effective mixed-mode buildings, however, presents a host of challenges for architects and mechanical engineers. Real and perceived barriers to mixed-mode buildings include general unfamiliarity with mixed-mode strategies, increased complexity in building operations and controls, fire and life-safety issues, energy codes, and humidity concerns associated with air-conditioned spaces that are not sealed.
Research Approach
The first phase of this project focused on investigating the issues of occupant satisfaction and control in four mixed-mode office buildings in Northern California. Each of these sites included air-conditioned office space where occupants have access to operable windows. Surveys were administered at each site to evaluate occupant satisfaction, operable window use, and occupant use of HVAC controls such as thermostats and adjustable air diffusers. Following the surveys, building operators and the building designers were interviewed to determine whether occupant response to the building met the operational and design intent.
The analysis of the field study made comparisons between each site, determining which design aspects of the building envelope and HVAC system were most successful in terms of occupant satisfaction and control. This study was published in Engineered Systems, May 2000, as “Mixed-mode Ventilation: HVAC Meets Mother Nature,” (link below).
In a subsequent phase of this research, we developed a web-based library of mixed-mode buildings, including a database of approximately 200 buildings and eight case studies. These case studies were created using existing information from publications, drawings and documents gathered from design firms, and from personal interviews. Every case study includes a consistent set of basic information about the building and HVAC design, operation and control characteristics. The website was launched in June 2006.
The subsequent focus of this research area was a study of control strategies for mixed-mode buildings, with a focus on understanding issues that guide the decision-making process for mixed-mode buildings, and documenting examples of control algorithms used in existing buildings. We examined buildings that use a combination of manual and automatic control of windows and mechanical system components. and the indoor and/or outdoor environmental conditions used as inputs in control algorithms. Results were published in October 2007.
Publications and Reports
Brager, G., S. Borgeson, and Y. Lee, 2007. Control Strategies for Mixed-Mode Buildings. CBE Summary Report, October.
Weeks, K., D. Lehrer and J. Bean, 2007. A Model Success: The Carnegie Institute for Global Ecology. Center for the Built Environment, University of California, Berkeley, May.
Brager, G. 2006. Mixed Mode Cooling. ASHRAE Journal, pp. 30-37. August. (Link to eRepository)
Brager, G., E. Ring, and K. Powell, 2000. Mixed-mode Ventilation: HVAC Meets Mother Nature. Engineered Systems, May, pp. 60-70.
