About Mixed-Mode
What is Mixed-Mode?
“Mixed-mode” refers to a hybrid approach to space conditioning that uses a combination of natural ventilation from operable windows (either manually or automatically controlled), and mechanical systems that include air distribution equipment and refrigeration equipment for cooling. A well-designed mixed-mode building begins with intelligent facade design to minimize cooling loads. It then integrates the use of air-conditioning when and where it is necessary, with the use of natural ventilation whenever it is feasible or desirable, to maximize comfort while avoiding the significant energy use and operating costs of year-round air conditioning.
How do the air-conditioning and operable windows work together?

There does not seem to be a “standard” mixed-mode approach in practice today – each building continues to be unique. Yet there are a number of classification schemes that describe the integration of natural ventilation and air-conditioning control strategies, usually in terms of whether they exist in the same space, or operate at the same time.

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Concurrent (Same space, same time)
Concurrent mixed-mode operation is the most prevalent design strategy in practice today, in which the air-conditioning system and operable windows operate in the same space and at the same time. The HVAC system may serve as supplemental or “background” ventilation and cooling while occupants are free to open windows based on individual preference. Typical examples include open-plan office space with standard VAV air-conditioning systems and operable windows, where perhaps perimeter VAV zones may go to minimum air when sensor indicates that a window has been opened.

Change-over image
Change-over (Same space, different times)
Change-over designs are becoming increasingly common, where the building “changes-over” between natural ventilation and air-conditioning on a seasonal or even daily basis. The building automation system may determine the mode of operating based on outdoor temperature, an occupancy sensor, a window (open or closed) sensor, or based on operator commands. Typical examples include individual offices with operable windows and personal air conditioning units that shut down for a given office anytime a sensor indicates that a window has been opened; or a building envelope where automatic louvers open to provide natural ventilation when the HVAC system is in economizer mode, and then close when the system is in cooling or heating mode.

Zoned system image
Zoned (Differed spaces, same time)
Zoned systems are also common, where different zones within the building have different conditioning strategies. Typical examples include naturally ventilated office buildings with operable windows and a ducted heating/ventilation system, or supplemental mechanical cooling provided only to conference rooms. For many mixed-mode buildings, operating conditions sometimes deviate somewhat from their original design intent (e.g., a building originally designed for seasonal changeover between air-conditioning and natural ventilation may, in practice, operate both systems concurrently).

What are the potential advantages of mixed-mode buildings?

Mixed-mode buildings offer a variety of advantages over sealed-air conditioned buildings:

Reduced HVAC energy consumption
A well designed and properly operated mixed-mode building can scale back or eliminate the use of mechanical cooling and ventilation systems throughout much of the year, with associated reductions in pollution, greenhouse gas emissions, and operating costs. Ventilation with cool outside air can reduce a commercial building’s energy use by 15 to 80%, depending on climate, cooling loads, and building type.

Higher occupant satisfaction
Occupants typically want windows that can open. Mixed-mode buildings have the potential to offer occupants higher degrees of personal control over their local thermal and ventilation conditions, as well as a greater connection to the outdoors, which should lead to increased occupant satisfaction and reduced potential for IAQ problems. Past research has found that building occupants prefer a wider range of indoor thermal conditions when they are provided with some measure of personal control.

Highly “tunable” buildings
Mixed-mode strategies provide inherent flexibility and redundancy in the space conditioning systems of a building, resulting in potentially longer life, greater adaptability to changing uses, and reduced lifecycle costs. With the careful application of mixed-mode cooling and ventilation, one can anticipate somewhat smaller mechanical systems and extended HVAC equipment life.

What are the potential disadvantages of mixed-mode buildings?

Mixed-mode strategies also have the potential to add cost and complexity to a building, and in the worst case might yield frustrated occupants and excess HVAC energy consumption. Because there is less familiarity, more design time might be needed than with conventional buildings with standard HVAC systems. There is a concern in the industry that concurrent mixed-mode schemes may result in wasted energy if air-conditioning and natural ventilation are occurring in conflict with one another, yet there have been no studies to determine under what situations this might occur. The need for humidity control in some climates may also exacerbate this conflict between the benefits of a sealed and permeable envelope. In addition, it is recognized that natural ventilation may be undesirable in some situations due to air-borne pollutants and allergens, or outdoor noises.

Why aren’t we seeing more mixed-mode buildings?

Although mixed-mode buildings are more common in Europe, there are relatively fewer examples in the U.S. There are several potential barriers (real and perceived) to more widespread adoption of mixed-mode schemes, which are often based on a lack of understanding about these systems. These barriers, or gaps in our knowledge, fall into the broad categories of:

Building Design Issues
The U.S. building design industry is generally unfamiliar with mixed-mode cooling strategies, there is a lack of case studies and design tools to facilitate their education, and existing design standards leave little flexibility for unconventional or innovative HVAC designs.

Building Operations and Controls Issues.
Mixed-mode buildings generally require integrating automatic and manual control strategies for both HVAC and building fenestration systems, which can be significantly complex. Commercial building designers and operators also share a general lack of familiarity with operable windows (and other permeable building envelopes) and a concern about their associated maintenance requirements.

Fire and Safety Concerns
The potential for smoke migration in a commercial building designed to incorporate wind-driven or stack-driven ventilation is at odds with many local building codes. There may be further concerns about building security and occupant safety for commercial buildings with operable windows.

Energy Code Concerns.
California Title-24 and other energy codes tend to limit designers to fairly conventional HVAC systems. Standards generally deter the installation of operable windows and mechanical cooling systems for the same zone.

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