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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.
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 (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 (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 becoming increasingly
popular in Europe and Japan, there are relatively few 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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