As most HVAC engineers already know, there is a major focus on energy efficiency in the building services industry, with significant reduction measures embedded in model energy codes such as ASHRAE 90.1-2010. Coupled with the Architecture 2030 challenge now adopted by the American Institute of Architects and the U.S. Conference of Mayors, the drive toward net-zero energy solutions by 2030 will require severe reductions in energy use as compared to business-as-usual.
This often leads design teams to consider the use of natural ventilation as a method to offset refrigerant-based cooling during a portion of the year. It is important to remember that natural ventilation is but one of many lower energy design concepts in the engineer’s toolbox and is most likely the one with the greatest constraints on its applicability.
Acknowledging that design teams are considering natural ventilation in greater numbers, this article is an overview of a number of practical concerns that must be seriously addressed prior to and during the design of natural and/or mixed-mode ventilation schemes. The article is loosely organized into three main categories of questions:
Who and what: This section summarizes the main sources of knowledge related to natural ventilation and gives a brief overview of the key definitions related to this portion of the field.
When and where: This section shares the results of a climate analysis study that maps the percentage of time during which natural ventilation may be of benefit as an alternate cooling source, and lists the main issues that limit the use of natural ventilation.
Why and how: This section describes why natural ventilation schemes might be applied in different contexts (and why they are not), and how they are generally configured.
Who and what
The prevailing expert in the United States in the field of natural ventilation is Dr. Gail Brager of the Center for the Built Environment at the University of California at Berkeley. Her field study-based research led to the development of the adaptive comfort standard that is incorporated into ASHRAE Standard 55. The UC Berkeley team maintains a website with a searchable database of naturally ventilated and mixed-mode buildings as well as case studies and reports of ongoing research in this part of the field.
According to 2009 ASHRAE Fundamentals, “Natural ventilation is the flow of outdoor air caused by wind and thermal pressure through intentional openings in the building’s shell.” ASHRAE Standard 62.1-2010 section 6.4 sets the opening sizes and configurations required for an area to be defined as naturally ventilated, and advises that mechanical ventilation systems are required to be present in conjunction with natural ventilation systems except when an engineered natural ventilation system is provided or an unconditioned zone has permanently open openings during all times of expected occupancy.
Lastly, ASHRAE Standard 55-2010 sets occupant-access and temperature-based limitations on the use of an extended range of acceptable temperatures to define comfort under natural conditioning (i.e., natural ventilation that is controlled by occupants to adjust thermal conditions in a space). This extended allowable range of indoor temperatures is known as the adaptive comfort model, which was developed on the basis of 21,000 data points from empirical field measurements in naturally ventilated buildings. It appears to occur based on a combination of occupant discretion in apparel, physiological effects, and psychological factors, with cognitive acknowledgement of the cooling source’s temperature limitation partially influencing subconscious expectations of indoor environment.
It is often the case that the free inflow of outside air alone cannot meet the thermal needs of a space. In these situations, many designers apply a mixed-mode (or hybrid) strategy in order to gradually increase the amount of cooling possible through ever-increasing energy use intensity. For instance, a space may go from an early morning heating demand to a beautiful mid-morning temperature that causes the occupants to open up the windows and enjoy the fresh air. As the day heats up after lunch, ceiling fans or an uncooled mechanical ventilation system may turn on to assist with occupant comfort. When the day get so hot that these measures are unsuccessful, then most mixed-mode systems will revert back to a condition of either traditional air conditioning or a scheme that uses radiant cooling coupled with limited amounts of natural ventilation. As the day wears on, energy savings can be accrued by monitoring the falling outside air temperatures and encouraging occupants to re-open the space as soon as possible.
As ASHRAE Handbook Fundamentals points out, air-side economizers can technically be considered to be a hybrid ventilation control scheme. The advantage that natural ventilation has over most economizer schemes is that both fan energy and cooling energy can be saved, and its disadvantage is the lack of filtration and the inherent fluctuation of temperature, humidity, and pressure relationships within the space.
When and where
Key questions that are always raised with respect to natural ventilation or natural conditioning are when and where it can be used; these questions typically initially revolve around local outdoor air quality and the local climate.
The disadvantage to a natural conditioning scheme is that there is rarely a particulate filter on an exterior window opening. Whatever pollutants are present outdoors, by definition, are present in at least the same concentrations indoors with a natural ventilation design. In order to understand what pollutants may exist outdoors per ASHRAE 62.1, the designer should review with the owner the U.S. Environmental Protection Agency’s maps showing the local area’s National Ambient Air Quality Standards “attainment” or “non-attainment” status for the monitored pollutants and compare them to prevailing Occupational Health and Safety Administration standards. In addition to this published data for the municipality, the designer should identify any unusual sources of pollution near the site (refineries, industrial plants, major highways, landfills or garbage collection areas, etc.) and assess the air quality through testing as necessary to ensure health standards. Additionally, the designer and developer should assess the needs of the user/tenant population, especially with regard to those with allergies or sensitivities that may require accommodation in an enclosed space with a more traditional filtered and air-conditioned indoor environment under the Americans with Disabilities Act requirements.
The second major set of criteria in this category revolves around the climate in which the project is located. While many of the ASHRAE standards take care to limit the sole reliance on natural ventilation to use in unconditioned spaces, there is little guidance on the appropriate use of natural ventilation in a mixed-mode configuration. While rules of thumb for early phase assessment can be reviewed for gross applicability, each engineer must understand the unique climate characteristics of the site before pursuing a scheme relying on natural ventilation for cooling.
One of the most frequently employed mixed-mode approaches is called “changeover,” which tends to be appropriate if outdoor air conditions are suitable for natural ventilation only during a portion of the year. The key project-specific question with this approach is whether there are a sufficient number of hours in the year to justify the expense of the secondary operable window system over and above the first cost of the base building’s HVAC system. Except in areas with greater than 30% of hours of deferred cooling, it is rare for annual energy savings alone to generate a sufficiently short payback period to be of noticeable economic benefit.
Nevertheless, operable windows are often a desired amenity, and there may be other drivers besides energy to engage with this design strategy. Therefore, it is important to understand specifically and seasonally when the use of open windows would be possible. Figure 2 summarizes for the same ASHRAE climate zones the percentage of hours in four typical months in which natural ventilation or natural conditioning may be possible. Most climate zones have the possibility of changeover types of systems for at least one season in the year.
In addition to the climate, it is also important to assess whether the project site has access to moving air. In some downtown venues where prevailing winds are diverted around whole blocks of construction, it is sometimes not possible to get air movement around the building itself, much less through openings in the façade. Similarly, it is useful to mine the *.epw data in order to understand the prevailing wind conditions so that the building massing can be reviewed and adjusted to capture natural breezes effectively. Lastly, it is important to determine in mid-rise and high-rise buildings in strong wind regimes what the indoor effects would be of opening the windows even a crack, since velocity increases significantly with height.
An additional consideration that often arises with natural ventilation schemes is the problem of noise breaking in from surrounding roads or neighboring buildings. The first thing to accept is that traffic noise in urban environments will break in through any wall opening. In a recent acoustic measurement study of a downtown New York office space, noise measurements 3 ft in from the open windows coupled with occupant surveys showed that the internal noise environment could support good speech intelligibility, despite measured noise gain from traffic. It is worthwhile to assess the potential noise break-in from outdoors whenever natural ventilation is considered to understand if increased background noise indoors will adversely affect the concentration of occupants. It is often the case that the increase in traffic noise is offset by the elimination of typical HVAC system noise during the natural ventilation mode. Additionally, it is also necessary as well to understand the occupants’ cultural expectations related to neighbor-generated background noise and speech privacy whenever any whole-building natural ventilation schemes are proposed. On one recent project, speakers for a white noise generator were integrated into the lighting fixtures to add back in masking noise to ensure speech privacy, as there was the possibility of cubicle-to-cubicle sound reflections off of the building’s concrete ceiling slabs (which were left exposed to engage thermal mass cooling).
Once the decision has been made that the outdoor air and climate are of a quality to support natural ventilation, then the last issue related to when and where is that of occupancy type. Care must be taken to ensure that the indoor air quality and cleaning standards are appropriate to a naturally ventilated building—sensitive electronic equipment, fragile art objects or musical instruments, or patients with respiratory distress should probably not be treated with natural ventilation schemes due to both particulate and humidity concerns. Similarly, areas of the building requiring tight pressurization or flow-direction relationships (such as laboratories) will not achieve their occupant-protective function in a naturally ventilated space. Lastly, it is of crucial importance for the designer to clearly articulate and predict the range of indoor temperatures and humidities so that this may be reviewed with tenants and any of their associated unions.
In this author’s opinion, it is a professional responsibility of the engineer of record to provide dynamic heat transfer and comfort modeling analysis in order to predict likely ranges of temperatures anticipated within the space. This data must be shared first with the original investor/developer in a simple and transparent way, and then through training with the tenants, so that it is clearly understood that statistically speaking, there will be days and hours during which the naturally ventilated areas of the building will exceed the traditional upper limit on indoor conditioned air temperature. Similarly, in mixed-mode systems, it is absolutely necessary to calculate and state the “time to recover” from the natural ventilation mode to the fully air conditioned mode, and to write the controls sequence of operations to advise occupants of when it is time to start shutting the windows, particularly in more humid climates.
One aspect that people often forget is that natural conditioning presumes that there is sufficient cooling airflow from the outdoors to absorb the heat generated within the space. It is important for the designer to use normal convective heat transfer calculations to determine what air-change rate is required to achieve the indoor space temperature intentions (even with the adaptive comfort model). Most clients will be interested in weighing loss of comfort against potential energy savings and potential first costs.
For instance, in a recent project, a lifecycle cost analysis was performed to evaluate adding a chilled-water fed air conditioning system to the kitchen area of an otherwise naturally ventilated space with radiant secondary cooling. The results reported the first cost, annual energy savings, percentage daytime hours above 70 F/ 74 F/ 78 F, and number of U.S. Green Building Council LEED points achieved through modeled energy savings for each of the design permutations. In the end it was determined that the food service areas not only would require a high level of filtration, but also that it was not acceptable for permanent employees working over the hot cooking surfaces to be forced to have a further excursion in maximum temperature, whereas the short-term sedentary students in the dining areas would not be adversely affected by larger ranges of temperatures and may appreciate the open atmosphere that a naturally ventilated space would bring.
Why and how
There are often reasons beyond energy savings that are the drivers for natural ventilation schemes. It is crucial for the design team to understand whether natural ventilation openings are being provided as an amenity of convenience to the occupants or whether the openings must be of a size and placement to ensure significant removal of heat from the space. The relative sizes and distribution of openings between these two approaches are very different.
As noted in the definition of natural ventilation and its associated equations from the ASHRAE Handbook- Fundamentals, there are two primary physical mechanisms that drive flow between openings of different pressures. The mechanisms tend to lend themselves to the typical configurations as follows:
Localized thermal buoyancy effects: This usually manifests itself as single-sided ventilation, which is usually applied at a discrete perimeter zone level. It is particularly effective if high and low level openings are provided or if one tall opening is provided to allow two-way flow. This configuration can be effective even when the zone is not open to the entire floor plate, but the interior zones will usually continue to require mechanical ventilation systems.
Whole building thermal buoyancy effects: This usually involves the use of light-wells, chimneys, or atria to generate a stack effect across the height of a building sufficient to create airflow from the perimeter, across internal spaces, and into the large-volume stack zone from which the heat leaves the building. These schemes usually must have engineering design and analysis, and have some level of automated operation, as there can be adverse effects on upper levels with respect to high-level heat back-spilling into occupied spaces. Additionally, it is rare that these systems can isolate the effects of wind from those of buoyancy. Therefore, care must be taken to examine both what happens on a still day, when there is insufficient heat generation within the space to drive flow, and what happens on a windy day, when thermal buoyancy effects are negligible as compared to wind pressures.
Whole building cross-ventilation: This usually requires a relatively shallow floor plate in the direction of prevailing wind flow. If there is no strong wind predominance, then the fall-back design scheme tends to invoke atria and chimneys with wind catchers or other monitor devices that take advantage of the Venturi effect to create velocity-generated suction pressures at the top of an air volume. The concerns that must be addressed through engineering and analysis include the control of air speed and direction within the occupied space.
When natural ventilation is coupled with auxiliary cooling, the following definitions are commonly in use:
Changeover mixed-mode ventilation: In this scheme, mechanical cooling and natural ventilation will serve the same space but at different times during the year. This type of configuration is primarily exploiting the seasonal results of the climate analysis above, and is usually triggered by relying on human choice or through outdoor temperature tracking, depending on the sophistication of the system. In general, keeping these systems simple but providing feedback to the users about appropriate interaction with the building façade is beneficial. Some recent schemes employ a red-light/green-light system to remind users that the outdoor air is sufficiently cool to open the windows, while others use e-mail or text messaging to provide similar information. Usually, window-switches are incorporated into changeover schemes, especially when occupants are in charge of opening and closing windows, to ensure energy savings. Similarly, motorized windows that open in response to outdoor air temperature may be appropriate in transient areas with shared occupancy (lobbies, atria, dining areas, conference rooms, etc.), as areas with no permanent ownership tend to lack active occupant participation with windows unless the base-condition upon entering is considered to be inadequate. Air-based cooling systems are appropriate for changeover mixed-mode systems as the HVAC operates in an on/off mode and there is less risk of air conditioning the outdoors as compared to other mixed-mode ventilation types.
Concurrent mixed-mode ventilation: In this scheme, mechanical cooling and natural ventilation will serve the same space at the same time. This type of configuration is most often seen incorporating a form of radiant cooling or heating as supplementary heat transfer in the space to assist with human comfort. Radiant cooling has benefits over air-based cooling systems in the concurrent mode because it saves fan energy and limits the energy waste associated with dilution of cooling air by the air from outdoors. With radiant cooling as a predominantly independent heat transfer mechanism as compared to convective cooling through “wind-on-skin,” the space can be allowed to have a dry-bulb temperature rise up to 80 F or higher, with radiant heat absorption occurring due to an artificially lowered mean radiant temperature, thereby reducing perceived temperature.
Zoned mixed-mode ventilation: In this scheme, mechanical cooling is provided to certain spaces, while other spaces in the same building experience natural ventilation. This scheme is often used with deep floor plates, as natural ventilation has a limited applicability only to the ~20 ft deep perimeter zones that have direct access to window openings. Additionally, there are often high heat load areas, such as server rooms or high-density occupancy conference rooms or classrooms, that may not be able to achieve sufficient cooling based just on outside air temperature. These schemes must take care to avoid adverse effects on the energy-consuming interior space HVAC systems due to pressure variability in the naturally ventilated zones. Additionally, some schemes employ a temperature reset algorithm for the interior HVAC systems in order to avoid excessive differences in perceived temperature between naturally ventilated zones and those zones with constant air conditioning.
As design develops for a natural ventilation scheme, it is important to also bear in mind the following construction phase issues:
- Specification responsibility for window elements
- Constructability and trade-coordination of window elements containing motorized devices and control switches
- Provision of highly detailed controls sequence of operations if motorized window actuators are used, due to contractor unfamiliarity with the nonstandard comfort-conditioning device
- Commissioning of natural ventilation systems, including confirmation of airflow speeds and direction and temperature stratification patterns as compared to original design intent
- Education of the occupants on their role in engaging with the façade to achieve their own comfort
This article has attempted to summarize the key aspects to be explored when considering a natural ventilation or mixed-mode scheme, particularly emphasizing the use of climate data in the evaluation of natural conditioning potential by season and climate zone. As designers understand the applicability of this age-old design method now coupled with newer supplementary mixed-mode approaches, it may be possible to offset some portion of energy use associated with air conditioning indoor spaces. How much energy could be saved is, of course, unique to each individual project based on the amount of perimeter space, the proposed natural ventilation design, and the climate constraints. Nevertheless, it is hoped that the presentation of this consolidated material will be of use to designers considering natural ventilation as a possibility.