By Michael P. Toohey
With an appropriate strategy and design, a naturally ventilated structure can be comfortable for all occupants, including those in a multi-storey building. This article examines how natural ventilation fits into mixed-mode setups, safety considerations for building occupants, and esthetic features, and also explores how it can relieve engineers of issues associated with pairing an off-the-shelf actuator with a standard window.
History of ventilation
Ventilation—the process of supplying and removing air through an indoor space—has been applied in structures for thousands of years. Igloos, teepees, Roman atria, and Persian wind-catchers are examples of how airflow has been harnessed to moderate environmental conditions or eliminate smoke.
Natural ventilation (NV) was the only kind of building ventilation prior to the development of the first mechanical fan. This was thought to date back to the 1500s in the Middle East, where a ceiling-suspended canvas-covered-frame would be actuated by servants pulling ropes to move the air. In the late 1800s, mechanical ventilation technologies and systems developed through the industrial revolution began making their way into large buildings, including Great Britain’s Houses of Parliament.
However, Willis Carrier, the father of air-conditioning, had perhaps the most dramatic impact on mechanical ventilation when he designed the first modern system in 1902 as a method of solving humidity-caused issues for a Brooklyn-based printing company. By 1915, he and six other engineers formed the Carrier Engineering Company, and human environmental comfort levels changed forever.
Natural ventilation’s second wind
As mentioned, it is only over the past few centuries natural methods have been overtaken by mechanical systems to make buildings more comfortable for occupants. Mechanical airflow systems are in place worldwide, but European and other Old World citizens are quite familiar with naturally ventilated buildings. In North America, people are far more accustomed to tightly sealed buildings with mechanically controlled air flow. However, natural ventilation is becoming more important, as it offers various benefits to both building owners and occupants.
Naturally ventilated buildings can offer lower construction costs, reduced energy consumption and maintenance requirements, less environmental impact, healthier air flow, and better long-term compliance to the emerging building codes and standards, such as Leadership in Energy and Environmental Design (LEED).
Natural, mechanical, or both?
Natural air flow (i.e. wind) and buoyancy are the two natural forces driving air through a building, and each provides a unique natural ventilation strategy. The first, cross ventilation, involves air driven by wind into a building on one side to force air out of the other. The second, stack ventilation, relies on the temperature difference between indoor and outdoor air and involves warm air rising to create a vacuum effect to pull in the lower, cooler air. Even if the air being drawn in is only slightly cooler, occupant comfort can be enhanced simply by the movement of the air, as the principles of heat transfer and evaporative cooling still apply. Further, natural ventilation is self-compensating—the amount of ventilation is improved when warmer, more buoyant air and wind moves across a roof vent, generating negative pressure and effectively vacuuming the warm air out. However, it can be difficult to achieve the desired environment with natural ventilation because of uncontrollable changes in wind speed, direction, and outside temperature.
Rather than relying on natural forces, mechanical ventilation uses fans to move air that has been heated or cooled through ductwork. While there are downsides to the mechanical ventilation—including increased power consumption, higher operating costs, more noise, high-maintenance requirements, and sick-building syndrome from a lack of outdoor air circulation—mechanical ventilation provides exceptional control of internal environmental conditions such as temperature and humidity regardless of outdoor conditions. Unlike natural ventilation, mechanical ventilation is set-point-established rather than self-compensating, so it will only ever provide the designed flow rate, which will diminish if the system is not properly maintained.
Unfortunately, there is no single answer to determine which method is the best choice. This depends entirely on the application. However, there is one certainty—it does not always have to be an either/or situation. Since each system has its own set of advantages and disadvantages, it is possible the best selection could be choosing both and going with a mixed-mode system.
One particularly good illustration of a mixed-mode benefit involves night-purging, where passive ventilation is kept closed during the day, but opened in the evening, exhausting the warm air out and chilling the thermal mass with cooler nighttime air.
Every building application is unique, and there are numerous criteria that can impact the selection of a ventilation approach. The following are 10 areas to consider when evaluating the type of system to employ.
Natural ventilation works well in buildings of a height that provides a good thermal gradient—generally a minimum of 6 to 8 m (20 to 26 ft). In these conditions, warm air leaves the building’s upper levels and is replaced by cooler air from the lower levels. Atria, malls, industrial sites, office buildings with large open spaces, and transportation facilities are good examples of this type of structure.
Effective ventilation may also be achieved in low-rise buildings by employing shafts connected to roof-mounted vents. This serves to create a chimney effect until the desired interior temperature is reached, at which point the vents are closed. Roof ventilators can either be flapped or louvred, and are used as the termination to the chimney, providing control of the heat release as well as weathering the shaft.
Naturally, a particular area’s ambient weather conditions can be important. For example, Calgary’s warm season tends to last from June 18 through September 7, with an average daily high of 23 C (73.4 F) and a low of 8 C (46.4 F). Indeed, the period from July 3 to August 20 is considered the most comfortable time of year from a dewpoint perspective, and the wide high-to-low temperature swing is ideal for night-purging, seemingly making Calgary a good natural ventilation location choice.
New York City, on the other hand, has a similar warm season that runs from June 4 through September 15, also with an average daily high of 23 C, but an average low of only 19 C (66.2 F). Further, the period from July 3 through August 26 features muggy and uncomfortable dewpoints. Neither of these facts preclude New York City buildings from using natural ventilation, but it does suggest mechanical cooling would be a better choice during the Big Apple’s hot August days.
It is important to factor the building’s location into the ventilation plan. Even in examples of perfect weather conditions, naturally ventilating a building located near an area where the air features high particulate concentrations (e.g. near stone crushing or cement plants), offensive odours (e.g. feed lots or water treatment facilities), or facilities generating a lot of noise (e.g. airports or rail yards), might not be the best overall choice for occupant comfort, even if the temperature remains cool.
Architects and HVAC contractors have a history of camouflaging mechanical ventilation systems, which can certainly be eyesores, even though their presence is generally accepted. Natural ventilation products can actually enhance a structure’s visual appeal and be used as part of a building’s overall design.
Products are available for glazed building façades to add a new dimension to a design, and can be supplied in numerous styles and colours. Units can be glazed in, effectively replacing a glass panel, or fitted onto steelwork.
Certainly, the initial cost to equip a building with two types of ventilation systems can be expected to be higher than had only one been selected, but it is quite feasible the additional initial costs may quickly be offset by reduced building operating expenses. Even if a natural ventilation system is engaged only 30 per cent of the time, months or even years of more useful service could be gained over a mechanical system that is always engaged. Lower maintenance can also be expected, along with reduced energy costs.
Typically, the payback on natural ventilation compared to mechanical ventilation can be less than three years. It is also important to consider performance of the actual ventilator—a thermally broken model will result in low heat loss and minimum air leakage. Maintenance on natural ventilators is low, and should be checked occasionally to ensure everything is working properly. With mechanical systems, filters need to be replaced and refrigerant levels need to be monitored.
Health, safety, and internal air quality
The Canadian Center for Occupational Health and Safety (CCOHS) and the U.S. National Safety Council (NSC) identify insufficient outdoor air intake as a common cause of internal air quality (IAQ) problems. In the United Kingdom, the National Health Service (NHS) states:
office workers in modern buildings without opening windows and with mechanical ventilation or air-conditioning are most at risk.
However, the benefits of fresh air intake through open windows needs to be weighed against the potential safety risks to the occupants of high-rise structures as well as to individuals on the ground. High-rise buildings may have restrictions to the amount windows can be opened due to fall risk. In this scenario, building owners can choose a glazed louvre model in order to restrict a large opening, but still maintain ample free area.
There are numerous types of building façade products and some natural ventilation systems that feature off-the-shelf actuators field-fabricated to fit operable windows and accomplish the ventilation objectives. While such executions may provide acceptable adequate ventilation, a units’ size, weight, and power requirements can negatively impact functional area usage, or damage window hardware and seals not originally intended for mechanical actuation. An alternative to this potentially dysfunctional design is a casement ventilator. This can allow design professionals to eliminate the need to retrofit or customize standard windows in the field.
Facility layout considerations
Natural ventilation systems use little electricity, resulting in a minimal impact on a structure’s carbon footprint. Careful consideration does, however, need to be paid to air flow and ‘free areas’—the effective area of ventilation, not just the physical area of the unit—in the initial design and configuration of the building’s interior to assure ambient conditions and heat loads are adequately addressed.
There are numerous applications when simple set-point activation—where the vents are actuated at a particular temperature—are all that are required to control ventilation. Other applications may require more complex systems connected to a building’s automated management system.
This approach allows different vents to open and close at various temperatures to control and blend fresh air input depending on the season. Advance knowledge of ambient conditions and occupant expectations can help determine the best control method for a particular application.
The earlier in the design process ventilation is considered, the more likely the best results will be achieved for virtually all the considerations mentioned. For example, if natural ventilation is known to be a primary objective of the structure before its design, louvre placement, and building orientation can be set to make the best use of prevailing weather patterns.
Venting façades can be placed to fully achieve the esthetic value of these products and designs may be created in such a way as to accommodate multiple venting options. For instance, vents can be placed away from pollution sources, floor plans may be developed to encourage rather than impede air-flow, and similar steps may be taken to ensure the structure is efficient, comfortable, and esthetically pleasing.
As the world moves toward healthier living and working environments, sustainable building practices, and lessened carbon footprints, natural ventilation systems are becoming more prevalent in designs, but strictly natural ventilation systems may not be ideal for every building application. A fully mechanical system may be the best option in applications requiring a specific internal temperature and humidity level. In many cases, however, a mixed-mode system using natural ventilation during certain periods and mechanical ventilation during temperature extremes, may be the best option. Knowing the environmental conditions and building-owner’s expectations early in the process is critical to selecting the right climate control system for a building.
Michael P. Toohey is the director of architectural product sales and marketing for the Bilco Company. He has more than 25 years of experience in the construction industry and has dealt with specialty access products for more than two decades. Toohey is a member of the National Roofing Contractors Association (NCRA). He can be contacted at email@example.com.