What is Natural Ventilation?

Proper ventilation is extremely important for occupants in any kind of building, no matter whether we have in mind a small family house or a huge skyscraper. Ventilation system is responsible for the health and thermal comfort of people inside of the building. It helps to eliminate moisture, smoke, cooking odors, allergens, and other pollutants.

One of the biggest issues in the buildings is moisture. Moisture is especially visible in wintertime when during the night the heating systems are off and the temperature is going down. Locally the saturation of air can reach 100% and water appears on some of the objects, for example on the windows. If the moisture is not removed by the ventilation system, it creates good conditions for mold to grow. In the long term perspective, the comfort of living in such a building will be very low due to a characteristic odor. But far worse are health issues that might appear.

Also, the ventilation system can help to eliminate allergens like pollen mites and other seasonal allergens which can lead to asthma or are very dangerous for people suffering already from asthma.

There are three major strategies for ventilation:

  • passive or natural ventilation
  • forced ventilation
  • displacement ventilation

The Reader can learn more about displacement ventilation and CFD modeling in SimFlow in this article (link do Displacement ventilation). Forced ventilation is described in the following article
(link do forced ventilation).

What is natural ventilation?

As the name suggests, natural ventilation lacks a controlled, mechanical process to define the airflow through the house or any other building. Instead, it is provided by thermal, wind, or diffusion effects through windows, doors, or any other intentional openings in the building.

Missing mechanical components create the potential for natural ventilation to significantly reduce the const required by the mechanical ventilation (both investment and maintenance, as well as energy cost). Because of the lower cost, natural ventilation is often proposed for so-called “green” buildings. But the fact that natural ventilation lacks the control of the airflow and is strongly dependent on building design or weather conditions, results in not sufficient ventilation analysis done by the engineers or designers to properly evaluate the performance criteria. Here CFD solvers such as SimFlow become handy because they allow us to evaluate a lot of different concepts and variations of the ventilation system in parallel. Based on the numerical results, engineers and designers can choose the most optimal solution.

Purpose of ventilation

Ventilation of any kind is used for:

  • air quality control – keeping the air inside the building clean, by diluting the indoor-polluted air with cleaner outdoor air,
  • direct advective cooling – when condition outside are favorable, warm indoor air is diluted with cooler outside air,
  • direct personal cooling – outdoor air can be driven inside the building in such a way and at a certain velocity level so that it can be used for cooling the occupants of the building,
  • indirect night cooling – to indirectly cool buildings interiors by pre-cooling thermally massive components of the building fabric or a thermal storage system with cool nighttime outdoor air.

Natural ventilation strategies

There are three main approaches to natural ventilation:

  • wind-driven cross ventilation,
  • buoyancy-driven stack ventilation,
  • single-sided ventilation.

Wind-driven cross-ventilation

It occurs when there are openings (typically windows) on both sides of the enclosed space. A schematic sketch of this type of natural ventilation is given in Figure 1.
For this kind of natural ventilation, it is very important to lower the resistance in the direction of the flow, created by any objects that might be put inside the ventilated space. This together with an appropriate level in wind pressure between inlet and outlet are the key factors for sufficient airflow.

Figure 1. Wind-driven cross-ventilation sketch. Source: National Institute of Standards and Technology Internal report 6781 (NISTIR 6781)

Buoyancy-Driven stack ventilation

This type of ventilation is realized by the chimney or atrium in the central part of the ventilated building. It relies on density differences to draw in the cold outdoor air through small openings on the sides of the building and exhaust warm, polluted indoor air. The main idea behind this ventilation strategy is to use buoyancy force to achieve the proper airflow. However, even the small wind acting on the building’s walls will generate the flow inside the building. Therefore, the wind effect is usually stronger than the buoyancy forces. Good design should take into account both effects. The schematic sketch of the buoyancy-driven stack ventilation is given in Figure 2.

Figure 2. Buoyancy-driven stack ventilation sketch. Source: National Institute of Standards and Technology Internal report 6781 (NISTIR 6781)

Single-sided ventilation

It is usually the least attractive ventilation strategy because the airflow is driven by room-scale buoyancy effects, small differences in envelope wind pressure, and/or turbulence. This kind of ventilation is used as a local ventilation solution that can be adapted for different rooms inside of the building. Because of its local nature, usually the driving forces are relatively small and highly variable. Nevertheless, single-sided ventilation can serve individual offices/rooms. The sketch of single-sided ventilation is given in Figure 3.

Figure 3. Single-sided ventilation. Source: National Institute of Standards and Technology Internal report 6781 (NISTIR 6781)

Depending on the building type, but in practice it may turn out that just a natural ventilation strategy is not enough. Therefore a mixture of two or three solutions is used. An example of such a solution is given in Figure 4.

Figure 4. Mixed global and local ventilation. Source: National Institute of Standards and Technology Internal report 6781 (NISTIR 6781)

Advantages and disadvantages of natural ventilation

There are pros and cons of both ventilation strategies: natural and mechanical. During the decision-making process, several different factors should be considered:

  • Energy savings for cooling and its limitations
  • Fan power and heat recovery
  • Control and reliability
  • Occupant health and comfort
  • HVAC equipment and cost
  • Duct cleanliness and filtration

Energy savings for cooling and its limitations

It should be stated clearly that it is not always possible to use natural ventilation for effective cooling of the buildings. If the temperatures and/or humidity are too high, cooling by natural ventilation cannot be realized. On the other hand, when the climate conditions are suitable, cooling by natural ventilation can bring significant benefits.

Fan power and heat recovery

In mechanical ventilation, fans can consume significant amounts of energy for cooling. The energy necessary for powering the fans is estimated to be at the level of 15% of the total energy used by office buildings. This creates great potential for natural ventilation, but as stated before, it can only be applied if suitable climate conditions are given.
On the other hand, mechanical ventilation has the advantage of using heat recovery units in cold climate conditions to recover heat from exhaust air through air-to-air and so-called run-around heat exchangers. It must be also noted that mechanical ventilation based on low-pressure systems tends to minimize the energy consumption by fans.

Control and reliability

Since natural ventilation strongly depends on wind and buoyancy forces, it is considered to be less controllable than mechanical ventilation. Because of changing conditions outside of the building, natural ventilation at times may under-ventilate, resulting in overheating or unacceptable air quality conditions, over-ventilate, resulting in unnecessary energy consumption to condition indoor air or provide unacceptable air distribution, resulting in local thermal discomfort.

Occupant health and comfort

Regarding health and comfort, studies show that in office buildings with more sophisticated HVAC systems, people tend to complain more about health issues, indoor air quality, and thermal comfort. It is not exactly clear why this is the case and there some hypotheses saying that sophisticated HVAC systems contain more potential sources of indoor air pollution like filters, cooling sections, and humidifiers.
Natural ventilation may seem to create better conditions for occupants. One of the reasons is the fact that often mechanical ventilation is optimized in terms of temperature comfort and not air quality. On the other hand, proper mechanical ventilation systems may provide both good air quality and thermal comfort.

HVAC equipment and cost

Mechanical ventilation requires significant costs for all the devices needed. The cost can be significant even up to 30% to 40% of the entire construction cost. It should be also noted that mechanical ventilation equipment also requires a significant amount of space that needs to be reserved for that purpose. In this context, natural ventilation seems much more attractive than mechanical ventilation.

Duct cleanliness and filtration

In mechanical ventilation indoor air quality depends strongly on the cleanliness of ducts and filters. It is necessary to provide proper maintenance of those components. On the other hand, natural ventilation circumvents this problem replacing ductwork with habitable spaces that serve naturally to drive the flow.

The role of CFD in Ventilation

The conventional natural ventilation approach is based on utilization of a wind-rose diagram, the interpretation of climate zoning requirements and the experience of the architect. It actually misses good scientific basis and detailed analysis. CFD analysis in SimFlow offers refined simulation and more scientific approach for the architects. In a very convenient and easy way, architects can perform a very detailed simulation of the building taking into account optimization process in order to propose a well-defined solution of the ventilation system.

SimFlow is software offering well-established CFD technology, that involves fluid mechanics, computing methods, visualization and many other disciplines. In CFD simulations, a numerical (computer) model of the building is created based on the geometry of the object and the numerical simulation is performed in order to investigate influence of the surrounding environment and its conditions on the site. Based on this information architects can predict and describe the building wind environment and design the optimized ventilation system.

In previous sections we mentioned that natural ventilation is very sensitive to environmental factors. This is why architects need to carefully examine the site landform and impact of site surroundings on wind environment using CFD solver like SimFlow. There are three aspects that must be taken into account when designing the natural ventilation system: site plan, building shape and building envelope interface. SimFlow can help to detect undesirable wind environments and find the optimal solution.

In the site planning phase SimFlow helps the architect to optimize the relations between a building and the project site, surrounding buildings, prevailing wind direction and building clusters. Information obtained from this phase is important to optimize conditions for natural ventilation. CFD analysis can help to identify the regions of low and high airflow speed. Additionally, factors such as daylight, noise, building coverage, orientation and spacing can be also determined. Analysis of airflow circulation, wind direction and speed can help to identify the regions of high velocity (over 5m/s) and low velocity (below 1m/s). Both regions should be avoided. The first one (region of high speed) creates uncomfortable conditions for people. On the other hand, low airflow speed creates undesirable conditions for natural ventilation – Figure x.

Figure x. Airflow speed around the cluster of buildings
Another aspect of natural ventilation design that architects can do in SimFlow is the building shape and interior design. When optimizing the shape and interior design, three main factors must be considered. The first one is functional requirements. The second factor is a proper building form to deflect and guide the wind in such a way to generate optimal passage for the wind. The third aspect is to create spatial forms inside of the building to enhance natural ventilation.
SimFlow allows calculation of the pressure differences between sides of the building, which is fundamental information for wind pressure ventilation design. Additionally, SimFlow can be used to optimize the airflow paths, building shapes and openings, eliminate or minimize the calm zone and estimate the maximum wind speed affecting the comfort Figure x.

This article was based on the following sources:

  1. Emmerich, S., Stuart Dolls. E., Axley, J., (2001), Natural ventilation review and plan for design and analysis tools, National Institute of Standards and Technology, NISTIR 6781
  2. Guo, W., Liu, X., Yuan, X., (2015), Study on Natural Ventilation Design Optimization Based on CFD Simulation for Green Buildings, 9th International Symposium on Heating, Ventilation and Air Conditioning (ISHVAC), Procedia Engineering 121, p. 573 – 581
  3. U. S. Department of Energy, Improving fan system performance a sourcebook for industry, https://www.nrel.gov