5.1. Designs for natural ventilation and hybrid ventilation systems
This section outlines the main design categories of natural ventilation and hybrid (mixed-mode) ventilation systems.
5.1.1. Natural ventilation systems
As previously defined, natural ventilation is the use of natural forces to introduce and distribute outdoor air into or out of a building. These natural forces can be wind pressures or pressure generated by the density difference between indoor and outdoor air.
There are four design methods available for natural ventilation systems:
cross flow (no corridor) — the simplest natural ventilation system with no obstacles on either side of the prevailing wind (i.e. windows of similar size and geometry open on opposite sides of the building);
wind tower (wind catcher/wind extractor) — the positive-pressure side of the wind tower acts as a wind catcher and the negative-pressure side of the wind tower acts as a wind extractor;
stack (or buoyancy), simple flue — a vertical stack from each room, without any interconnections goes through the roof; this allows for air movement based on density gradients; and
stack (or buoyancy), solar atrium — a large stack that heats due to solar radiant loading, which induces air movement due to density (temperature) differentials; without radiant loading, the atrium provides minimal ventilation.
5.1.2. Hybrid (mixed-mode) ventilation systems
As previously defined, hybrid (mixed-mode) ventilation relies on natural driving forces to provide the desired (design) flow rate. It uses mechanical ventilation when the flow rate is lower than that required to produce natural ventilation.
Three design methods are available for hybrid ventilation systems.
Fan-assisted stack — when there is insufficient solar radiant loading on the stack (i.e. evenings and inclement days) the ventilation rate is supplemented by extraction fans. Inlet air is heated and cooled to maintain comfort for building occupants.
Top-down ventilation (fan-assisted stack plus a wind tower) — when there is insufficient solar radiant loading on the stack (i.e. evenings and inclement days) the exhaust ventilation rate is supplemented by extraction fans while the supply ventilation rate is supplemented by the wind tower (wind scoop). Inlet air is heated and cooled to maintain comfort for building occupants.
Buried pipes — when land is available, ventilation pipes (ducts) can be buried. If air remains underground for long enough, the air will approach the steady-state underground temperature (i.e. warming or cooling the outside air). This system is not ideal for high ventilation rates.
illustrates the different systems of natural and hybrid ventilation.
Different natural ventilation and hybrid ventilation systems. Source: Courtesy of Professor Martin Liddament, VEETECH, Coventry, UK.
5.2. Basic design concepts for natural ventilation
Developing the design concept for a naturally ventilated building that incorporates infection control involves three basic steps, described in more detail in section 5.4.
Specify the desired airflow pattern from the inlet openings to the outlet openings.
Identify the main available driving forces that allow the desired airflow pattern to be achieved.
Size and locate the openings so that the required ventilation rates can be delivered under all operating regimes.
A general procedure for natural ventilation starts from the architectural design, system layout and component selection, vent sizing and design-control strategy. The procedure is concluded by detailed design drawing.
Converting an existing building or designing a new building to use natural ventilation for controlling airborne infection would, ideally, include the presence of single-bedded isolation rooms with operable windows and ensuite toilets. However, in resource-poor contexts, the number of such isolation rooms may need to be limited, with additional cohort isolation being provided, when necessary, by contingency facilities (e.g. outdoor isolation tents open to the wind).
There is a need to develop effective and appropriate engineering technologies and innovative architectural features to maximize the use of natural ventilation for different climatic conditions worldwide.
Unlike other types of buildings, when the prevailing wind direction and average velocity may be used, the design of natural ventilation for infection control should consider the worst situation — that is, when the wind is absent, and where supplementary mechanical ventilation may be needed.
5.3. Climatic and other considerations in ventilation design
A number of factors need to be considered when designing a building to effectively use natural ventilation for infection control.
High air-change rates are needed when infection control is the main building design objective. The impacts of the high air-change rates on the overall indoor environmental conditions should be considered; these include thermal comfort, indoor air quality and fire safety. Other likely unfavourable ambient environmental factors such as noise and air pollution, and their impacts on indoor environmental quality have to be assessed before building design starts. In cold climates, the need for warmth inside the building can be at odds with the high air-change rate needed for infection control. In transient seasons of hot and humid climates, moisture condensation in the ward interior can lead to wet beddings and floors, rainy ceilings, and mould and mildew growth — resulting in unpleasant and unhealthy conditions. However, large openings in the building envelope make it easier for insects, wild animals and other unwanted intruders, and may also cause problems relating to security and vector-borne infectious disease control.
5.3.1. Maintaining thermal comfort
In temperate and warm climates and under good ambient air quality conditions, a higher ventilation rate is good for both thermal comfort and indoor air quality. However, this is not true for cold climates where outdoor air infiltration should be minimized for thermal comfort. When the ambient air temperature stays above 30 °C, the thermal conditions in a naturally ventilated ward may become intolerable. Therefore, in a naturally ventilated building, more effort needs to be spent on the architectural and envelope design to achieve acceptable indoor thermal comfort than for a building with mechanical ventilation. This includes the selection of windows, proper external shading, envelope insulation and the properties of external surface materials with regard to solar absorption and thermal radiation. A design engineer must also understand that a final design is a compromise between the conflicting requirements in hot summer and cold winter conditions. Thermal performance simulation tools are available to help quantitatively assess and compare theeffectiveness of different design options. A more detailed explanation of the technology options and simulation techniques are provided in ASHRAE (2009).
5.3.2. Considerations for hot summers
Architectural design features
When the land area allows, active use of ground-to-sky radiation will greatly reduce the effective radiant temperature. Semi-open architectural design is preferred, and should allow direct long-wave radiation from ground to sky to occur. The semi-opening should be on the shade side of a building to avoid direct solar irradiation — this is how a sunshade works (see ).
Semi-open design allowing ground-to-sky thermal radiation can greatly improve the thermal comfort in hot summer.
Solar heat gain should be minimized by using proper external shading or the more sophisticated glazing systems. The buoyancy effects of the solar heat on airflow can be used to lead the warm air to the higher levels of the building. Fortunately, this is in line with the desired airflow patterns for infection control.
Low-energy mechanical cooling
A high air change rate may be favourable for thermal comfort in cold weather; however, on muggy and calm days, high air change would make conditions uncomfortable. In low-wind conditions, air change that is caused by buoyancy may not generate enough internal air movement.
Mechanical cooling fans
In addition to hybrid (mixed-mode) ventilation systems, air movement can be improved using electric cooling fans — although improving air movement by introducing an artificial “breeze” does not necessarily increase the air-change rate.
ASHRAE (2009) provides design guides that use the widely accepted predicted mean vote (PMV) model, which takes into account the air temperature, radiant temperature, air velocity, clothing level and people's activity level. Designers can use the PMV model to estimate the raised air velocity required at higher air temperatures. When the temperature is below 30 ºC, acceptable thermal comfort can be achieved using elevated air speed (Xia et al., 2000). Temperatures above 30 ºC will inevitably cause thermal stress in the building's occupants.
Cooling fans with multiple speeds should be used, and people should be able to adjust the fan speed as needed. Using fans in this way greatly reduces energy consumption, compared with air-conditioning.
On hotter days when air temperature is above 30 ºC, using a cooling fan only would not be sufficient to keep the building cool enough for occupants. Instead, a low-cost evaporative cooling method can be used — and is particularly useful when there is a high air-change rate. This strategy also has a relatively low cost, compared with a full air-conditioning system (Zhang et al., 2000).
5.3.3. Considerations for winter
In cold winter conditions, a high air-change rate is not desirable for thermal comfort, particularly as windows may be closed to keep the building warm. Even if normal heating is introduced, with a high air-change rate the effects might be insignificant, and energy efficiency will be low. Therefore, heating strategies must be planned carefully. Building envelope design should be able to capture the solar heat and minimize conduction loss through the wall. Proper insulation of walls and the use of double glazing are desirable. For extremely cold climates, a rigorous assessment using simulation techniques should be undertaken, so that the degree of coldness can be quantified. This can be used to determine whether the natural ventilation strategy could be adopted for the climate being considered.
When considering active heating strategies, targeted radiant or direct near-body heating methods are more effective, and are preferred for two reasons. First, due to buoyancy effects, the warm air from the common convective radiators tends to float to the upper part of a space. Second, at a high air-change rate, the heat loss is tremendous. Modern, electric radiant heaters are readily available, and are a better option than other commonly used electric radiators.
Electrically heated mattresses are also available and typically use about 50–100 watts. They are effective for in-bed patients, and may allow patients to tolerate much lower in-ward air temperatures associated with the high air-change rate. They also help to avoid the excessive energy consumption associated with the ordinary space-heating methods.
5.3.4. Maintaining healthy indoor air quality
With a higher air-change rate, the indoor air quality is more linked to the ambient air quality. The benefit is that the indoor air quality is less likely to be affected by the presence of common indoor pollutant sources, such as the off-gassing from common building materials.
5.3.5. Managing ambient air pollution
With the high air-change rate of untreated outdoor air, indoor air quality will be more affected by the ambient air pollution (Weschler & Shields, 2000; Ghiaus et al., 2005). In regions with severe ambient air pollution problems, the location of an infectious disease hospital should be chosen carefully. A hybrid (mixed-mode) ventilation design may be the only option. Solely relying on ordinary window openings will expose the occupants to a high ambient pollutants level.
5.3.6. External noise
As pointed out in CIBSE (2005), the presence of significant noise sources is one of the main barriers to using natural ventilation. However, this guideline recommends two solutions: one is to place the ventilation inlets on the sides of the building away from the principal noise sources; the other is to integrate acoustic baffles into the ventilation opening. However, this second solution will reduce the air-change rate, and is therefore best combined with hybrid (mixed-mode) ventilation where a mechanical fan can avoid the increased pressure loss over such a vent.
5.3.7. Selecting low-emission interior materials
A comprehensive understanding of air pollutant emissions from interior building materials has developed over the years (Levin, 1989; Li & Niu, 2007). Designers and contractors should be aware of the standards and regulations on building materials for indoor use. In particular, materials that can potentially release airborne respiratory-tract irritants should be avoided.
5.3.8. Humidity and mould growth
Condensation can occur on ceilings, walls, floors and beddings for many reasons. For example, in buildings with a heavy structure and that use natural ventilation, a sudden change of weather with warm, moist ambient air may induce condensation when the surface temperature is lower than the dew-point temperature of the moist incoming air (Niu, 2001). While the conditions are a discomfort and annoyance during the condensation period, mould may also grow — which is a health hazard.
When designing buildings with natural ventilation for a hot and humid climate, lightweight and insulated walls should be used. The surface temperature of a lightweight construction or a wall with internal insulation will respond rapidly to changes in air temperature, limiting the rise of surface and internal relative humidity when the sudden warm and humid air comes in contact with the wall (e.g. in the transient spring season).
For existing buildings with massive concrete or masonry walls, several retrofitting, operation and maintenance strategies may be needed if a natural ventilation strategy is to be adopted. The first option would involve the interior surface treatment, which can either be long term or short term.
5.3.9. Security and vector-borne disease spread
Large openings in natural ventilation without any protection increase the risk of security breaches and the spread of vector-borne diseases. Purpose-designed barred windows and semi-transparent mosquito meshes can be used in these situations.
5.3.10. High-rise considerations
Locating respiratory wards on the top floors may be desirable for high-rise buildings to minimize the possible re-entry of the exhausts into adjacent floors. This re-entry is caused by buoyancy as the exhaust air is normally warm and tends to flow upwards after leaving the wards (Wehrle et al., 1970).
5.3.11. Fire safety considerations
Designing a building with openings connecting rooms may conflict with fire-safety and smoke-control requirements. Naturally ventilated buildings may also be zoned to be in line with the compartmentalization requirements for smoke control. Ventilation openings may also be shut during a fire. The fire escape route also needs special attention, because natural ventilation design also has an impact on smoke flow pattern.
5.4. Designing for natural and hybrid ventilation systems
When developing the design concept for a naturally ventilated building for infection control, three basic steps are involved.
Specify the desired airflow pattern from the inlet openings, through the wards and other hospital spaces such as corridors, to the outlet openings. This is associated with the form (single corridor, central corridor, courtyard, etc.) and organization (relative location of the nursing station, offices, storage, etc.) of the building, which in turn depends on its intended use and site conditions, such as prevailing winds.
Identify the main available driving forces that enable the desired airflow pattern to be achieved. The effective strategies for infection control tend to be mostly wind driven, although the stack-driven strategy may also work if designed properly. A combined wind-driven and stack-driven flow needs to be considered where necessary and feasible. In some cases, hybrid (mixed-mode) ventilation may be used and these natural forces can be supplemented by fans. In a good design, the available dominating driving forces are in synergy with the intended flow pattern.
Size and locate the openings so that the required ventilation rates can be delivered under all operating regimes. This is, in itself, a three-step process. First, the ventilation rates need to be determined based on the infection control requirements as specified in Part 1 of this document. Second, the openings need to be sized and located to deliver these airflow rates under design conditions. Third, a control system needs to be designed to maintain the required flow rates under varying weather and occupancy conditions.
A general procedure for natural ventilation design includes several components.
Architectural design — architects and engineers must initially set the global geometric configuration of the system (e.g. siting of the building and landscape configuration, overall building form, and approximate positions of fresh air inlets and air exhausts), considering both dominant and prevailing wind conditions, as well as unusual conditions by time of day and season.
System layout and component selection — the designer will then lay out the airflow paths from inlet to outlet that will achieve the desired airflow objective (e.g. for the purpose of infection control and thermal comfort) and then select the types of airflow components (e.g. windows, doors, vents, solar chimneys) that will provide the desired control of airflow.
Opening (door, window, vent etc) size — the designer will then size the components selected considering the ventilation requirements and relevant climatic conditions. Both the indoor and outdoor design conditions (or design criteria) need to be considered.
Design control strategy — the designer must then develop a strategy for controlling ventilation flow to the design objectives when the operating conditions vary. At this stage, both hardware and software for control may need to be chosen to implement the control strategy if a high-tech natural ventilation strategy is used.
Detailed design drawing — finally, the designer must develop detailed drawings so that the systems can be built.
5.4.1. Vent sizing
Vent sizing refers to the process of estimating the area of openings to achieve the required ventilation flow rates based on certain geometry, climate and other data of the building design. The sizing of openings is also a function of the opening distribution, which is a part of the ventilation strategy.
There are two methods for estimating the size of the vents required.
Direct methods are derived for simple buildings where the ventilation flow rate is a simple function of the governing parameters.
Allard (1998) discussed five of these methods.
Indirect methods use network models to try different opening size combinations and identify the best one. One promising design method is the loop pressure equation-based method suggested by
Axley (1998).
After the necessary ventilation flow rates in each zone of a building are estimated, these methods can be used to design the main flow paths and size ventilation openings to satisfy ventilation requirements in each zone. When designing large buildings, designers might also need to know different design options, how natural ventilation compares with mechanical systems, etc.
When a building is designed and operated with a configuration of openings and flow paths, the ventilation flow rate will mostly be determined by the available natural driving forces. At the design stage, it is important to harness the prevailing winds and to enhance and control stack (or buoyancy) forces in the building. This can be done by carefully positioning and sizing the openings, and by innovative use of devices to increase natural forces, such as wind towers and solar chimneys.
Transient high ventilation allowances
Allowing a transient ventilation rate that is much higher than the minimum ventilation rate specified in Part 1 is one of the benefits of natural ventilation. When the outdoor temperature is comfortable and the air is clean, it is effective to allow more outdoor air into the building. For some climates and buildings, a transient high ventilation rate can also be used for summer cooling. A transient high ventilation rate might also be needed when there are renovation activities in the building, which generate a high amount of pollutants in the air.
5.4.2. Three major design elements of natural ventilation
Designing natural ventilation requires more than just estimating vent and window sizes — it also requires innovative design and significant attention to detail. Priolo (1998) presented a comprehensive design guideline for natural ventilation. This section gives a brief overview of the three layers of the design process related to natural ventilation design:
site design — building location, layout, building orientation, landscaping;
building design — type of building, building function, building form, envelope, natural ventilation strategy, internal distribution of spaces and functions, thermal mass, heating, ventilation and air-conditioning if it exists; and
vent opening design — position of openings, types of openings, sizing of openings, control strategy.
Site design
Site design involves integrating the buildings with the surrounding topography and buildings. For some situations, minor changes to the local site may be allowed, within the limits of environment and wildlife protection.
For natural ventilation, it is best to use the natural airflow patterns of the site to increase the potential of natural ventilation.
If the building needs summer cooling and minimum winter ventilation, investigate the summer and winter prevailing wind directions, and locate the building to receive more summer winds and protection from winter winds.
When several buildings are being built on one site, make sure each of the buildings is exposed to summer winds, but not to winter winds in cold climates.
As discussed in section 4.1, the driving wind pressure is not just the positive pressure at the windward openings, but also the negative pressures at the leeward openings. Building form and orientation should result in an increase in the negative pressures in the wakes of airflows. Aynsley, Melbourne & Vickery (1977) provide a useful explanation of downwind wakes caused by different building forms.
Vegetation also affects air movement around the buildings through wind sheltering, wind deflection, funnelling and air acceleration. Air quality and conditions are also changed when travelling beneath canopies of vegetation (e.g. trees).
Building design
For simple buildings, follow the guidance of Priolo (1998) on roof design, aspects ratios and the use of overhangs, wind walls and recessed spaces. For large and complex buildings, use computational fluid dynamics (e.g. Fluent, 2003) to investigate various design options for improving the natural ventilation potentials, and to avoid cold draughts. Take care to ensure pedestrian comfort at the outdoor ground level.
Internal space distribution is also important. For example, relatively “dirty” spaces should be located on the leeward side to avoid back flow of polluted air and odours into other spaces. Large windows for other living spaces in the windward side, such as the wards, can create a funnel effect to induce more incoming air. Interior partitions and furniture should not block the airflow.
For infection control, a single-row ward layout works better than a double-row layout with a central corridor in terms of natural ventilation and daylight. Large, open spaces should always have large windows in opposite walls. With the central corridor layout, natural ventilation may be improved by combining cross-ventilation with stack (or buoyancy) ventilation through corridor vents or through shafts in multistorey buildings.
For multistorey hospitals, stairwells and other shafts can work as exhaust ventilation systems to avoid warm air entering the upper-level apartments or offices. The outlet openings of the shafts should be located on the leeward side of the building, above the top floor level, with the inlet openings on the windward side of the building.
As the penetration depth of wind-driven natural ventilation is limited, the width of the building is limited (CIBSE, 2005). However, the use of wind towers may permit deeper buildings.
Vent opening design
In any design, the smallest opening area (the bottleneck) controls natural ventilation flow rate. Inlet and outlet openings should have as near equal dimensions as possible to maximize the airflow rate.
The position of openings needs to be considered with care, because of the possible conflict between cross and stack (or buoyancy) ventilation, human cooling or thermal mass cooling, etc.
Proper selection and design of openings such as windows, screens, louvres, solar chimneys, passive stacks, is also important. Proper sizing may be done using the vent sizing methods discussed earlier.
There are some other aspects to consider.
Furniture and internal portioning — ventilation openings should not be blocked, and furniture layout and internal partitioning must not restrict the intended flow path and opening access.
Ward depth — unlike mechanical ventilation, naturally ventilated buildings need to be narrow. The natural air currents may penetrate deeply into a building. The rules of thumb for the ward depth are available from
CIBSE (2005) (see ).
Shading — blinds, overhangs and projections (including deep window reveals) may be used. Self-shading by the building itself and remote shading (e.g. by another building or trees) may also work if properly considered. Retractable external blinds are desirable.
Daylight and glare control — windows may be provided with a screen to avoid the direct sunlight. The shape and the position of the window openings are also important. The colour and the finishes of the surfaces must also be chosen properly for a comfortable level of lighting and glare control.
Heating and cold drafts — during slightly cold weather, localized heating may be used to provide some thermal comfort. However, care should be taken if a greater indoor and outdoor air temperature difference is caused, because this can, in turn, increase the driving force. Natural ventilation may not be possible for ventilation control during very cold weather.
Cooling — during hot and humid weather, local spot cooling or personalized cooling systems may be used (e.g. by using ceiling fans or desk fans).
Noise and acoustics — external noise may be avoided by locating the windows and other ventilation openings away from the primary noise courses. Absorbent partitioning, ceiling banners, etc., may also be used to absorb noise.
Fire safety — designing a building with openings that connect rooms may conflict with fire-safety and smoke-control requirements. Ventilation openings may need to be closed during a fine. Fortunately, naturally ventilated buildings can be designed to be in line with the compartmentalization requirements for smoke control. The fire escape route needs special attention, because natural ventilation design also has an impact on smoke flow pattern.
Security — security risks may be created with opening windows, particularly on ground floors.
The rules of thumb for the depth of the ward for three different ventilation strategies. (A) Cross-ventilation. (B) Single-sided ventilation driven by buoyancy forces alone (i.e. stack (or buoyancy) ventilation, which is not effective for airborne infection (more...)
5.5. Types of natural ventilation systems
Natural ventilation systems are classified by their basic architectural design elements (corridors, courtyards, wind towers, chimneys, etc.). These building elements define the routes of airflow, as well as the basic natural ventilation strategy.
There are six basic types of natural ventilation systems:
These systems are described in the following sections. It is possible to combine some of these systems to suit the local climate and particular hospital needs. Annexes F–I describe the natural ventilation systems used in four hospitals in different countries.
This guideline considers only simple natural ventilation systems, and designers will need to consider other aspects (e.g. control) when they are designing high-tech natural ventilation solutions.
5.5.1. Single-side corridor type
In the single-side corridor type of natural ventilation system, the corridor is on one side of the ward (see and ). The airflow is a single directional flow either from the ward to the corridor or from the corridor to the ward, depending on the wind incident direction. This single directional flow can help to prevent cross-infection. The design of the windows is crucial for this type of design: it is better to position the windows in line with the ward door to create the path for cross-ventilation (Allard, 1998).
Wind-driven natural ventilation in the single-side corridor type hospital with wind entering the ward. Note: This conceptual drawing should be used with care, and realistic limitations need to be considered.
Wind-driven natural ventilation in the single-side corridor type hospital with wind entering the corridor. Note: This conceptual drawing should be used with care, and realistic limitations need to be considered.
F Beer is credited with designing the first corridor hospital, where all the rooms were arranged alongside internal walkways. His hospital in Bern, built between 1718 and 1724, was the first of this type.
5.5.2. Central corridor type
The central corridor type of natural ventilation system is derived from the single-side corridor type by adding another series of wards on the other side of the corridor. The possible airflow path would be from one ward to the corridor, and then to the ward on the other side. When the wind is parallel to the windows, adding a wing wall helps to drive the outdoor air to enter the wards first, and exit from the central corridor. A central corridor type of floor layout would result in possibly contaminated air moving from the upstream ward to the downstream ward. At present, this guideline does not recommend this type of design.
5.5.3. Courtyard type
Courtyards are traditionally enclosed zones that can help to channel and direct the overall airflow and thus modify the microclimate around the buildings. Based on the relative position of wards and corridor to the courtyard, this type of natural ventilation system can be divided into the inner corridor and outer corridor subtypes (see and , respectively). This system can supply more ventilation than the others, as long as the courtyard is sufficiently large. The outer corridor type has an advantage over the inner type, because it can avoid cross-infection via connected corridors by delivering clean outdoor air into the corridor first.
Combined wind and buoyancy-driven natural ventilation in the courtyard type (inner corridor) hospital. Note: This conceptual drawing should be used with care, and realistic limitations need to be considered.
Combined wind and buoyancy-driven natural ventilation in the courtyard type (outer corridor) hospital. Note: This conceptual drawing should be used with care, and realistic limitations need to be considered.
The first hospital of this type was Ospedale Maggiore, built in Milan in 1456, and designed by Antonio Averulino (better known as Filarete). The hospital is a symmetrical building with a large central courtyard; on both sides, the wings of the building delineate four smaller courtyards.
5.5.4. Wind tower type
A wind tower type of natural ventilation system can capture the wind at roof level and direct it down to the rest of the building (see and ). Weatherproof louvres are installed to protect the interior of the building and volume control dampers are used to moderate flow. Stale air is extracted on the leeward side. The wind tower is normally divided into four quadrants, which can run the full length of the building and become air intakes or extractors depending on wind direction.
Wind tower design. Note: This conceptual drawing should be used with care, and realistic limitations need to be considered.
Wind-driven natural ventilation in the wind tower type hospital. Note: This conceptual drawing should be used with care, and realistic limitations need to be considered.
5.5.5. Atrium and chimney type
An atrium or chimney can help to increase the natural ventilation potential. An atrium or chimney type of natural ventilation system can be a side-atrium or chimney type, or a central atrium or chimney type, depending on the relative position of the wards, and the atrium or chimney (see ). Outdoor air is sucked into the wards through the windows by the stack (or buoyancy) effect. After diluting the contaminated air in the ward, the hot and polluted air converges in the atrium or chimney and discharges through the top openings. The applicability of this type of design will mainly rely on the height of the chimney, the indoor–outdoor temperature difference and its interaction with the background wind. This approach may be combined with motor-driven dampers and pressure sensors to control airflows and overcome some of the limitations of natural ventilation.
Buoyancy-driven (including solar chimney) natural ventilation in the solar chimney type of hospital. Note: This conceptual drawing should be used with care, and realistic limitations need to be considered.
5.5.6. Hybrid (mixed-mode) ventilation type
A limitation of natural ventilation is that it can sometimes depend too much on the outdoor climate. For example, if the outdoor wind speed is too small or the outdoor temperature is too high, the availability of natural ventilation will be reduced. To overcome this, hybrid (mixed-mode) ventilation can be used. In a simple hybrid (mixed-mode) ventilation system, mechanical and natural forces are combined in a two-mode system where the operating mode varies according to the season, and within individual days, reflecting the external environment and taking advantage of ambient conditions at any point of time.
The main hybrid (mixed-mode) ventilation principles are:
switching between natural and mechanical ventilation
fan-assisted natural ventilation
concurrent use of natural and mechanical ventilation.
Each of the natural ventilation solutions discussed above (single-corridor, central corridor, courtyard, wind tower, and atrium and chimney) may be combined with mechanical fans to create a hybrid (mixed-mode) system. Of course, like all the systems that use natural or mechanical ventilation, design and control are critical.
5.6. Applicability of natural ventilation systems
Natural ventilation systems should be designed to take into account the local climate. There are four major climate types: hot and humid, hot and dry, moderate and cold.
Design of a natural ventilation system can also have one of three major objectives: to provide thermal comfort, to control airborne infection or indoor air quality, or to save energy.
When a ventilation type is evaluated against a climate type, both thermal comfort and infection control should be considered, but not energy-saving performance.
The performance is star-rated.
View in own window
| ★ | The performance in either thermal comfort or infection control is unsatisfactory. In terms of infection control, it means the magnitude of the ventilation rate. |
| ★★ | The performance is fair. |
| ★★★ | The performance is acceptable, but compromise may be needed in terms of thermal comfort. |
| ★★★★ | The performance is good in terms of both thermal comfort and airborne infection control. |
| ★★★★★ | The performance is very good (satisfactory) in terms of both thermal comfort and infection control. |
contains a comparison of the performance of different types of natural ventilation systems in the four major climate conditions.
Potential applicability of natural ventilation solutions in ideal conditions (consensus of a WHO systematic review).
5.7. Commissioning, operation and maintenance
The performance of a ventilation system depends crucially on design, operation and maintenance — collectively known as commissioning. These determine the performance and reliability of the ventilation system and are important whatever the level of technology used in the building's ventilation system. Proper construction and commissioning are needed to ensure the desired ventilation performance is achieved under different (climatic) circumstances, while proper operation and maintenance are needed to ensure the desired ventilation during the system's lifetime.
5.7.1. Commissioning
It is important that, even for a very low-tech system using grilles and vents, for example, the documentation describing the reasons for the design, how it works and how it should be maintained be handed over to the building manager or operator. For example, design and maintenance documentation describing why vents are of a certain size and in certain places will enhance the understanding of the system and help to ensure it is maintained properly.
The designers need to provide documentation to the personnel managing the building and its ventilation system:
about the design strategy and expected operation of the natural or hybrid (mixed-mode) ventilation system;
on the operation of the natural or hybrid (mixed-mode) system in day or night time, during different seasons, in extreme weather conditions, and when adapted for emergency conditions;
for the patients and health-care workers explaining how the building works, is operated, and who has the right to open windows, etc;
describing the operation and maintenance of the for the ventilation system, developed jointly with the commissioning personnel (i.e. an operating and maintenance manual); and
explaining all the above (i.e. commissioning documentation).
It is desirable that the people using the system have the opportunity to provide feedback to the designers, however simple the system. Feedback and fine-tuning are essential to iron out potential problems in the system, and should continue for the first year of operation.
The commissioning process acts as a checking procedure to ensure that:
the ventilation system is installed and operated as designed
the system can be operated correctly and safely
the system may be adjusted to satisfy the ventilation requirement at different climatic conditions
ventilation rates under different weather conditions are appropriate.
This process should be maintained at least for the first year of operation.
5.7.2. Operation and maintenance
Operation and maintenance personnel should understand how the systems operate, and have some knowledge of infection control. Special attention is needed for the documentation and instructions for these personnel.
Operation personnel need training for the procedure to follow in special weather conditions, such as heavy rain, typhoons and heavy storms.
Patients are generally not permitted to operate the system unless instructed to do so (this includes opening windows).
Natural ventilation or hybrid (mixed-mode) ventilation usually has many distributed components, such as windows and fans. Detecting faults in these components can be time consuming.
It is crucial for any hospital designed for infection control to be reconsidered in terms of ventilation design when the occupancy patterns are changed.
Regular occupant surveys and checks will help to identify potential operational problems, as well as deal with complaints.
In naturally ventilated hospitals, the satisfaction of the patients and health-care workers may be improved if they understand how the system works.
5.8. Summary
Designing a naturally ventilated building for infection control follows three basic steps: selecting the desired airflow pattern, identifying the main driving forces, and sizing and locating openings. Although these steps are common to designing all such buildings, local conditions, such as the year-round climate and the impact this has on infection control, must also be taken into account.
At a more specific level, the main design elements of natural and hybrid (mixed-mode) ventilation systems are dictated by the specific components used. Aspects of different ventilation systems can be selected and combined as needed to suit the local climate and the requirements of each individual hospital.
