7 Integrating ventilation systems
In cooperation with NKUA
7.1 General principles on ventilation systems
Ventilation is a primary determinant of exposures to indoor air contaminants. The European Concerted Action’s Report No 11 [CEC92] defines ventilation as “…supply to and removal of air from a space to improve the indoor air quality (IAQ)”, whereas the ASHRAE Standard 62-1989 [ASH92] defines ventilation as “The process of supplying and removing air by natural and mechanical means to and from a space. Such air may or may not be conditioned”. Following the definition of ASHRAE there are two types of ventilation: natural and mechanical.
Natural ventilation includes the movement of outdoor air through intentional openings such as doors and windows, and through unintentional openings in a building’s shell (such as cracks) which result in infiltration and exfiltration. Obviously, temperature control on a year round basis in an occupied space is difficult if the space is ventilated by natural means only. A certain amount of infiltration and exfiltration occurs in every building since it is impossible to construct a building that is 100 % “air tight”.
Mechanical or forced ventilation is intentional ventilation supplied by electric fans. These fans are usually part of the building’s air handling unit, which heats and cools the air supplied to the building (and to certain extent also filters and (de)humidifies the air). Mechanical ventilation, when associated with heating and cooling, is able to control the building temperature actively.
There are several functions of ventilation systems [MCN84]:
§ Provide a healthy and comfortable IAQ environment
§ Provide space temperature control
§ Provide indoor humidity levels by adding or removing moisture to ventilation air
§ Remove excess heat, humidity and contaminants by exhausting to outside part of return air. In certain cases local exhaust systems may be needed to exhaust contaminated air directly to outside
§ Regulate building pressure for control of outdoor air infiltration
§ Confine or exhaust smoke, heat and toxic fumes in case of fire
7.2 Integrating natural ventilation
7.2.1 Points of attention
Natural ventilation is caused by pressure differences across a building’s envelope resulting from wind and air density differences between indoor and outdoor air. Thus, design of naturally ventilated buildings requires a thorough understanding of airflow patterns around buildings and of the factors affecting these airflows. The objective of a successful design is to ventilate the largest possible part of the indoor space. Fulfilment of this objective depends on many parameters including window topography, landscaping location of surrounding buildings, building shape and orientation, interior design, and wind characteristics.
In Figure 80 a Mashrabiya (prototype used for the control of natural ventilation and lighting in the East) is presented.
Figure 80: Mashrabiya of the Jaml-Ad-Dhahabi house [SAR06].
7.2.2 Rules of thumb
§ Airflow patterns around buildings depend on surrounding terrain and location of nearby buildings. At the leeward side of the building (side opposite of that facing the wind) a wake region (a region with high turbulence) exists, the extent of which depends on a building’s shape and the wind direction. For a typical house, the average wake length is four times the ground to eave distance. A building is considered to be within the wind shade of a neighbouring building when the distance between the two buildings is smaller than the wake length. Available natural ventilation potential is reduced in buildings located within the wind shades of other buildings.
§ Windows and openings should be located on both the windward and leeward sides of a building. Ventilation of interior spaces increases when openings are located on opposite walls and minimizes when openings are located on adjacent sides because air is forced to change direction.
7.2.3 Practical examples
Several methodologies have been proposed in order to calculate the necessary surface area of openings, as well as their placement in the building in order to obtain natural ventilation. Existing methodologies can be classified into the following categories:
§ Simplified empirical methods. These methods are based on simple formulas for estimating surface areas of openings based on known airflow rates. An example of these methods is the Florida Solar energy method, according to which inlets and outlets should be of equal size [CHA86]. The main advantage of these methods is that they can immediately give an approximation of the situations at hand.
§ Computational network methods. These methods are based on the network approach and can be used to determine the area and orientation of large openings on a building’s shell so that pre-determined airflows can be achieved [AWB91]. The network approach is a mathematical concept that assumes that each zone of a building can be represented by a pressure node, and the environment outside the building can be represented by boundary nodes. Nodes are interconnected by airflow paths, such as cracks, windows and doors, thus forming a network. Then application of the mass conservation between different zones and flow paths allows the determination of the airflow. Network airflow models combine the effects of both the wind and temperature.
§ Computational fluid dynamic (CFD) methods, by which the flow as well as the temperature field inside the building can be visualized. The main advantage of these methods is that they are more trustworthy when comparing with the other two methods. On the other they are more time consuming and more input data are required in order to obtain reliable results.
7.2.4 Practical examples
§ Smoke control in case of fire is more difficult and may require special equipment and/or variances in codes.
§ Outdoor noise is difficult to manage in a building that relies on operable windows or vents.
§ Acoustic separation between spaces can be difficult to achieve.
§ Low pressure differences often require large apertures for desired airflow rates.
§ Outdoor air must be clean enough to introduce directly into occupied space. If filtration is required, mechanical ventilation is necessary.
§ Air to air heat recovery is not possible
§ Air to water heat recovery is possible in some cases with a heat pump situated in a centralised extraction duct
7.3 Integrating mechanical ventilation
Mechanical ventilation is usually associated with one or more of the following processes: heating, cooling, filtration and humidity control. The terms heating, ventilating and air-conditioning systems (HVAC systems) and air-handling units (AHU) refer to equipment that provides one or more of these processes. There are various HVAC systems which can be summarized constant volume mechanical ventilation systems or variable air volume mechanical ventilation systems, and in single or multi-zone systems. (For an extensive overview of these systems see [ALE94]).
The basic components that should be considered during integration of mechanical ventilation systems are the following:
§ Outdoor air intake is an intentional opening through which air outdoor air is introduced into the system. The amount of the outdoor air is controlled by dampers which are either manually or automatically controlled.
§ Air filters (for particle removal)
§ Heating/cooling coils, humidification and evaporative coolers. Heating (and/or cooling) coils are used for addition (or removal) of heat in the supply airstream under forced convection conditions. Heating/cooling media include water, aqueous glycol, and halocarbon refrigerants. Humidification is needed if relative humidity is below a comfortable level. Evaporative coolers are an alternative inexpensive cooling method but with limited power.
§ Fans are used to move air through the duct distribution system and to induce air motion within a building.
§ Ducts are used to distribute conditioned air from an air-handling unit to occupied spaces.
§ Terminal devices and cooling towers.
7.3.1 Points of attention
Advantages of mechanical ventilation
In general, outdoor weather conditions (temperature and humidity) may contribute noticeably on the indoor microclimatic conditions, which may in turn result to potential damage. For example, as moisture moves from a warm area to a colder one from vapour pressure, condensation may be occur on indoor materials in the colder area. High humidity levels during wintertime cause moisture to collect onto the cold surfaces, (e.g. windows), or to migrate into walls. Thus, wooden windows or walls and wooden structural elements may be damaged,
On the other hand, too low humidity levels can dry and crack historic wooden or painted surfaces.
In such cases the use of mechanical ventilation is preferable comparing to natural ventilation. The major advantages can be summarised in the following:
§ Indoor microclimate is independent of the outdoor weather conditions.
§ Heat can be extracted from the exhaust air and be used in order to preheat the fresh air supply.
§ The air supply can be preheated or pre-cooled. It can also be humidified or dehumidified
§ The air can be cleaned by filters
Ventilation effectiveness
It is usual to design systems based on maximum design occupancy. However, advanced systems can also take the real number of people inside a space and the dynamic behaviour of the indoor air pollution, for example with CO2 control, into consideration. This may however be misleading if the fresh air distribution inside the building is not well designed, i.e., if the fresh air is not delivered where it is really needed. This concerns, first, the distribution amongst the various spaces inside the building, and, second, the distribution within each space. If the distribution among the various spaces is not proportional to the real needs, e.g., supply air is proportional to design loads in constant airflow rate systems and to actual loads in VAV systems, indoor air quality is affected where not enough fresh air is supplied. To correct this, the occupants or the systems managers usually increase fresh air supply rates to the whole building, with the ensuing increase in energy consumption.
Within each space, supply and exhaust grilles must be conveniently selected to ensure that the fresh air goes through the occupied space, i.e., the portion of the space where occupants really are: from the floor up to a height of about 2m. This is best characterized by the concept of "Ventilation Effectiveness". Once again, low ventilation effectiveness will result in the bypass of a large portion of the supply air directly to the exhaust grilles. Thus, to ensure thermal comfort and indoor air quality, either the air supply is increased, or the room thermostat is set at a lower temperature in summer, or both. Displacement ventilation is an interesting option that is especially well suited for air- conditioning systems. It consists of supplying all the cooled air at floor level, thus 100 % within the occupied space. It then rises around the occupants (and other heat sources) and the polluted air is then extracted at ceiling level. This system has the highest possible ventilation efficiency, but it requires special considerations if the space also needs heating during winter.
7.3.2 Points of attention
§ Air infiltration (through the exterior envelope) should be carefully examined prior to the placement of the mechanical ventilation system.
§ Indoor temperature and humidity should be monitored for at least one-year period in order to determine microclimate problems.
§ Equipment deficiencies that must be fixed prior to the installation or upgrading of mechanical systems.
§ Architectural interventions that require radical reconfigurations of historic spaces are inappropriate for the building.
§ Spaces and specific features with architectural significance should be identified and evaluated in order to ensure their preservation. This includes existing mechanical systems or non-mechanical features such as cupolas, transoms, or porches.
§ Non-significant spaces where mechanical equipment can be placed should be located. Such places might include attics, basements, false ceiling or floor cavities.
§ Attention should be paid in order not to place extra stress on historic building materials through the vibrations of the mechanical equipment.
Figure 81: Historic building material destroyed by installation of through-the-wall air conditioners.
7.4 Related chapters
Part 1 Chapter 1: General principles of the building envelope
Part 1 Chapter 2: Penetrations and additions
Part 2 Chapter 1: Indoor user comfort
Part 2 Chapter 8: Integrating heating and cooling emission systems
7.5 References
[CEC92] CEC. 1992. Guidelines for Ventilation Requirements in Buildings. Report No. 11. European Concerted Action : Indoor Air Quality and its impact on man. EUR 14449 EN. Commission of the European Communities. Luxemburg: Office for Publications of the European Communities.
[ASH92] ASHRAE Standard 62-1989
[MCN84] McNall PE and Persily, AK. 1984. “Ventilation Concepts for office buildings” Ann. Am. Conf. Gov. Ind. Hyg., 10: 49-58
[SAR06] Saranti K., 2006. “Air moving in and through building: historical prototypes and contemporary applications”. International Workshop on Energy Performance and Environmental Quality of Buildings, July 2006, Milos island, Greece.
[CHA86] Chandra S., Fairey PW and Houston MM, 1986 “Cooling and Ventilation” Solar Energy Research Institute Report, SERI/SP-273-2966. DE86010701, Golden, Colorado
[AWB91] Awbi, HB. 1991. Ventilation of Buildings. Publisher: Chapman and Hall.
[ALE94] Alevantis l and Xenaki-Petreas M. “Indoor Air Quality in Practice”. In: Energy Conservation in Buildings, M Santamouris and DN Assimakopoulos, (Editors), Central Institution Energy Efficiency Education, University of Athens, Athens, Greece.
[NPS08] http://www.nps.gov/hps/tps/standguide/