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Passive ventilation relies typically on using both convective air flows that result from the tendency of warm air to rise and cool air to sink and taking advantage of prevailing winds. Air moves naturally due to the buoyancy effect when a temperature difference exists and less dense air rises. This is called the stack effect It is ventilation without the assistance of fans or other mechanical air moving equipment. The size and placement of these openings can be used to guide air into and through your home.PASSIVE VENTILATIONTo encourage cool air flow, you'll need larger windows opening to the breeze and smaller, higher windows on the walls on the opposite side of the house

Passive ventilation can only work if air has clear, uninterrupted pathways through your home. You can maximize air flow by designing open plan areas or having high vents or other openings between rooms. In general, windows should be larger on one side of the home than the other in order to encourage air flow.PASSIVE STACK VENTILATIONPassive Stack Ventilation (PSV) is a system of vertical ducting from room ceilings to roof outlets providing ventilation by stack effect and wind passing over the roof.

This is a non-mechanical form of ventilation whereby vents are located in wet zones and use the principle of convection to allow the movement of air via currents through ducts measuring around 100-150mm. Background vents need to be installed in conjunction with passive stack.This works due to the natural mechanism of the 'stack effect' caused by temperature differences between the inside and outside: cold air enters the lower part of the building and rises to exit at the top. Flow can reverse in summer when the interior is cooler than the outside. Vents in the ceilings of 'wet rooms' such as the kitchen and bathroom connect via ducts to outlets on the roof, and allow the warm moist air to escape. This is replaced by cooler dry air entering via trickle vents in 'dry' rooms, such as living rooms and bedrooms, and air leakage, which disperses to all parts of the building. Dispersal is assisted by gaps at the bottom of internal doors.The layouts shown in Figure 1 are considered to be suitable for the majority of dwellings of up to three storeys. Separate ducts are taken from the ceilings of the kitchen, bathroom, utility room or WC to separate terminals on the roof. There are broadly two suitable positions for the duct outlet terminals.

DESIGN OF PSV SYSTEMSI. Ducts with ridge terminals (Figure l(a))Placing the outlet terminal at the ridge of the roof is the preferred option for reducing the adverse affects of wind gusts and certain wind directions. This layout does, however, introduce bends into the ducts because of the room configuration. Sharp bends will cause a resistance to the flow of air and so reduce the effectiveness of the system. Any bends should therefore be of the 'sweep rather than the 'sharp' type and no more than two in number. In addition, no section of duct should be at an angle of more than 45 to the vertical so that, in general, exhaust grilles should be as close to directly below the ridge as is practicable (Figure 2).

II. Ducts penetrating the roof (Figure l(b))An alternative layout is to have the ducts running vertically and penetrating the roof away from the ridge with the centre line of the PSV within 1.5 m of the ridge. They should extend above the roof level to at least ridge height to ensure that the duct outlet is in the negative pressure region above the roof.

Ducts that terminate on the slope of the roof (tile vents) should not be used because, under certain wind conditions, air flow will almost certainly be reversed (Figure 3). This would result in moist air being transferred to other rooms in the dwelling and may cause discomfort. Also, such positioning of the outlet leads, particularly in the bathroom, to a significant loss of stack height which is one of the main driving forces of the system.

ADVANTAGESSuitable for retro-fit during major refurbishmentNatural ventilation uses no energy or little energy therefore reduces building running costs. Silent in operationLow maintenance - periodic cleaning of grilles and checks of ducting and roof outletsDISADVANTAGES

Requires careful design and installation to work effectively.Performance dependent on weather. conditions - but flow can be assisted by an extract fan.No heat recovery possible although performance enhanced by humidity-sensitive inlets

ACTIVE VENTILATIONMechanically assisted extract ventilation system -through an air handling unit or direct injection to a space by a fan. A local exhaust fan can enhance infiltration or natural ventilation, thus increasing the ventilation air flow rate. This may be because the number of individual ducts would be excessive, i.e. too space consuming and obtrusive with several roof terminals. A low powered (40 W) silentrunning fan is normally located within the roof structure. It runs continuously and may be boosted by manual control when the level of cooking or bathing activity increases. Humidity sensors can also be used to automatically increase air flow.Ventilation systems that use fans to draw fresh outside air into the building, the systems provide consistent air flow to interior spaces and can be designed to maximize the indoor environment quality by controlling factors such as air speed, air quality, temperature , and humidity. A well-insulated, well-designed home may only need to use active ventilation for rooms where moisture is generated (bathroom, laundry andkitchen), while passive ventilation will be sufficient for maintaining air quality through the rest of your home.Active ventilation may also be needed to get warm air into cooler, damper areas such as south-facing rooms - for example by heat transfer systems

Dampers can be used to control air entering and/or exiting a natural ventilation system. These dampers could be linked to occupancy sensors, temperature sensors, time switches and other weather sensors to give automatic control of ventilation which is the key to a useful system.The diagram below shows some of the features of a Natural Ventilation system for a four-storey building.Mechanical ventilation with heat recovery MVHR is a development of MAVS to include energy recovery from the warmth in fan extracted moist air from bathrooms and kitchens. The heat recovery unit contains an extract fan for the stale air, a fresh air supply fan and a heat exchanger. This provides a balanced continuous ventilation system, obviating the need for ventilation openings such as trickle ventilators. Apart from natural leakage through the building and air movement from people opening and closing external doors, the building is sealed to maximize energy efficiency. Up to 70% of the heat energy in stale air can be recovered, but this system is not an alternative to central heating. A space heating system is required and MVHR can be expected to contribute significantly to its economic use.

A mechanical inlet and mechanical extract system can be used to regulate and balance supply and emission of air by designing the duct size and fan rating specifically for the situation.Mechanical ventilation systems are frequently applied to commercial buildings, workshops, factories, etc., where the air change requirements are defined for health and welfare provision. There are three categories of system:1. Natural inlet and mechanical extract2. Mechanical inlet and natural extract3. Mechanical inlet and mechanical extract

In large buildings where smoking is not permitted, such as a theatre, a downward air distribution system may be used. This provides a uniform supply of warm filtered air.

Internal sanitary accommodation must be provided with a shunt duct to prevent smoke or smells passing between rooms. In public buildings, duplicated fans with automatic changeover are also required in event of failure of the duty fan.

Basement car parks require at least 6 air changes per hour and at exits and ramps where queuing occurs, local ventilation of at least 10 air changes per hour. Duplicate fans should be provided with a fan failure automatic change over.

Natural inlet and mechanical extractHYBRID VENTILATION SYSTEMMixed Mode Ventilation or Hybrid Ventilation- utilizes both mechanical and natural ventilation processes. The mechanical and natural components may be used in conjunction with each other or separately at different times of day. The natural component, sometimes subject to unpredictable external weather conditions may not always be adequate to ventilate the desired space. The mechanical component is then used to increase the overall ventilation rate so that the desired internal conditions are met. The simplest often used definition of hybrid ventilation is "ventilation system that uses natural air intake through wall inlets in combination with mechanical extraction"

The illustration shows how a fan can be used to increase airflow. In this example, the cross-ventilation principle is utilised as long as the wind speed is sufficient to create airflow inside the building. The window on the side of the building that faces the wind is opened less than the window on the sheltered side.

Where there is insufficient airflow in the building the fan operates to increase airflow. Insufficient airflow may be due to:- low wind speeds or outdoor temperatures being higher than indoor temperatures - the building structure preventing natural airflowAnother hybrid ventilation system uses the sun to assist air movement. The vertical shafts in the building are glass fronted so that the sun heats up the air inside and causes it to rise out the openings at roof level. The high level openings in this case are stainless steel chimneys. As air flows out of the chimneys at roof level replacement air is drawn from the rooms into the shaft and thus naturally ventilated.

Low energy fans for use on still air daysGlass for solar heating of thermal chinmeyBRE OFFICE,SCOTLANDThe main hybrid ventilation principles are: Natural and mechanical ventilation This principle is based on two fully autonomous systems where the control strategy either switches between the two systems, or uses one system for some tasks and the other system for other tasks. It covers, for example, systems with natural ventilation in intermediate seasons and mechanical ventilation during midsummer and/or midwinter; systems with mechanical ventilation during occupied hours and natural ventilation for night cooling. Fan assisted natural ventilation This principle is based on a natural ventilation system combined with an extract or supply fan. It covers natural ventilation systems which during periods of weak natural driving forces or periods of increased demands can enhance pressure differences by mechanical (low-pressure) fan assistance. Stack and wind supported mechanical ventilation This principle is based on a mechanical ventilation system that makes optimal use of natural driving forces. It covers mechanical ventilation systems with very small pressure losses where natural driving forces can account for a considerable part of the necessary pressure.