Shower room ventilation - Extractor Fans

Whenever a shower is installed it is always advisable to fit an extractor fan to remove excessive water vapour and condensation. This is especially important in rooms where there are no windows or en-suites where the moisture-laden air may cause discomfort if not removed.

The extractor fan can be turned on via a pull cord, which must be beyond the reach of anyone using the shower, a humidistat which automatically turns on when needed or a switch connected to the lights. The latter is usually used in rooms with no windows and is a building regulation requirement.

Most bathrooms will be fitted with a through the wall extractor but a shower enclosure, or a shower in a confined space e.g. basement will require and in line extractor which can extract the moist air over a longer distance.

There are many issues facing the designer of ventilation installations in today's buildings says Chris Atkinson of Domus Ventilation Ltd. They include the assessment of correct extraction rates, the power and correct placement of the extractor, establishing the source of return air, taking into account energy saving issues and the correct choice of ducting through which air must flow efficiently.

Effective ventilation is now a primary consideration in the design and construction of all buildings. Greater use of sealed heating systems, double glazing, and increased insulation combined with the moisture created in kitchens and bathrooms, makes efficient ventilation a high priority.
Reported in PHAM news April 2001

One of the main problems that can reduce the efficiency of an extractor fan is poor placement There are a number of basic rules that can be applied in order to ensure that a fan is correctly sited:

• Always install the fan in the furthest window wall or ceiling from the main air inlet point and at a high level. This will ensure maximum airflow of fresh air throughout the whole room.

• Always ensure that there is sufficient provision for replacement fresh air into the room by using internal grilles in the door, ceiling or wall.

• If installing a fan in a room containing a fuel-burning device, which has a non-balanced flue, there must be sufficient replacement air to prevent fumes being drawn down the flue when the fan is on maximum extract. (See BS 5440 -Building Regulations for specific requirements.)

• Exhaust air must not be discharged into a flue used for exhausting fumes from appliances supplied with energy other than electricity. When deciding the method for exhaust air discharge and intake flow rates, the requirements of all relevant authorities must be strictly adhered to.

• Do not site fans where temperatures are likely to exceed 50°C, or above a cooker hob or eye-level grille.

• Wiring should be carried out in accordance with current IEE Regulations (UK) or standards of the country in question. Ensure that the instructions provided are followed during installation, with particular care taken to check the supply voltage, fuse rating and wiring are correct.

• IEE regulations require mains voltage fans to be positioned out of arms reach of the person using the bath or shower. i.e., Outside zone 0, 1 or 2 as stated in amendment 3 of BS7671:1992. Mains voltage fans and controls should also be sited away from potential water spray.

• Always use an In line Duct or Safety Extra Low Voltage (SELV) fan to ventilate a shower cubicle (zone 1). The mains voltage transformer control unit with SELV output must be positioned away from any potential water spray and out of arms reach (outside of zone 0, 1 or 2) of the person using the bath or shower.

• If the duct passes through an unheated roof void the duct should be insulated to reduce the formation of condensation. A vertical duct should also be installed with a weatherproof roof cowl of sufficient free area for the air volume. A condensation trap must be fitted in order to release the condensation build up. Horizontal ducts should fall away from the fan unit.

Because it is normally hidden, a ducting system is often not considered an important part of the construction or refurbishment of a building.

It is not always the fault of the cooker hood/fan manufacturer when dreams of fresh rooms and low noise fans are not achieved. It can often be that the wrong size or configuration of ducting has been selected by the installer or system designer. In ventilation installations the aim normally is to move the air from its source to the outside of the house as efficiently and quietly as possible. Common sense decrees that, to do this effectively the shorter the distance the air has to travel, the quicker the aim is achieved. It is also common sense that if an obstacle is put in the way such as a bend, the process will become more difficult as the air will lose its momentum and may require more powerful assistance in order to achieve the aim. It is worth remembering that many cooker hood manufacturers' guarantees will be invalidated it the correct size of ducting is not used to match the power of the hood.

Types of ducting

• Round pipe ducting
  A round duct provides optimum airflow performance and should be used if installation space allows.

• Rectangular ducting (flat channel)
  Rectangular or flat channel ducting is an excellent alternative when space is restricted. Its low profile enables the ducting to be easily concealed in the ceiling void or along the top of wall units.

• Flexible hose ducting
  Flexible hose is available in round and rectangular form to complement the rigid systems. It provides a solution to overcome awkward situations where a rigid system cannot be installed. Flexible hose is most effective for slow moving air, such as with tumble dryers, but can be used with higher volume appliances (with due care taken to ensure that its use is limited and that the hose is kept as taut and straight as possible).

The performance of the fan system is dependent on the performance capability of the fan itself, the size of the duct, the ducting configuration and the type of wall terminal installed.

Each ducting run can be planned in advance and its performance levels evaluated so it can be ascertained if the desired installation will meet the required level.

Technical data is available for the extractor fan/ cooker hood installer or system designer in order to achieve the optimum overall extraction rate. This data highlights how much pressure (measured in Pascals) is generated in the ducting run and will show if the required airflow rate can be achieved.

The performance of a ducting system is greatly affected by both the overall length of the ducting, the number of bends contained within it and the type of outlet. The total resistance of the duct system can be calculated by simply adding up the resistance of each component. Each product within the Domus range has a measured resistance value for the three UK Building Regulation flow rates.

Note: For every extractor fan or cooker hood installation, provision should be made for make up or return air. In most dwellings this is catered for by natural leakage. However, some high performance products may require the installation of an air supply set.

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