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
Where to site the fan
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
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.
Choosing and installing the correct ducting
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
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 diagram shows a typical pressure flowrate
curve for an extractor fan or cooker hood (flow rate information should
be available from the fan/cooker hood manufacturer). The information it
contains is obtained by a series of laboratory measurements starting with
free flow at zero pressure then, by steadily increasing the pressure that
the fan has to operate against, the volume flow and pressure at each point
is derived in order to plot the curve.
In order to achieve the current UK
Building Regulation flow rate requirement for a kitchen of 60 I/s (216 m3/hr),
or 30 I/s (108 m3/hr), for a hood within 3OOmm of hob centreline, the maximum
duct system resistance when using this particular cooker hood can be no
greater than 150 Pascals (Pa). Therefore, by reducing the resistance of
the duct system, the extract performance of the fan or cooker hood is increased.
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.