Flueing and Vents

In four Groups use the Post-it notes on the wall to show your knowledge

9189-05 Unit 317 Specific

domestic core safety for natural

gas

L.O 12 Know the ventilation

requirements of gas appliances

installed in dwellings.

In this session we

will:

Describe the tests

and checks to

confirm the

suitability and
performance of
the ventilation.

Open Ended

What do you know about Ventilation for gas appliances?

Checking existing ventilation

•During installation of an appliance,
it is essential that the ventilation
provision is checked for compliance.
•When an appliance such as an
open flued boiler is located within a
room, it must have sufficient air
from outside to burn properly. The
supply of air is commonly provided
via a vent located on an outside
wall of the property.

Confirming ventilation performance

The British Standard that gives

guidance and advice on

ventilation is: BS 5440 – Flueing

and ventilation for gas

appliances of rated input not

exceeding 70kW.

An adequate supply of air is

essential for the safe operation

of gas appliances.

Ventilation is also required in the
following circumstances:
• To dissipate heat and provide cooling for

appliances contained within
compartments.

• Removal of condensation by providing

air changes within a room where
flueless appliances are installed.

• The provision of relief air to overcome

the effects of mechanical extraction and
fans.

Home

Open in new tab

Copy link

Confirming
ventilation
performance –
Categories

•Ventilation is required for all
the appliance categories
below:
• Flueless appliances.
• Open flued appliances.
• Open flue appliances

located in a compartment.

• Room sealed appliances

located in a compartment.

Confirmin
g
ventilation
performan
ce –
Ventilation
types

•There are three main types of
ventilation and these are as follows:

•Permanent ventilation
•Provided for an appliance to aid
combustion and heat dissipation.

•Adventitious ventilation
•Provided to enable the required air
changes within a building to meet the
building regulations.
•
•Compartment ventilation
•Provided for an appliance sited within a
compartment to supply air for
combustion and cooling.

Fill in the Blank

What type of ventilation does the following passage describe?

‘Provided for an appliance to aid combustion and heat dissipation.’

Confirming ventilation performance – Ventilation
types

Adventitious ventilation
Buildings admit draughts from the outside (through gaps around windows, beneath doors
and via window vents) and this is called ‘adventitious ventilation’ which equates to about
35cm2 of free air.
Permanent ventilation
When calculating the free area required for the appliance installed
within a room we use 5cm2 required for each kW (net) of gas
used.
Owing to the presence of 35cm2 for adventitious ventilation, this
means the
first 7kW of the input rating of a gas appliance can be deducted.
Example:

A 16kW (net) open flue boiler is installed in a
room.

16kW – 7kW = 9 kW.

Ventilation for 9kW = 9 x 5 = 45cm2

A vent providing a minimum of 45cm² of free air should therefore
be located on an outside wall to provide air supply for combustion.

Confirming ventilation performance
– Ventilation types

•A compartment is an enclosure designed
to house a gas appliance.
•These installations require high and low-
level ventilation unless the manufacturer’s
instructions state otherwise.
•The purpose of vents located on an
appliance compartment is to provide air
for complete combustion, enabling the
correct operation of the flue. Also for
appliance cooling.
•There is no adventitious air
allowance in compartments.

Confirming ventilation performance – Compartment
ventilation

Both of the air vents
must communicate

with the same room or

space.

Both of the air vents
must be on the same

wall to outside.

If a compartment

contains two or more
appliances, the heat
inputs must be added

together when

calculating the vent

sizes.

Range rated

appliances must be

calculated using their
maximum heat input

rating.

Compartment ventilation for open flued
appliances

•If we calculate the ventilation requirement for an open flued
appliance of 15kW (net) maximum input installed in a compartment
with vents facing externally, we would use the formula of multiplying
the lower vent by 10cm² times the kilowatt input of the appliance
and the higher level multiplied by 5cm². Therefore:
•10 × 15 equals 150cm² at low level and 5 × 15 = 75cm² high
level
•If the vents in the compartment are based internally, the following
equation applies. Input rating of the appliance times 20cm² for the
lower vent and 10cm² for the higher vent. Therefore:
•20 × 15 = 300cm2 at low level and 10 × 15 equals 150cm² at
high level

Gross and net

When working out your ventilation requirements, we calculate either in gross
or net values.

Grossis the the higher heating value which takes into account the latent heat
of vaporisation of the water in the combustion process, whereas netassumes
that the latent heat vaporisation of the water in the fuel has not been
recovered.

Therefore, when calculating in net, allow 5cm2 per kilowatt input.

And, for gross, allow 4.5cm2per kilowatt input.

• To convert gross to net divide by 1.11.
• To convert net to gross multiply by 1.11.

Vent calculations for open flued
appliances

In a room

One vent required in either position a, b, c or d direct to outside
air. (5cm²/kW of rated heat input above 7kW.)

In a compartment

Ventilated direct to outside air 10cm²/kW of rated heat input at low
level and 5cm²/kW of rated heat input at high level.

In a compartment

Ventilated via a room or internal space 20cm²/kW of rated heat input at low
level and 10cm²/kW of rated heat input at high level. The room is ventilated as
in 1 above.

Open flue ventilation in series

In some cases, open flued

appliances are located some

distance away from an outside wall.

In order to provide air for

combustion, it will be necessary to

provide ventilation via any other

wall between the appliance and the

outer wall.

This method of is called installing

air vents in series and the following

rules must be adhered to:

• When venting through one wall to

an appliance, the internal vent
needs to be the same size as the
one located on the outer wall.

• If venting through more than one

room, the internal vents need to
be 50% bigger than the external
vent.

Open flue
ventilation in
series

•If air is provided from the outside air,
as shown in the example below, both
internal and external vents remain
the same size.

Multiple Choice

If an open flued appliance is to be installed in a room supplied with air from another, as in the image below, how should the ventilation opening in room 1 be sized?

• The vent in room 1 should be 50% larger than the vent between rooms 1 and 2

• a)    This situation is not permissible

• Both internal and external vents should be the same size.

• The vent in room 1 should be 100% larger than the vent between rooms 1 and 2.

Open flue vents
in series

•In the situation, shown below, the
external air vent 1 remains the same,
but both internal vents 2 and 3 are
50% bigger than the external vent.

Multiple Choice

If an open flue boiler of 28kW gross was installed in the situation below, what size should the vents be?

• 92.28mm² for both the external and internal vents.

• 92.28mm² for the internal vent and 105mm² for the external vent.

• 105mm² for the external vent and 210mm² for the internal bank.

• 28mm² for the external vent and 56mm² for the internal vent.

Open flue vents
in series

•In the case of compartments, the vents in
series rule does not apply.
•Vents A and B are sized for the compartment
as normal. However, both the internal vents 2
and 3 are still required to be 50% bigger
than vent 1.

Calculations for room sealed appliances

Located in a room

No vent is required.

Compartment

Ventilated direct to outside air 5cm²/kW of rated heat input
at both high and low level.

Compartment

Ventilated via a room or internal space 10cm²/kW of rated heat
input at both high and low level.

Many manufacturers now state that ventilation is not required for compartment installations because
of the low surface temperature of their room sealed boilers.

This extract is taken from a
room sealed Baxi
combination boiler
installation manual …

Multiple Choice

A room sealed combination boiler is installed within a kitchen. The net input of the appliance is 32kW. What are the ventilation requirements for boiler?

• 125cm² at high level only.

• 125cm² at high and low level.

• 160cm² at low level only.

• No ventilation required.

Vents

Ventilation grilles must allow free air to pass and NOT be closable. Air
must NOT come from bath or shower rooms. Vents should NOT
incorporate any gauzes or screens.

• Air vent openings should be not be larger than 10mm and no smaller

than 5mm.

• External air vents should be located so that they will not become

blocked.

• Air vents on internal walls should not be fitted any higher than

450mm.

• An air vent should never penetrate a protected shaft or stairway.
• Vents can be made from a range of materials such as terracotta,

plastic, brass and aluminium.

Hit-and-miss-type vent with gauze,
which is not suitable for ventilation
for gas appliances.

Suitable

ventilation

devices

Core vent – Fixed ventilation
size, installed across the full
width of the cavity wall
using a core drill to drill
through a wall.

Black hole ventilator – Fixed
ventilation size, installed
across the full width of the
cavity wall.

Suitable ventilation devices

Ventilation grille – Fixed
ventilation openings of a
certain size, normally fixed to
the fascia of a wall or internal
door to provide ventilation.

Brass metal ventilation grille –
Fixed ventilation openings of a
certain size, normally fixed to
an internal cupboard door to
provide ventilation.

Unsuitable ventilation devices

•Ventilation devices must
NOT incorporate a closing
device or fly screen. Should
the vent be closable it could
cut off the ventilation supply
to the room. The presence of
a fly screen could also
restrict the ventilation
openings providing less free
air than specified on the
main vent.

Vents

High- and low-level vents should be located at
the greatest vertical distance apart.
If a duct comes from high level, the duct should
terminate below the level of the burner.
It is not acceptable to duct from low to high
level.
On louvre doors the total free area should be
equal to that of the calculated high- and low-
level vents.

The louvre door, shown
left, requires each gap
between the louvres to
be measured individually
to calculate the free air
that can pass through
them.

Ventilatio
n
cupboards

Compartments
containing gas
appliances should be
labelled accordingly.

Ducting requirements

•A duct which connects two air vents should be no longer
than 3m in length and should have no more than two 90-
degree bends in the design, unless a special allowance is
made for the restriction that they impose.
•This can be achieved by increasing the cross‐sectional
area by 50% for each successive 3m or part thereof in
excess of the initial 3m run (BS 5440 Part 2: 2009).
•Flexible ducting should not be used, in order to avoid
sagging low points and the potential of condensation to
form.

How to measure ventilators

For louvred ventilators, measure the
height opening of the ventilator
aperture as shown.
Then measure the length of each
aperture and multiply by the number
of apertures on the ventilator to
obtain the total free area in cm2.

Length x height x number of apertures = Free area in
cm2