Fireplaces and chimneys
Function of fireplace and flue
• To secure maximum heat for the comfort of the occupants;
• To provide adequate protection against the spread of fire;
• To ensure that the products of combustion are removed;
• To prevent downdraught;
• To burn the fuel efficiently and safely.
How an open
fire works:
Air is sucked in as
the Smoke rises.
Smoke escapes up
the narrow flue
The grate lets air in
And allows ash to drop
Into the tray below
Smoke and gasses are
Vented to the outside
Collins 2001
The air that is drawn into the flue from the room must
Be replaced. There must be some means to allow air
Changes to occur in the roon. Michael Hannon 2003
The average open firplace requires approximately
140 m3 of air per hour
Possibilities include.
• Controlled vents in wall.
• Vent of openings in the
windows.
• Underfloor air supply
Pivot window Michael Hannon 2003
For a fireplace and flue to aperate
efficiently the chimney must have
adequate:
• Strength and stability
• Weather resistance
• Thermal insulation
• Fire resistance
• Chimneys for use with solid fuel appliances
should be capable of withstanding a
temperature of 11000C.
Chimney strength and stability
Same as that for a wall. As it will be higher
and thicker than the wall in which it is
situated, allowances must be made for this
in the foundations.
Extra allowance
in the foundation
• The chimney must be self-supporting above
the roof, where it will be subject to wind
pressures. There are specific regulations
controlling the height of chimnsys to ensure
that they will be stable enough to resist
wind pressures.
The height and width of a chimney:
(H) Should not exceed 3.5 times width (B) Michael Hannon 2003
4.5m min
Chimney heights in bungalows HomeBound 2001
Chimney pots are tapered to
Reduce the entry of rain.
The taper improves the draught
And flow of gases. Set chimney
Pots ¼ the length (150) mm
Into the stack for adequate
Support.
Michael Hannon 2003
Flue to
House B
Flue to
House A Party
wall
1st floor
Ground floor HomeBund 2001
• Rafter and roofting and flooring timbers,
such as joists, are trimmed around the
chimney. Keep all timbers a minimum of
40mm away from the outside face of the
chimneystack.
Timber joist
40mm min. gap
If blockwork
Is less than
200mm.
Chimney stack HomeBund 2001
Trimmer Trimming
joist
Party
wall Joist parallel to chimney stack HomeBond 2001
Trimming
joist
Trimmer
Party
wall
Joist at right angles to chimney stack HomeBond 2001
40mm gap
All around
Brian Porter 1991
Brian Porter 1991
No timber beyond
This line (exept
Flooring and trim
Eg. Skirting board.
Weather Resistance
The need to prevent the penetration of wind
and rain is the same for chimneys as it is for
external walls. Special care is needed at the
intersection of the roof with the chimney.
To make the chimney watertight with the
roof covering. This achieved using a gutter
and flashing at the top edge and an apron
flashing at the bottom edge.
Soakers and stepped flashings are used at the
sides. Lead is the main material used for
flashing. (or copper and aluminium may be
used).
The top of the stack must be detailed to throw
off water and prevent any moisture entering
the chimney at this point.
Front apron
Lead soakers
Slate roof
Weepholes
Stepped cover
flashing
Back gutter
Metal tray DPC
Michael Hannon 2003
50mm upstand tight
against flue liner
50mm upstand to
prevent water
flowing over edge
of tray
Drainage channel
Lead flashing
HomeBond 2001
Lead tray with 50mm
upstand at sides and
edges.
Stainless steel tray
incorporating
drainage facility.
HomeBond 2001
Stepped DPC
Lead soaker
Apron flashing
Michael Hannon 2003
Weepholes to
discharge over
flashing.
150mm
min.
Stepped
Flashing.
Metal tray DPC
fitted above
lower flashing HomeBond 2001
Lead flashing
Weepholes
Dampness, the result of an inadequately installed
DPC tray in a brickwork chimney stack. HomeBond 2001
Sand cement mix to
seal gap
Throating
50mm
Min. Dpc
under
coping 100mm
Min.
Thickness
Of wall
Between
Flues
Above
Roof level HomeBond 2001
HomeBond 2001
Precast concrete chimney capping
Thermal insulation
Locate chimneys and flues on internal walls,
where possible. This improves the
insulation provided by the flue. Insulation to
the flue is necessary to prevent the flue
gases cooling and resultant slowing down of
airflow up the flue.
Poor insulation will lead to condensation on
the walls of the flue.
Fire resistance
Construction must be such that there are
adequate distances between combustible
materials and fireplaces, flues and
chimneys.
Use non-combustible insulation
Over this area.
200mm min.
200mm min.
HomeBound 2001
recess.
300mm min. 200mm min.
Recess.
Michael Hannon 2003
Homebound 2001
Fender wall
Chimney wall construction 200mm thick flue between one flue and
another.
200mm min.
blockwork
between
flues liners.
HomeBond 2001
100mm thick between a flue and the outside air.
100 mm
Min.
HomeBond 2001
200 mm
Recess
200 mm min.
Solid separating wall Michael Hannon 2003
300mm min. 200mm min.
Recess.
Cavity wall Michael Hannon 2003
200 mm
Recess
100 mm Min.
Back to Back (same dwelling) Michael Hannon 2003
200 mm
Min.
100 mm Min. External wall
Michael Hannon 2003
• The flue outlet must be well above the roof
to prevent downdraught and to carry the
combustion by-products safely into the
atmosphere.
Not less
Than 1 m
Flat Roof Michael Hannon 2003
Not less
Than 600mm
Greater than 100
Michael Hannon 2003
Not less
Than 600mm
Greater than 100
Michael Hannon 2003
Not less
Than 600mm
Less than
600mm
Greater than 100
Michael Hannon 2003
Not less
Than 1 m
Less than 2.3 m Less than 2.3 m
1 m min.
Sky light
Opening window
Height of flue over openings Michael Hannon 2003
Less than 2.3 m Less than 2.3 m
Not less
Than 1 m
Not less
Than 600mm
Tank
room roof
Adjoining building
Height of flue over adjacent or adjoining
Buildings. Michael Hannon 2003
Constructional hearths
• Constructional hearths should be of soild, non-
combustible material at least 125mm thick.
• No cumbustible material should be used under a
constructural hearth unless:
1. It is to support the edges of the hearth.
2. Or there is an air space of at least 50mm
between the material and the underside of the
hearth.
Jamb
(chimneyBreast)
Structural hearth.
Non
Combustible
material
Construction studies 1988
Note: if fule walls are less than 200mm thick, a gap
Of at least 40mm is left as shown (50mm is usual)
Section through
hearth
Brian Porter 1991
Hearth
Homebound 2001
Constructional hearth
Brian Porter 1991
Brian Porter 1991
Fireplace design
The fireplace consists of a rectangular recess to suit the fireback or appliance to be fitted.
The walls that project into the room to form this recess are called the chimney breast.
A fireplace must have some means of reducing the width of the opening to that of the flue. This is achieved bu using a pre-cast flue gathers.
The size and shape of the throat is important for the efficient removal of the by-products of combustion. The recommended size is 100 mm.
Opes to
accommodate pipes
From the back boiler
Blockwork nib to
Accommodate central
Heating pump and pipework.
Homebound 2001
Precast flue gatherer
Width of opening and
Depth of recess
To suit appliance Michael Hannon 2003
First floor
ground floor
Construction studies 1988
• The fireback’s function is to contain the
burning fuel, prevent heat damaging the
wall behind it and to project the heat of the
fire into the room by radiation.
The space behind the fireback is filled with a
weak backfill mix.
flue
Motar sloped to
Form flaunching
(throat) lintel
Sectional
fireback
Corrugated
Card leaves
An expansion
Gap when it
burns
Fireback has to
Be sealed to the
surround
Collins 2001
Back boiler
Back boiler is used at the rear of the fire to
heat water for central heating and for
domestic use.
Fireplace surround
Boiler flue
Flow and
Return pipes
Boiler damper
control
Boiler
Michael Hannon 2003
Deap ash pits
Ash containers for the normal foreplace are
placed above the hearth. Which means the
fire is raised off the floor.
For the fire to be at the level of the hearth,
sink the ash pit below this level. Seep ash
pits are sesigned to hold from three to seven
day’s ashes.
• The air supply for this type of construction
is below floor level. A valve, usually at
hearth level, controls it. Ducting from all of
the external walls id fed into a balancing
chamber in front of the fireplace. A cast
iron pipe connects the balacning chamber to
the ash pit.
Duct to
Outside wall
Deep
ashpit
Butterfly
valve
Balcncing
chamber
Duct to outside wall
Deep ash pit
Michael Hannon 2003
Other applainces
Log-burning stove Collins 2001
Collins 2001
Collins 2001
Approximate percentage heat output
for the following appliances
• Open fire = 20% to 40%
• Solid fuel = 40% to 50%
• Oil central heating boiler = 60% to 80%
• Gas = 80% to 90%
Flue angles
Build a chimney as straight as possible. This
ensures the best draw. Where an offset is
required it should be not less than 500 to the
horizontal (1270 to the vertical)
For maximum draw
Keep flues as
Straight as possible
Michael Hannon 2003
585 mm
300-600
1270
Michael Hannon 2003
Flue liner
• The flue liner is made form burnt fire clay
or acid resisting clay and may be rebated or
socketed and built with the socket end up.
The traminal pot or chimney pot is used on
the top of the chimney and designed to
assist draught. Flue liners are in sizes from
150-225mm internal diameter.
450
225
Socketed
Rebated
Terminal pot
Construction studies 1988
Instaling flue liners
• It is important to fit a liner that’s suitable for the
kind of heating appliance your using.
• Flue liners take the form of tubes, either one-piece
or in sections, and are made of metal or some
other ridgid non-combustible material, such as
pumice.
• Flue liner are jointed with 1:6 motar abd packed
around with lime motar as the work progresses.
Flue liner
Collins 2001
Safe access
Collins 2001
Collins 2001
cowl cowl
Collins 2001
Faulty chimneys
If a chimney does not function properly smoke can
be blown into the room through the fireplace
opening. Thid can result from a number of factors:
• Blocked flue
• Inadequate air supply
• Throat not properly designed
• Flue not properly gathered
• Chimney not high enough to prevent downdraught
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