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