Background
[0001] The present invention relates to a device which assists in the prevention of fires
in roofing and insulating materials used in building construction. More particularly,
the present invention relates to a device which prevents heat transfer causing the
ignition of flammable material with low thermal conductivity in contact with the outer
surface of a chimney or flue which contains hot gases.
[0002] A traditional method for roofing buildings is the use of thatch. Thatch comprises
bundles of straw or reed which may be secured to roof battens by means of yarn or
other ties. Thatch is a good insulator, but is problematic in that once alight, thatched
roofs burn persistently and are difficult to extinguish. Fire is perceived as being
a major hazard in buildings which have thatched roofs.
[0003] There has been a recent trend towards construction of new buildings with thatched
roofs. These are seen as being environmentally-friendly. However, the majority of
thatched buildings are old, many having been built before building regulations were
put into place. When these old buildings were constructed, chimney walls were usually
only one brick thick, e.g. about 100 mm. At the time that these buildings were constructed,
the fireplace would have been open-fronted and as a consequence, flue gases would
be diluted with large amounts of cooling air, thus lowering the temperature of the
flue gas. Traditional thatch maintenance techniques often require a top coating of
straw or reed to be applied over existing thatch. Over a number of re-thatchings the
depth of the thatch can reach over 1 metre, sometimes over 2 metres. The insulating
characteristics of deep thatch are such that little heat is lost through the body
of the thatch. However, the relatively thin chimney brickwork acts as a poor insulator.
Additionally, in many houses, the chimney and fireplace structure has been altered
to reduce the uptake of cooling air. Therefore, heat from the interior of the flue
is conducted into the thatch, which retains the heat.
[0004] Enclosed solid fuel stoves are becoming very popular with thatched-property owners.
Under normal operating conditions, flue gas temperatures from such stoves can reach
300°c. When a fire is burning strongly, temperatures can rise higher. It has been
found that the centre of a deep multi-layer thatch around the chimney flue can reach
85% of the flue gas temperature after one day of continuous use. There is little cooling
between the inner face of the flue in contact with the hot gas and the exterior brickwork
in contact with the thatch. Even if the stove or fire is used intermittently, there
is no significant reduction in the ultimate temperature reached in the body of the
thatch when this intermittent use is continued over several days.
[0005] Statistics available show that the probability of fire in any given thatched building
is half that of dwellings in general. Chimney fires occur more frequently in tiled
and stated roofed premises but are easier to extinguish without damage. Burning soot
exacerbates the problem of fire in chimneys. Surveys have shown that this is because
most owners of thatched properties are aware of the dangers and are very careful.
However, once started, a fire in a thatched roof is difficult to control. It has been
found that a spark or burning ember flying out of a chimney or flue is unlikely to
be sufficient to start a fire. The spark lands on the thatch but will usually smoulder
for a few seconds before extinguishing. The persistence of fires in thatched roofs,
once started, and forensic investigation of the nature of the fires, have indicated
that fire often develops because of the raising of the temperature in the body of
the thatch around the chimney above the combustion point (about 200°), and the gradual
development of a smouldering mass of material or hot-spot within the thatch. When
fire brigades attend such fires, they need to cut away the thatch to direct their
hoses on the hot-spots - but such cutting exposes the smouldering interior of the
thatch to oxygen which only serves to increase the ferocity of the fire.
Prior Art
[0006] A survey of patent literature shows that a number of fire detection systems are available
(e.g. as disclosed in GB-A-2203832, Graviner and US 4037463, Showa). EP-A-0449761
of Fiber Guard A/S describes an optical system of warning of fires in thatched roofs
and a connected fire alarm. EP-A-0779606 of CGA Direct describes a method for monitoring
the temperature in thatched roofs, wherein an audible warning of high temperature
is given to the householder when the temperature of the thatch reaches a critical
level. However, such monitoring methods are expensive to install and require continuous
monitoring. The probes and apparatus can be rendered useless if disturbed or if the
probes are displaced by only a few millimetres away from the flue. Furthermore, such
systems are not aimed at preventing fires, merely warning of a high-temperature situation.
In the case of fire detection systems, because of the nature of fire spread within
thatch, the alarm may be triggered too late to prevent the whole thatched roof being
lost.
[0007] The present applicants have developed a passive methodology of preventing the temperatures
of thatched roofs, where deep thatch is adjacent to a chimney, rising to dangerous
levels. The methodology involved can be used to prevent fires occurring in other situations
where a flammable insulating material surrounds a hot chimney or exhaust flue.
Statement of Invention and Advantages
[0008] According to the present invention, a collar of thermally conductive and non-flammable
material is inserted between the outer surface of the chimney flue and the flammable
material to be protected. Heat passing through the wall of the flue is conducted away
from the flammable material and dissipated harmlessly.
[0009] Suitable materials for the collar include aluminium, copper and lead. Aluminium and
copper are preferred, most particularly aluminium. The materials used should be, for
length of service, corrosion resistant and for ease of use, malleable. The sheets
may be flat or curved as most appropriate to fit the external surface of the chimney
or flue.
[0010] The conductive material preferably forms a continuous collar around the circumference
of the chimney. However, it has been found that a non-continuous collar will also
serve to dissipate heat, providing that sufficient, preferably over 50%, most preferably
over 80% of each surface of the brickwork chimney is covered where it comes into contact
with the roof covering. The non-continuous collar may be formed from rods or, preferably,
strips. Each strip may be from 50 mm to 300 mm, preferably about 120 mm wide.
[0011] The conductive material must extend the full depth of the thatch. Preferably the
chimney collar has a thickness of from 5 mm to 15 mm, most preferably 5 mm to 10 mm.
It is preferred that this thickness is made up of two or more layers of sheeting.
As well as the benefit of thinner layers being easier to handle than a single thick
and heavy layer, it has been found that an air gap between the sheets assists in dissipation
of thermal energy away from the thatch.
[0012] The thermally conductive collar can be used in conjunction with a temperature warning
system as described in the prior art referred to above.
[0013] It has been found that the use of the chimney collar prevents the temperature of
the thatch which is proximal to the outer surface of the chimney rising to dangerous
levels. It has been found that if thatching, reed or straw is exposed to temperatures
of around 200°C or higher, a process of charring, which may lead to flaming combustion,
can occur. If a 6 mm sheet of aluminium is used, the temperature of the thatch proximal
to the chimney does not reach 200°C, even after extended heating in the chimney flue
using a stove where the gases in the inside of the flue are over 300°C.
[0014] As described above, when surrounded by a thick layer of thatch or other insulating
material, repeated flow of hot gases through a chimney leads to a build-up of heat
on the external chimney surface - the heat does not dissipate quickly enough before
new heat is applied. However, using the collar of the present invention, the heat
quickly dissipates and thus there is no continual rise in temperature until the combustion
point is reached.
[0015] It has been found that the use of the collar helps to spread the temperature rise
of the outside surface of the chimney evenly across the brickwork. Without a collar,
there is an increase in temperature along the brickwork at the centre of the adjacent
thatch. However, if a collar is used, the temperature is more evenly spread, without
the high point previously found. Additionally, when the flue is not in use, the brickwork
cools more rapidly.
[0016] In a further embodiment of the invention, it has been found that a jacket can be
provided for the chimney, for example of metal, filled with a heat conductive material,
for example air or water. As with the metal sheeting, heat is dissipated away from
the outer surface of the chimney.
Specific Embodiments
[0017] A specific embodiment of the present invention will now be described by way of example
with reference to the accompanying drawings.
[0018] Figure 1 shows a perspective "transparent" view of a chimney passing through a thatched roof,
with the conductive material affixed to the faces of the chimney.
[0019] Figure 2 is a cutaway view through the thatch showing one face of the chimney flue surmounted
by conductive sheeting.
[0020] Figure 3 shows a section across one face of the brickwork with the conductive sheeting in
place.
[0021] Figure 4 shows a section across a chimney which is surrounded by two layers of strips of metal.
[0022] Figure 5 shows temperature traces of various parts of a flue and thatch when the conductive
collar according to the present invention is in place, when removed and when not in
place.
[0023] Figure 6 shows the temperature across the external face of the brickwork of a chimney flue
with and without the conductive sheeting in place.
[0024] In a structure according to the present invention, a chimney (5) passes through a
roof, onto which is attached by known means, bundles of thatch. Four sheets (1, 2,
3, 4) of aluminium, each 6 mm thick, or any other suitable corrosion conductive material
such as copper are placed directly adjacent to each outer surface of the chimney through
the depth of the thatched roof (6). The top of each sheet extends at least to the
upper surface of the thatch (6) and may project some distance above the surface of
the thatch. This has two benefits; there is increased conductivity of heat away from
the chimney into the air and there is also sufficient depth to enable further thatch
to be put on during re-roofing without the need to replace the panels. In another
embodiment, two layers of 3 mm thick sheeting are used in a laminate structure, with
a 1-2 mm air gap between the sheets.
[0025] The sheets may be fixed to the external surface of the chimney (10) by any of a known
number of methods, e.g. bolting each sheet to the chimney or allowing the sheeting
to rest on a small ledge. A preferred method is the use of turnbuckles (11) which
support the sheet (1) from below the thatch. These turnbuckles can easily be adjusted
from inside the roof space. It is preferred that the panels can be easily removed
when the thatch is being maintained in order that the chimney may be inspected and
repaired as necessary.
[0026] In existing thatched roofs, the sheeting can be slid up or down against the outside
of the chimney, whilst ensuring that the full depth of the thatch is protected. In
new roofs, the sheeting is attached prior to thatching. A standard flashing (8) secures
the top of the sheet to the brickwork and prevents the ingress of water between the
sheet and the chimney. Typically, a lead flashing is used in such an application.
A cement fillet flashing could be used, but it has been found that cement fillets
reduce heat extraction and can also be dislodged by thermal movement in the conductive
sheet. It is preferred that the protective sheeting and flashing are selected from
materials which are electrochemically compatible.
[0027] It has been found that the collar can be made of non-continuous metal strips. This
use of strips makes it easier to slide the metal down between the chimney and the
thatch when utilising the invention on roofs which are already thatched. Figure 4
shows such an arrangement, using two layers of strips (41) to increase protection
and so that thinner and thus more manageable strips can be used.
[0028] An experiment was carried out to show the effect of the thermal sheeting in reducing
the temperature of thatch in proximal contact with the chimney. Figure 5 shows the
results which were found using a number of temperature probes in the thatch, on the
inside surface of the chimney flue and on the outside surface of the chimney flue.
A protective collar according to the present invention was inserted between the outside
of the chimney and the thatch, ensuring that the sheeting extended the full depth
of the thatch. A fire was then lit at the bottom of the chimney and allowed to burn.
It was found that the temperature of the inner surface (9) of the chimney flue soon
reached approximately 300°C as hot gases rise. The temperature of the outer surface
of the chimney (measured on the brick surface adjacent to the protective sheeting
(1)) rose gradually to reach a constant temperature of approximately 200°C. The temperature
of the thatch nearest to the flue reached approximately 150°C. This is shown in Figures
5 and 6.
[0029] After approximately 24 hours of measurement, the temperatures being measured had
reached a stable condition. The thatch in proximity to the outside of the fluid was
below the combustion temperature of thatch. The fire was then extinguished. The temperatures
dropped over a six hour period. The temperatures of the inside of the flue, the outside
of the flue and the thatch all dropped to about 60°C. The protective sheeting was
then removed, and the fire was reintroduced. There was a rapid elevation in the temperature
of the inner surface of the flue back to about 300°C. The temperature of the outside
surface of the flue, which was now measured on the brickwork, rose to about 240°C.
The temperature of the thatch in touching proximity to the external brickwork also
rose rapidly to approximately 230°C, at which point the thatch began to smoulder,
heat up and then burst into flames. This rise in temperature occurred only four hours
after the fire had been reintroduced.
[0030] It is apparent from the experiment that the protective sheeting helped dissipate
the temperature of the chimney such that the thatch did not reach combustion temperature.
Even after a long period of heating, the thatch did not burn.
[0031] Although the present application has been described in relation to the use of external
thatch against the chimney flue, it is also expected that the methodology of utilising
a collar around the chimney flue could be used in situations wherein other combustible
materials surround a chimney, such as insulation material on loft floors which abuts
a chimney, or even the growing use of grass as an outer covering to roofs in environmentally-sensitive
locations.
1. A method for preventing fire in a thatched roof having a chimney which comprises inserting
a collar of heat conductive material between the chimney and the thatch of the thatched
roof, said collar extending the full depth of the thatch.
2. A method as claimed in Claim 1 wherein the collar is formed as a continuous metal
collar.
3. A method as claimed in Claim 1 wherein the collar is formed by strips of metal which
form a non-continuous collar.
4. A method as claimed in any preceding claim wherein the collar comprises two or more
layers around the chimney.
5. A method as claimed in any preceding claim wherein the conductive material is aluminium
sheet.
6. A method as claimed in any preceding claim wherein the thickness of the collar is
from 5 mm to 10 mm.
7. A method as claimed in any preceding claim wherein the collar is fixed to the chimney
by turnbuckles which support the collar from below the thatch.
8. A method as claimed in any preceding claim wherein the top of the collar is secured
to the chimney by flashing.
9. A method as claimed in Claim 1 wherein the collar comprises a jacket filled with a
heat conductive material.
10. A method as claimed in Claim 9 wherein the heat conductive material is water.