Fire prevention device

Abstract

 A device consisting of an appropriate number of flat or curved sheets or strips of thermally conductive material (1-4) which are secured around a flue which may contain hot gases. These sheets or strips are interposed between the external surface of the flue and thermally insulating flammable material around the flue, and serve to dissipate and prevent heat being transferred to the flammable material thus protecting it from ignition.

Description

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.

Claims

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.

Drawing