Fire Safety of Metal Chimneys in Residential Homes in Finland
|Tila||Julkaistu - 11 lokakuuta 2019|
|Nimi||Tampere University DIssertations|
started due to the ignition of flammable materials in the vicinity of metal chimney penetrations through floors, roofs and walls. In 2012, metal chimneys caused over 70% of all chimney-induced fires in residential buildings in Finland. The safety issue with metal chimneys is important, as they represent only 10% of all chimneys in Finland. To improve the fire safety of metal chimneys, an extensive research programme was conducted at the TUT Fire Laboratory of Tampere University of Technology (currently known as Tampere University) between 2010 and 2016. The study was mainly experimental. A series of laboratory and field tests were performed in order to determine the flue gas temperatures of fireplaces to be used in designing chimneys. The effect of the installation of metal chimneys and the effect of the smouldering combustion of the organic content of mineral wool on fire safety were studied using laboratory tests. Several reasons for chimney penetration-induced fires have been identified: higher actual flue gas temperatures onsite than those assumed in chimney design, incomplete or insufficient chimney installations and the smouldering combustion of mineral wool insulation. Fireplaces and chimneys are tested in accordance with EN standards. The standard tests are conducted in predefined laboratory conditions. The actual conditions onsite may be very different from these laboratory conditions. Site conditions vary, for example due to fuel type and chimney-draught conditions, which depend on site conditions, time, draught controls and the chimney length and installation. Regardless of this variation in conditions, chimney design based on EN standard tests should lead to a fire-safe solution. The flue gas temperature given on the CE marking of a fireplace may not always lead to a safe solution and should therefore not be used in designing a chimney. In the laboratory tests, the highest flue gas temperatures of the tested fireplaces measured in the temperature safety test were 124°C to 381°C higher than those given on the CE marking. In some field tests, the flue gas temperatures and chimney draught levels exceeded significantly those of the standard laboratory tests. The mean flue gas temperatures measured during the room heater and sauna stove tests were approximately 100°C higher than the flue gas temperatures given by the
manufacturers in the CE marking of the fireplaces. The study highlighted the differences between the conditions in real installations and those in the thermal performance tests prescribed by the standard for the certification of chimneys. It showed that the temperatures measured in the tests performed according to the standard can be lower than the temperatures that may occur in real installations. The standard’s weaknesses concern the position of the chimney in the test structure and the hot gas measurement point in the tests. For chimney testing, hot gas can drop by over 150°C in temperature between the standard measurement point and the chimney penetration, so the chimney may be
tested at too low a flue gas temperature. The highest risk is in the chimney thermal shock test as, in a soot fire, burning can occur just at the chimney penetration. The test results show that the flue gas temperature at the roof penetration may be 350°C lower than the test temperature. The position of the chimney in the test structure, in a corner of the roof and near two walls does not represent the worst condition in which a chimney may operate. In real installations, chimneys are usually completely surrounded by a roof that offers lower thermal conductivity than the walls of the test structure. In the test, the temperatures measured at the roof insulation were about 60°C higher than those measured on the walls. The temperature in the chimney’s roof penetration is affected by the smouldering combustion of mineral wool binder. Smouldering combustion generates additional heat in the penetration structure, which in turn increases the temperature of both the penetration insulation and the surrounding floor and roof structures. Experiments on mineral wool specimens show that smouldering combustion can increase the insulation temperature by hundreds of degrees, which in turn can increase the temperatures of the combustible roof construction materials located adjacent to the chimney penetration by over 100°C for a limited period of time. Several factors that can increase the temperatures in the chimney penetration were identified in this research. It has also been shown that the simultaneous action of several factors is also possible, which can increase the penetration temperatures to the level of the ignition temperature. The study presents a number of methods for increasing the reliability of current EN standard tests and thereby improving the
fire safety of metal chimneys.