PASSIVE AND ACTIVE FIRE PROTECTION OF THE AIR HEATED SOLAR HOUSES Tibor Harsányi and Lajos Takács Department of Building Structures, Technical University of Budapest, 1-3 Mûegyetem rakpart, 1111 - Budapest, Hungary Phone 36-1 463-1306, Fax 36-1 463-3949, email ltakacs@est.bme.hu Abstract - Since 1993 in Hungary, several pilot houses were and is being built with the Japanese OM-Solar hybrid air heating system. The heating system, the collector and the building structures were adapted to the Hungarian climatic conditions and building practice. The first house was monitored during a heating season and the consequences of this monitoring were used during the developing work. This paper will describe the adjustment of the air - heated houses to the Hungarian fire protection standards, taken into consideration the higher hazard of fire spread in case of air-heated solar houses. The fast fire spread can handicap the successful evacuation of the inhabitants and hinder the fire brigades in the efficient fire extinguishing. Both passive and active fire protection methods were combined and used in order to reduce the higher risk in case of accidental fire in solar heated residential and public buildings as well. 1. INTRODUCTION Hungary is in the temperate-continental climatic zone, between the 46-48 N, with frequent weather fronts at spring and autumn and large temperature swings. The 20/12 degree-days value is in average 3.100, the winter design temperature is between -11 and -15 C for the different zones. The number of sunny hours is about 1.900-2.100/year, which is quite favourable for various solar systems. The demanded inner temperature of residential houses is 20 C, which - added to the winter design temperature (-11-15 C) - results big temperature difference between the indoor and the outdoor area. The required specific heating energy therefore is large. This may cause also condensation problems at different building structures and elements, such as external walls and flat roofs, air collectors and air ducts at different solar systems. 2. GENERAL DESCRIPTION OF THE OM- SOLAR SYSTEM The OM-Solar system was developed by Prof. A. Okumura in Japan. The Hungarian adaptation was carried out by THERMO Ltd, A. Léderer. The architectural design of the houses was developed by the ROBITHA Ltd, with leader architect T. Harsányi and assistant L. Takács, the latter is the author of this paper. 2.1. Japanese OM-Solar houses The original Japanese OM houses have a twoshaped roof collector, the upper part of which is covered with glass, and the lower is covered by an ordinary, but good absorber roofing material, for example black coloured metal sheet. The sun radiating on the roof heats the fresh air in the cavity which is drawn through the inlet and moved upward into the roof. The glass surface at the upper part of the roof is simply placed over a metal plate in order to decrease heat losses from the warm air to the environment and improve the heat gain. At the roof top a thermostat is installed in the ventilated air gap. The warm air is channelled into the semi-circular duct. When the air temperature reaches 25 C, an axial fan is activated by the thermostat. During winter, this fan circulates the air down the vertical duct through underneath the floor to the air outlets of the different rooms. The system pre-heats the ventilation air or heats the house by the ventilation air. Japanese houses usually have a reinforced concrete slab foundation, which in OM-Solar houses is wellinsulated and also used as heat storage system. As room temperature decreases, the stored heat is slowly re-radiated, warming the entire floor. Then the air, which conducted the heat to the storage slab, is released into the rooms, stimulating fresh air circulation.
The only mechanical part of the system is the socalled handling box (which is an air controlling unit with an axial fan an checkdampers), situated at the roof-top. Its purpose is to regulate the air flow gathered from the roof top duct and to channel either to the north exhaust opening or to the vertical air duct. It contains two checkdampers moved by small electric engines, an axial fan and a heat exchanger, the latter produces domestic hot water. As the other solar systems, OM-Solar system is dependent upon solar energy, so auxiliary heating is required during cloudy or rainy conditions. Various auxiliary heating devices are proper to the OM-Solar systems, but the most practical is a fan convector, which results that the OM-Solar regardless of the heat source, solar or fan convector, the comfort level of the heating system is the same, and the stored heat in the concrete slab can be utilized. In this heating method, the air lets in to the air duct at the top of the flat through the so-called recirculation valve. Installing the recirculation valve to the system, the equal temperature is also ensured between the top of the gallery and the floor level in the groundfloor. In summer conditions, the heated air from the collecting air duct is exhausted outward through the duct to the north part of the house. Additionally, heat from the air induced through roof collector is transferred by the water heating coils to the domestic hot water storage system. 2.2. Hungarian adaptation The original Japanese houses are built with timber structures. Compared with the Hungarian heavyweight building structures (masonry walls, concrete slabs), the building structures of Japanese houses generally have a lower thermal mass, which is more proper to the air heating than heavy structures. The princip of the solar heating system of the Hungarian houses is the same as the Japanese ones, but there are some differences between them as a consequence of different building methods, climatic conditions and regulations: - the heat storage system is not the concrete foundation slab on the soil as in the Japanese houses, but the concrete floor slabs; - the auxiliary heating system is in the basement, an ordinary hot water gas boiler, heating the air in the Handling Box through a heat exchanger; - the upper glazed part of the roof is situated under different angle than Japanese houses in order to optimize the efficiency of winter sun radiation at Hungarian winter climatic conditions. In the advanced houses, the whole collector surface is covered with glass, improving the collector s efficiency for the higher temperature differences between the indoor and outdoor areas is Hungary; - the advanced Hungarian OM-Solar house reduces the originally four heat exchangers to two, using as auxiliary air heating exchanger the upper one in the handling box. This demands magnetic connection valves in the heating circuits and larger domestic hot water tank with overflow connection to the gas boiler; - and the use of the buried pipe earth cooling system, because of the Hungarian summer conditions, instead of artificial air conditioning. The principal section of the advanced Hungarian OM-Solar houses can be seen on Fig. 1. 3. FIRE PROTECTION PROBLEMS OF THE OM - SOLAR HOUSES 3.1. Regulations One of the most important developing work is the adjustment the air - heated houses to the Hungarian fire protections standards and the fire spread hazard increased by the air cavities and gaps. According to the fire protection standards, using of non-combustible materials and building structures is generally required, but only in case of the loadbearing structures and partitions. Use of any combustible materials which have lower ignition temperature than 150 C, except bituminous materials and dry adhesives, is forbidden in Hungary. The standards are based on passive fire protection principles, which means reducing the fire hazard and decreasing the speed of fire spread as it is possible. The protection of the human life and safety play a significant role in our regulations as well.
Since the air heating or the solar air heating is not common in Hungary, the standards contain no specific regulations or rules for these structures. Our developing task was to increase the fire protection safety of the Hungarian OM-Solar houses to the level of the ordinary houses, which have no air heating or solar air collector system. Active heat and smoke ventilation is required according to the Hungarian standards only in case of halls (at least 3,60 m headroom, one level building without attic) with more than 1.600 m 2 floor area. Automatic fire detection and automatic extinguishing systems are prescribed by the official fire department, so the need of active fire protection system is not defined by the standards. 3.2. Fire protection characters The fire risk and the hazard of fire spread is higher in case of air heated solar houses than the ordinary ones because of the followings: - The most widespread plastic, timber and timber based combustible materials have an ignition temperature of 260-350 C. Some bituminous materials and synthetic adhesives can have even lower ignition temperature than 200 C. In the air collector, the air temperature can reach 80-100 C in summer condition. This can have a result, that lower ignition energy is enough to burn a fire at the combustible materials of the collector when it works. - Air collector and the ventilation system can play a high role in widespreading of an accidental fire in the air heated houses. The rooms are connected with air ducts, which can spread the fire, because the air is channelled artificially and because of the different density of the air coming from the different temperatures. - Combustible air ducts and thermal insulations can accelerate the fire spread as well. - In case of an accidental fire, heat and the smoke can spread even faster than the fire. This can cause serious hazard of life for the inhabitants since the smoke coming from the different combustible materials is toxic. 4. FIRE PROTECTION TOOLS According to the chapter 3.2. mentioned unfavourable fire protection characters of the air heated solar houses, both passive and active fire protection design principles must be applied in order to reduce the hazards of air heated solar houses. 4.1. Passive fire protection Technical solutions which reduces the fire risk are used to call passive fire protection tools. This must have a principal role according to the Hungarian fire protection standards. The most dangerous elements of the OM-Solar systems are the air collector and the air ducts, which can contain combustible materials. The air collector is situated on the roof, which is a timber construction in Hungary. The air ducts are partly made together with the non-combustible reinforced concrete floor slabs (main ducts), partly plastic or aluminium pipes. The concrete air ducts are insulated in their internal side, in order to reduce their thermal mass and prevent the warm air from unfavourable heat losses. Non - combustible duct system and thermal insulation must be used therefore by the followings: - The use of any plastic based thermal insulation material inside the main air ducts are not recommended as polystyrene or polyurethane foam etc. Mineral wools cannot be applied because the small wool pieces can cause allergic diseases or cancer. So non-combustible thermal insulation is necessary, for instance pressed mineral wool stabilised by cement or any non-combustible adhesive. The necessary thermal resistance of the internal insulation is to separate the warm air from the loadbearing structures with high thermal inertia is 0,15 m 2 K/W, so the thickness is usually 1,0-2,0 cm depending upon the material). - Non-combustible ducts are recommended as well. There are no regulations, but a given fire resistance capacity must be ensured. Non-combustible and non-toxic material is necessary, as the above mentioned pressed mineral wool stabilised by cement or any non-combustible adhesive. These boards can have a fire resistance capacity of 30-90
minutes depending upon their thickness which can be between 1,0-10,0 cm. 4.2. Active fire protection After the fire broke out, really efficient extinguishing and evacuation is possible only during the first 10 minutes of the fire. After 10 minutes, the fire spread will be so fast (after the so-called flash-over), that even if the fire will be extinguished, large losses or human deaths will be the result. In Hungarian conditions, after the fire alarm, the official fire department must reach the burning house in 10-30 minutes, in case of the capital, Budapest, in 10 minutes. Therefore a quick and a reliable fire alarm is extremely important, especially in case of air heated solar houses, where the fire spread hazard is increased. 4.2.1. Automatic fire detection and alarm system The best solution is an automatic fire detection and alarm system with automatic connection to the local fire department. Since most of the solar air heated houses are residential buildings, selection and the positioning of the detectors is very important. In residential houses the smoke of cigarettes and the damp in the kitchen can result false alarms. The artificial ventilation generates slow air movements in the rooms so the smoke can miss the detector situated on the ceiling, so the fast fire alarm will not be ensured. There are two consequences of the above mentioned facts: - The type of the fire detectors are very important. Heat detectors or heat raise detectors are not favourable because the detection time is not quick enough. Smoke detectors (optical or ionisation) are better, but the sensitivity of the detectors must be adjusted to the residential circumstances (cigarette smoke, damp). - Detectors must be put in the ventilation system as well in order to reduce the alarm delays. The best positions are the OM-Solar Handling Box and the air regulation units in the basement, because the air is channelled towards them, therefore the density of the smoke here will be first so large that the detectors can indicate it. Reliable automatic fire detection and alarm systems require a regular and documentated official supervision with the correction of the eventual mistakes in the system. 4.2.2. Heat and smoke ventilation Heat and smoke ventilation have the following advantages in case of an accidental fire: - reduces the heat accumulation and the fire spread velocity, - reduces the heat load of the loadbearing structures, increasing their fire resistance capacity; - reduces the amount of toxic gases and smoke in the rooms, so the evacuation can be longer and safer. Heat and smoke ventilation is not required in case of any kind of residential houses. Heat and smoke ventilation is more important at multy storey residential buildings, sport halls, schools and other public buildings. The use of the OM - Solar air heating system as a mechanical heat and smoke ventilation system can be solved quite easily because of the presence of axial fan and the air ducts. Heat and smoke ventilation must be activated automatically, so in this case an automatic fire detection and alarm system is necessary. When the fire detection system alarms, it must switch on the axial fan in the OM-Solar handling box, independently of its momentary situation. The checkdampers in the OM-Solar Handling box must be in 'recirculation' and 'north exhaust' state: the heat and smoke lets in the system through the recirculation valve and transferred out through the north exhaust outlet. In order to decrease the inlet of the fresh air in the house, closing or tightening of the buried pipe system is also necessary. By this method, the heat and smoke ventilation will be ensured by mechanical way, but fresh air supply must be ensured as well in order to prevent that the air pressure drop inside the house, because this can stop even the heat and smoke ventilation. Too much fresh air is not favourable because this can spread the fire. The use of the OM-Solar system as a heat and smoke ventilation have the following conditions:
- From the recirculation valve to the north exhaust outlet, the system must contain only noncombustible materials (air duct, thermal insulation). - The parts of the systems must have a fire resistance capacity of 30, 60, 90 minutes adjusted properly to the estimated fire termin. - There cannot be a combustible roofing material around the north exhaust outlet, such as bituminous shingles. - The axial fan must carry a heat load of 400 or 600 C for the given fire term, according to the type and the fire load of the building. - The axial fan must have a significantly larger air transfer performance than normally required, because the heat and smoke ventilation demands much more air transfer capacity. The change between the 'normal' and the 'fire' speed of the fan must be switched off and on automatically. - The exhausted heat and smoke coming from the north exhaust duct cannot set on fire any another building or cannot cause a human life hazard. 4.2.3. Automatic fire extinguishing installation The installation of the automatic fire extinguishing systems are common in case of computer server rooms or central command rooms. It must be taken into consideration, that the fire will spread through the air ducts, so the extinguishing material must get in the air channels and ducts as well. The automatic fire extinguishing systems can be activated by the fire detection and alarm system as well. Very important to adjust the different active fire protection systems such as fire detection and alarm system, fire extinguishing system and heat and smoke ventilation system perfectly together. 4.3. The expected operation of the system In case of an accidental fire, the smoke must first reach the OM-Solar Handling box through the recirculation valve or the air regulating unit in the basement. The automatic fire detection system generates an alarm procedure with the followings: - automatic alarm for the local Fire Department; - activates automatically the heat and smoke ventilation function of the system; - activates - if it is installed - the automatic fire extinguishing system. With this, the evacuation can be started in time and Fire Department can start the extinguishing even in the critical 10-minute phase of the fire (before the flash-over). The human life hazard coming from the fire can be drastically reduced, and the lower heat load of the loadbearing structures can prevent the building from the large losses or collapsing. None of the Hungarian OM-Solar houses were tested in real fire conditions yet. 5. CONCLUSIONS The fire protection of the air heated OM-Solar houses can be provided by active and passive tools together. Passive tools are the use of noncombustible materials in case of the air ducts and thermal insulation. Active tools: automatic fire detection and alarm system, heat and smoke ventilation, automatic extinguishing systems. The optimal fire protection can be ensured with both passive and active tools. With this, the level of the fire protection can reach that of those houses which have ordinary air heating systems.
Fig. 1. The principal cross section of the advanced Hungarian OM-Solar houses