The Calculation And Analysis Of Fresh Air System Design For Ultra-low Residential Buildings

With the development of passive ultra-low energy buildings, thermal insulation and air tightness performance of the building envelope has been significantly improved and heating load and energy consumption generated by the building envelope are therefore greatly reduced. But the consequent problems, such as the lack of fresh air and high concentration of indoor pollutants that caused by the highly air tightness makes mechanical ventilation system a widely used solution in passive ultra-low residential building air supply problems. Thus increasing demand of fresh air became a great challenge under the heating load limitation of no more than 15 kWh/(m2a) per year and heat recovery devices are becoming an important measurement of cutting down the heating load and reducing building energy consumption. How to determine the system air flow rate under ordinary pressure state according to the building air tightness test results is important fresh air system heat load calculation and system design.Focusing on the problems that mentioned above, this paper takes a fitting analysis on the test results and determines the characteristic curve of the building air tightness.Then a relationship of air infiltration change rate between building air tightness test results and operation status has been obtained, thus a correction of air flow rate calculation is carried out. This paper also takes a sensitive factor analysis on energy efficiency of the heat recovery system. To solve the frosting problem of heat recovery system in cold regions, this paper proposed the preheating temperature operation control strategy with the conception of preheat opening temperature and preheat setting temperature, thus to specify the calculation of preheat load and solve the frosting problems that caused by the underestimate of preheat load. Meanwhile this paper also proposes a method for calculating the correction system loads the actual operation rules to reduce calculation errors.The results show that the building air tightness characteristic curve can represents the capacity of building envelope on reducing air infiltration under various differential pressures. Building air infiltration and its air tightness test results under normal operating conditions are in line with universal linear relationship and thus can be obtained by a corresponding penetration ACHn. The frosting point at exhaust side of heat recovery devices can be calculated via the efficiency of the devices and the outside temperature in cold regions and thus can further help determine whether choose sensible or total heat recovery devices. Since the existing method of fresh air heating load determination is not suitable for high air tightness constructions, the infiltration correction method in this paper can solve this problem. Taking Harbin as example, the penetration load air infiltration of passive ultra-low residential building accounts for 9.3-14.1% of the total load, and an optimized preheat preheating temperature strategy can reduce40% heating load compared to conventional operating mode.