НАУЧНИ ТРУДОВЕ НА РУСЕНСКИЯ УНИВЕРСИТЕТ – 2012, том 51, серия 1.2 - 60 - Modeling of indoor air composition time variation during the night in naturally ventilated occupied spaces Detelin Markov ”Modeling of indoor air composition time variation during the night in naturally ventilated occupied spaces”: presents a closed non-steady state one dimensional mathematical model for evaluation of indoor air composition time variation in occupied spaces with non-organized non-controllable natural ventilation. It presents as well an analysis of the mathematical model by which are identified both the factors and the level of there impact on the variation of system mass and composition Key words: natural ventilation, metabolic CO 2 time variation ”Моделиране на изменението във времето на състава на въздуха през нощта в обитаеми помещения с естествена вентилация”: представя затворен нестационарен едномерен математически модел за определяне на състава на въздуха (газовата смес) в обитаеми помещения с неорганизирана неконтролируема естествена вентилация. Тук е пресдтавен, също така, и анализ на математическия модел, чрез който са идентифицирани факторите и степента на тяхното влияние върху изменението на масата и на състава на газовата смес в помещението. Ключови думи: естествена вентилация, изменение на метаболния СО 2 във времето. INTRODUCTION Tracer gas technique with metabolic CO 2 as a tracer gas is used widely over the last decades for ventilation measurements in occupied spaces [3, 4, 5, 7, 8, 9] even with natural ventilation. Typically its application is based on the CO 2 mass balance equation in volume units r r r r a a r r V V X V V X V G dt dX & & & - + = 3 10 , (1) where X r and X a are CO 2 volume fractions in the room air and outdoor air (ppm), respectively, V r is room air volume (m 3 ), G & is CO 2 generation rate (l/s) at the room conditions, r V & is the volume flow rate at which the indoor air escapes from the room (m 3 /s), and a V & is the volume flow rate at which the outdoor air enters the room (m 3 /s). System mass balance equation is taken implicitly in the form a r V V & & = , (2) In the ASTM E 741-06 standard [1] the tracer gas technique is described with details and the limitations of its applicability are presented and explained. The ASTM D 6245–07 standard [2] presents the conditions under which metabolic CO 2 could be used for ventilation measurements following the ASTM E 741-06 tracer gas technique. There is stated that only the decay technique can be used for ventilation measurements in a single zone, i.e the system exchanges air only with outdoors. Unfortunately the metabolic CO 2 tracer gas technique, based on equations 1 and 2, is applied for ventilation measurements in occupied spaces with natural ventilation even for build-up intervals without analyzing the potential uncertainties, which could affect the result significantly since in naturally ventilated occupied spaces very often 5 . 0 ≤ = r r V V ACH & . MATEMATICAL MODEL Schematic diagram of the system and the surroundings is presented on Figure 1. The indoor air (r) is the system under investigation. The system exchanges mass with the surroundings, i.e. some amount of indoor air escapes to outdoor ( a r m , & ) and/or to the