This paper is describing the first results of the Annex 27 work aiming at developing simplified tools for evaluating domestic ventilation systems by using sophisticated simulation programs studying pollutant concentration either for each person or in an individual room. Assumptions based on previous research works are set up for a number of parameters. The total number of combinations are about 17500 and have been reduced to 174 by the statistical method "fractional factorial analysis". With this reduction it is possible to make all the runs even with sophisticated multicell models. The result is presented as coefficients for the various pollutants.