In order to smooth the ambient air convection system exhaust volume = fresh air. 1, the volume of building space size: Residential Indoor People's breathing activity with a vertical height of 2.4m; commercial buildings wife's breathing activity with a vertical height of 4.5m, accounting for the entire volume of breathing space with a ratio of about 0.26-0.47. 2, residential, office buildings which fresh air is not less than 30m3 / hp. Considering the number and minimum fresh air ventilation to two factors, whichever calculation of the maximum fresh air as selection basis. 3, stadiums, conference hall, theater, etc., according to the number of ventilators combined attendance really determine the selection of fresh air, people staying three hours or less, 50% of attendance to determine fresh air. 4, shopping malls wait for central air conditioning system space, central air-conditioning system by 30 percent the total air volume of fresh air is determined for selection. 5, factories, workshops and other toxic and harmful substances disseminated place, according to the dilution of the air volume required to determine the fresh air, with the number of ventilators for selection central air system can solve the problem: (1) residential per capita fresh air can not reach health standards. (2) newly renovated house for a long time did not dare stay. (3) move into a new, pregnant women can produce discomfort, fatigue and other strange symptoms. (4) house too stuffy closed window, window noise, dust and heavy; and result in increased energy consumption of air conditioning. (5) home, kids, elderly loved by secondhand smoke poison. (6) indoor furniture moldy, damp walls, and microbial growth. (7) deal with the haze of the health threat. (8) the recovery of polluted indoor air exhaust away energy (cold or heat - heat recovery units).