If the exhaust air takes anti-backflow measures, 6w.

York Air Conditioning Maintenance Q&A: How to set up fire prevention zones in air conditioning systems?

Ventilation and air conditioning systems should be set up horizontally according to each fire prevention zone, and vertically should not exceed 10 floors. When there are measures to prevent backflow on the exhaust ducts and each floor is equipped with an automatic sprinkler fire extinguishing system, the fresh air ducts are not subject to this restriction. Currently, in buildings such as tourist hotels, hospitals, and office buildings, most use fan coil units plus fresh air. The duct cross-section is small. If measures are taken to prevent backflow of the exhaust air, the fresh air duct can vertically cross more than 10 floors.

Methods to prevent backflow: The vertical height of the exhaust branch pipes on each floor can be increased so that the exhaust branch pipes on each floor cross two floors.

Data source: Beijing York Air Conditioning Maintenance Center http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3745369.