Installation of optical cables in ducts by blowing (with air) or by floating (with water) are widely used and well known techniques. However, some mysteries remain, e.g. what is the effect on the installation length when a (HDPE) duct is hanging, fixed with regular intervals (spans), and how does this depend on the span? In this paper an analysis is made. It is found that for constant temperature the blowing length is hardly affected for a span of 2 m or less (for 32/26 mm duct, span scaling proportionally to duct diameter) and when the duct is hung under tension this is also true for larger spans. But, when the duct is in an environment with varying temperatures (e.g. under bridges) a span of 1 m can already be critical (blowing length drops by ~1/3rd). The initial tension only helps a bit here. Creep of the duct and cable sag also have an effect and are treated qualitatively. Tests are proposed to check all details. Floating the cable helps to reach longer installation lengths, also because it may be used to cool the duct.