# Duct Size Question

• posted on January 30, 2009, 3:04 pm
I need to extract 247 cubic metres per hour from my bathroom. I will be using a 150mm (6") ceiling fan capable of extracting 250 m3/ph (Solar and Palau Silent 300). I have a 1.5 metre run. Currently I have 100mm (4") circular flexible hose already installed. Will this existing hose be sufficient for the job? If it is I intend to use a 150mm – 100mm reducing spigot. I have read that 100mm is sufficient for up to 250m3/ph but i have not been able to confirm this.
Increasing the diameter of the hose will tricky because of the configuration of the void space through which it will travel. The wider the diameter the bigger the problem. If I go larger than 100mm hose I will need to use rectangular flexible hose. The ideal option (180mm x 90mm) is not available as rectangular flexible hose. It seems that the only rectangular flexible hose options are options available are (150mm x 70mm) or (204mm x 60mm). Assuming the existing hose i have is not sufficent, is the 150mm x 70mm enough or do i need to use the 204mm x 60mm.
Many thanks for you help

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• posted on January 31, 2009, 12:25 am
On Jan 30, 3:04 pm, snipped-for-privacy@gmail.com wrote:

The 100mm flexi hose at 1.5 meters will offer a lowest resistance of 25Pa if fully extended with no bends. The transition 150 to 100 might offer another 5Pa (off the top of my head). So you fan would have to be capable of at least overcoming 30Pa. Grills and diffusers worst case might be another 20Pa each.
The 150mm x 70mm will amount to precisely the same thing.
The 204 x 60 would have a pressure drop 80% that of the 100mm flexi. But transitions and grills etc remain.

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• posted on January 31, 2009, 3:55 pm
On Jan 31, 12:25 am, "Marc O'Brien - ACRTC"

Actually, looking at my recent copy of the NEBB TAB book it lists spiral duct roughness as 3mm and not as 0.3mm presented to me a few years back in a suppliers catalogue.
Changing my software to reflect this I instead no calculate that 250m³/ hr through 100mm spiral over 1.5m will have a pressure drop of 72Pa
So your fan will have to be able to do 250m³/hr against at least 100Pa external static.