WEISS PRODUCT FAQ's

Duct Size


Duct sizes should generally be kept as large as possible a small difference in diameter makes a huge difference to the amount of restriction, as the back pressure from friction losses is inversely proportional to the power of 4. This means that halving the size of the duct increase the pressure lose by 32 times!

At the very minimum, the duct size should be at least a large as the size of the fan, i.e. a 150mm fan uses 150mm ducting. This is more important for axial fans, where it is the blade tips that do all the work. If the duct is smaller than the fan inlet, then or is not in line, then effectively the fan is unable to get any air to the fan tips, so it is only the inner section doing any work. If the exhaust duct is smaller than the fan outlet, then the air is likely to suffer from turbulence, and hence is less efficient.

If the fan is in a wet area (eg shower), then you will need a 12v fan with an isolating transformer. (Weiss FV140)