3"...">
276°
Posted 20 hours ago

POWERTEC 70145 4-Inch to 3-Inch Reducer

£7.675£15.35Clearance
ZTS2023's avatar
Shared by
ZTS2023
Joined in 2023
82
63

About this deal

For 3 different "axial" (propellor-type) fans, power did not change at all with restriction - neither more nor less. While that might negatively affect a marginal design due to inadequate cooling with a severely restricted airflow, a mere 4"->3" reduction would be unlikely to have that effect, and many bathroom fans are, in fact, one of the other types I tested. The idea that there's "more load (or strain) on the motor" is, however, false, as measured by the power consumption of the motor. Less mass (and volume) of air is moved, and in physics terms that means less work (physics definition of work) is done. A hypothesis I have not tested is that the axial fans are likely moving the same mass of air because it's circulating around the tips of the fan blades when a restriction is applied, so they are moving it in circles (and will also be heating it up.)

For a "mixed flow axial" fan (radon fan, specifically) unrestricted 4" pipe 18W, restricted down to 2" 16W. The less smaller duct you have, the better - so run 4" as far as possible before reducing to 3" for the wall penetration - that reduces the extent of the friction loss from smaller duct, and thus reduces the performance lost.

Since this misconception that restricting fans puts more load on the motor comes up rather frequently, I made some tests.

Asda Great Deal

Free UK shipping. 15 day free returns.
Community Updates
*So you can easily identify outgoing links on our site, we've marked them with an "*" symbol. Links on our site are monetised, but this never affects which deals get posted. Find more info in our FAQs and About Us page.
New Comment