Hello and need advice again,
I have 3" conduit for bathroom ventilation and would like to increase the speed that the humidity is cleared from bathroom after a .shower. How do I do this ? Thanks for helping!
There are many factors that affect velocity of air flow, change any of the parameters and you will change the rate of flow.
1 - Duct size, larger will carry more air, however, you need a fan that can push it.
2 - Duct length, the longer the run, the slower the air will move.
3 - Duct type, smooth surfaces allow air to flow faster, with less turbulence, rippled surfaces slow flow and increase turbulence.
4 - How straight is the duct, straight = fast, corners = slow.
Then you get to fan parameters, what is the size and type of fan, what is the speed in which it turns? Small = slow, larger = fast. blade fans are not as efficient as squirrel cage fans.
Dirt build up affects fan efficiency as well. In a bathroom with all that moisture, dust can collect on the inner surfaces of the fan and the ducting, slowing air flow.
Cubic volume of the space versus the maximum cubic volume capacity of the fan, the larger the space, the longer it will take a fan to exchange the air within it. Make sure your fan is sized correctly, too large is more detrimental than too small because it can affect exhaust drafting of gas powered appliances, creating a toxic environment for you to be breathing in.
Most exhaust fans now have 4", some 6" ducts. Just increasing the duct won't help, you need a bigger fan. The big box stores have fan displays and recommended exhaust fan sizes.
On yours, make sure the end cap and damper built into the fan work properly.
Too long a duct run increases the air friction and lowers the efficiency. A larger fan may be needed to overcome this.
Houses often get the smallest, cheapest fan installed when they are built.
Check out the Panasonic lineup of vents.
Add one vote for Panasonic vents
Check for proper sizing first, then when you need to replace the fan (which might be now) go Panasonic.