Ducting and Accessories
Importance of Right Ducting
The importance of using the proper ducting and grilles can have a big effect on the flow rate of a fan. No matter how good the flow rate of the fan you have installed is rated, you can reduce the fans flow rate very easily with poorly design or installed ducting. It is all about installing the right fan with the correctly sized ducting and a very low resistive external grille. Many people are concerned about the long distance the ducting has to be installed to reach the outside. But ducting in a straight line has a very low resistance and it is the number of bends and the exterior grille that will cause the greatest resistance. The resistance of the duct work can be halved by increasing the duct diameter. Instead of installing a larger fan to double the air flow which will only quadruple the resistance.
All fans have a power curve which will tell you how they will perform with different duct work and the expected final air flow rate. It also depends on how much air you are trying to expel into the atmosphere. Constant running fans operate at a low rate and can sometimes cope with resistance better that an intermittent fan which is trying to expel a far larger volume of air over a short period of time.
Uninsulated duct work in a cold attic space will usually cause moisture to condensate in the ducting and if there is loose flexible ducting installed this will allow the water to pool and eventually block the fan. All ducting installed in a cold space should be installed with 25mm duct wrap which is made of fiber glass and has a foil backing. Constant running fans will also reduce this problem as the constant flow of warm air from the house keeps the duct work warm and helps stop condensation. If the ducting consists solely of flexible ducting, then you should really consider replacing it and installing new ducting.
We have a large range of ducting accessories available and we are here to help you choose the right products when purchasing a new fan. We also have premade lengths of ducting made specially to go out through soffit boards along with prewrapped insulation. This ducting goes from 100mm solid ducting to solid and flexible 204 x 60 ducting which is the equivalent of 125mm round ducting. This allows the installer to go between the wall plate and roofing felt while reducing the resistance going through the soffit board. Many installers use just 100mm flexible ducting and squash it down in size to go between the wall plate and the roofing felt. This greatly increases the resistance of the ducting and hence the flow rate of the fan.
If you require any ducting accessories while purchasing a fan please call us for advice. We stock a large range of ducting products from Verplas. Below is their brochure which will give you a range of there ducting products but also the static pressure drop in pascals at different flow rates. Many fans can only cope with 10 to 20 pascals of a pressure drop.