Rangehood Ducting
Choosing The Correct Ducting
Nobody wants to see ducting in their homes so it’s always hidden away. This does not mean however, that it’s not important. If your cooker hood isn't vented correctly it probably won't work properly, be inefficient at moving air and be more noisy. The noisier your rangehood the less likely you are to use it, which is bad for your home. For quiet, efficient movement of air from your kitchen to the outside you not only need the correct extractor fan or rangehood but also the right ducting.
In any ventilation installation the aim is normally to move the air from its source to the outside of the building as efficiently and quietly as possible. All ducting causes resistance to air movement by friction so it makes sense to reduce the length of travel and minimise any obstructions, such as bends and back-draft dampers. With a longer the ducting run and more obstacles, the moving air will lose momentum and so require a bigger fan to achieve the required flow.
Extractor fans vary in power from model to model just as cars do and the results are reflected in much the same way. Just as a high powered car engine should be quieter, last longer and generally provide the driver with more comfortable ride, the same can be said of the fan motor. And in the same way a car travels better and more efficiently on a smooth straight road, so it is for the rangehood. A bigger motor will allow the passage of more air with less effort and therefore less fuss and noise so long as the ducting is as smooth and straight as possible.
Recommendations
A rangehood silencer or muffler will reduce noise considerably over normal ducting. Ducting kits are available that incorporate such a muffler, such as Rangehood Tamer.
In summary:
- Choose the shortest and most direct route from the rangehood to the outside where the air exits.
- Ensure that bends and obstructions are kept to a minimum.
- If the duct system is boxed in or completely inaccessible for maintenance, ensure that all joints are properly sealed with duct tape of ventilation sealant.
- Check the extraction capability and connection size of the fan and choose an appropriate size duct. Choose a fan with higher air capacity than you think you need rather than lower capacity.
- Air travels better through a rigid round duct as the air will travel at a lower speed resulting in less turbulence, vibration, noise and energy loss. So use round duct as much as possible.
- Rigid duct is more efficient than flexible duct and should be used whenever possible.
- If flexible hose must be used it should remain as taut and as straight as possible. Be especially careful to ensure that it doesn't interfere with the rangehood back draft flaps as this will reduce airflow, increase noise and strain the fan motor
- Choose the correct size of exit grill or louvre and avoid back draft dampers if possible.
- Use a ducting kit that incorporates a muffler(silencer)
More about ducting kits < Here >
More about Rangehood Tamer < Here >
Rangehood ducting is available online from: ApplianceSmart
The pictures above show how the back-draft flaps of the rangehood operate to prevent air coming back from the outside when the unit is switched off and open to allow air through when it’s switched on. It’s always advisable to check the functioning of these before the ducting is installed to ensure that they installed properly and can open fully to allow air through. To test just switch the rangehood fan on before ducting.
The picture on the left shows clearly how the crinkled flexible duct can prevent the flaps from opening properly. It’s also clear that the flimsy construction and ridged surface of the duct will cause air turbulence, back pressure and noise.
This type of flexible ducting should be avoided at all costs. It’s used far too often because of its cheapness and is too often found to be the cause of poor rangehood performance. Buy a decent rangehood kit from a reputable source to avoid these problems. Remember that cheap can be costly in the long run.
Kitchen ventilation advice for New Zealand kitchens
Copyright Tom McKernan 2017