Our Quick Guide contains more than enough information for most people but here we will go into a little more depth about ducting for those who really want to know more!

Extraction Power & Ducting sizes

When choosing your cooker hood you ideally need it to be able to move the air in your kitchen 12 times an hour though you should aim for at least 8.

A good formula to follow in order to help you calculate the power that you need is to multiply the volume of your room by 10. (Volume = Length x Width x Height). An average room is approximately 25mˆ3, therefore the extraction rate required for the average room is 250mˆ3/h.

Different size hoods will have different size outlets depending on how powerful they are. It is also worth noting that if you wish to travel further than 3 meters then it would be best to convert to the size up.

We would say our ducting is for the following outputs

  • 100mm (4") - Up to 250mˆ3/h - Suitable for low powered cooker hoods and tumble dryers on short, simple runs.
  • 125mm (5") - Up to 500mˆ3/h - Suitable for medium powered cooker hoods, efficient on long runs.
  • 150mm (6") - 500mˆ3/h & Above - Suitable for high powered hoods, most efficient ducting.

More info on ducting and recycling

Generally with all cooker hoods you have a choice, to duct to an exterior wall OR to use carbon filters that recirculate the air. Ducting is ALWAYS the preferable option. It is only recommend if ducting out is IMPOSSIBLE. Most hoods will usually have a grease filter fitted on the front usually as a metal grease filter that can be washed, this is regardless of whether the hood is vented or recirculating.

When using a hood as a recycling model, air is drawn into the hood, fats are trapped in the grease filter, the air then passes through the charcoal filters to help absorb cooking odours, the cleaned air is then recycled back into the kitchen.

It is realistic to assume that using a hood on recycling mode will circulate at least 30% less air than by ducting out – this is because the charcoal filter will restrict a certain amount of airflow. With almost all hoods charcoal filters are an optional extra and do cost, the price can range from anything from £5 to over £100. Manufacturers also recommend that they are changed every 6 months or earlier with frequent use. These charcoal filters are NOT required if you are ducting out.

Recirculating with charcoal filters will reduce cooking odours but will NOT reduce steam and condensation. If you wish to reduce odours, steam and condensation, then you MUST duct out to expel these out of your home.


Try to plan your hood and cooker onto an outside wall in the kitchen where possible. By doing this you will minimise the length of the ducting and maximise the hood performance. Hoods over a griddle or chef top

Choosing and Installing the Correct Ducting

A ducting system is often not considered an important part of the construction or refurbishment of a building. However, the importance of the correct ducting is becoming recognised by the building industry. It is not always the fault of the cooker hood manufacturer when dreams of fresh rooms and low noise fans are not achieved. It can often be that the wrong size or configuration of ducting has been selected by the installer or system designer. As it cannot be seen, air movement is often made more complicated than it actually is. Air moves just as water moves and it is sometimes useful to visualise a problem by substituting air with water.

In ventilation installations the aim is normally to move the air from its source to the outside of the house as efficiently and quietly as possible. It is, therefore, common sense that to do this effectively, the shorter the distance the air has to travel, the quicker the aim is achieved. It is also common sense that if an obstacle is put in the way, such as a bend, the process will become more difficult as the air will lose its momentum and may require more powerful assistance in order to achieve the aim. It is worth remembering that many cooker hood manufacturers’ guarantees will be invalidated if the correct size or an inferior specification of ducting is not used to match the power of the hood.


Cooker hoods can be noisy, especially when using charcoal filters, but this noise doesn't come from the motor it is actually the noise of the air moving. Therefore the simpler the ducting runs (less turns, shorter runs with round ducting) the less noise it makes.

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A few key installation guidelines can be applied to help ensure that the ducting selection is correct and installed in a way that optimises the extractor’s performance.

  1. Choose the shortest and most direct route to the outside or the point where the air is to be released.
  2. Ensure that bends are kept to a minimum.
  3. If the duct system is boxed in or completely inaccessible for maintenance, it is recommended that all joints are secured, using Ventilation Duct Sealant, for increased peace of mind.
  4. Check the extraction capability and connection size of the fan and choose an appropriate size duct.
  5. Air travels better through a round duct, as the air will travel at a lower speed, resulting in less turbulence, vibration, noise and energy loss. However, the installation space available may prevent this ideal.
  6. A rigid duct is more efficient than a flexible duct and should be used when possible.
  7. If flexible hose is used, the hose should remain taut and as straight as possible.
  8. Special consideration should also be given to the selection of the wall terminal and the use of back draft dampers, as these also increase air resistance.
  9. In practice, the actual installation may mean that all the above points cannot be adhered to, but Polypipe Ventilation has the products to provide the best solution.