Ventilation & Ducting Sizes Explained

In depth

Ventilation and ducting sizes are often confusing for homeowners looking to improve air quality or install extractor fans. The sheer number of measurements – square inches, cubic feet, litres per hour (lph) – can be overwhelming, leading many people to buy equipment that’s either too small or, worryingly, far too large for their space. Understanding the terminology and how it relates to your home's dimensions is key to a successful installation and avoiding costly mistakes. It begins with recognising that ducting isn't simply about diameter; it’s fundamentally about airflow volume, which is dictated by the size of the fan itself and the design of the ductwork connecting it.

The most common way ventilation manufacturers describe their products is through lph (litres per hour) ratings. This figure represents the amount of air a fan can move – essentially, how quickly it replaces stale or contaminated air with fresh air from outside. However, lph alone isn’t enough to determine if a particular ducting size will suit your needs. You need to factor in the length and number of bends in the duct run. Every bend introduces resistance to airflow, reducing the effective lph rating. A straight 10m (33ft) run of 100mm (4in) duct will move air at a significantly higher rate than a convoluted, multi-bend system of the same size. Manufacturers often provide ‘equivalent length’ figures, which attempt to account for this by adjusting the lph rating based on the complexity of the ducting layout. Don't just rely on these figures; consider your actual installation when assessing suitability.

When it comes to diameters, you’ll typically find sizes ranging from 80mm (3in) to 150mm (6in). Smaller ducts – 80mm and 110mm (4.3in) – are generally used for extractor fans in kitchens or bathrooms, where the airflow requirements are relatively low. A 110mm fan might be adequate for a small bathroom, but it will struggle to move air effectively through a longer duct run or in a larger kitchen. 125mm (5in) and 150mm (6in) ducts are more commonly found in commercial kitchens or larger domestic installations where higher airflow is needed – perhaps for removing steam from a large cooking area, or for connecting multiple extractors. The larger the duct diameter, the less resistance to airflow there will be, but also bear in mind that larger ducts require significantly more powerful (and therefore often more expensive) fans to achieve the same lph rating.

Measuring your space accurately is crucial before making any purchase. Start with the room you intend to ventilate. Measure the length and width of the room in metres – this will give you the floor area in square metres. Then, consider the ceiling height. A low-ceilinged room will significantly restrict airflow if a large duct runs directly above it. It's far more effective to route ducting along walls or across ceilings where there’s more space. Next, think about the distance between the fan and the point where the air needs to be exhausted – for example, the length of the duct run from your extractor hood to a window or external vent. This is critical when calculating equivalent length adjustments. Finally, factor in any obstructions - furniture, walls, built-in appliances – that could impede the flow of air.

A very common mistake people make is assuming that a larger room automatically requires a larger ducting size. While it’s true that a larger space will generate more heat and potentially require higher airflow, simply increasing the duct diameter won't necessarily improve ventilation performance. In fact, excessively large ducts can actually *reduce* efficiency by creating excessive static pressure, leading to reduced airflow and increased energy consumption for the fan. The correct approach is to accurately assess the room’s volume (length x width x height) and then select a fan with an appropriate lph rating based on that volume – typically around 50-75 litres per hour per cubic metre of space, depending on usage.

To help you determine the right size for your needs, consider these general guidelines. A single extractor fan in a small bathroom (up to 4 square metres) will likely be fine with 125mm ducting. For larger bathrooms or kitchens (over 8 square metres), 150mm ducting is recommended. If you're installing multiple extractors, such as one over a hob and another over a sink, ensure that the ductwork connecting them is appropriately sized to handle the combined airflow. Remember to add extra length for bends and turns – ideally, keep runs as straight as possible to minimise resistance. Finally, always check the manufacturer’s specifications regarding maximum run lengths before purchasing ducting; exceeding these limits will significantly impact performance.