Fans
Fans used for ventilation and air movement work by one of two different methods. The most common is the axial or propeller fan, similar to those used on stands for moving air in a room or used in computers for cooling power supplies. They can move large volumes of air at very low pressures.
The other type is a centrifugal unit, and these are similar to a centrifugal water pump. They have a plate with radial vanes and the air is forced out by the centrifugal force. These are either an in- line fan, where the air goes straight through parallel to the fan shaft, or the other type is the "squirrel cage" fan where air is drawn into the centre of the fan and exhausted through the side casing, perpendicular to the impeller axis. The latter is rarely used in domestic fans.
Axial fans are the most common, as they are the cheapest to produce and more compact. One disadvantage they have is that they cannot develop high pressures, with a limit around 60Pa-80pa for household sizes.
Centrifugal fans with the same size inlets as axial fans can develop pressures up to 4 times that of an equivalent axial fan, however, they also usually cost a minimum of twice that of an equivalent axial fan. Fans can either be surface mounted or in-duct, where it is in the middle of a duct run.

Fan Curves On a few fans, we show a graph of airflow vs static pressure (back pressure, sort of). This means that if you have a certain pressure that the fan is pushing or pulling against, then you know the likely airflow rate.

Ducting The usual materials for ducting are either a steel spiral sheetmetal rigid pipe or a flexible PVC or aluminium foil tube supported by spiral steel wire. The latter is easiest to install and is cheap, however it is very restrictive to airflow. The steel pipe requires special tools to be installed properly, which most homeowners or plumbers do not have. The most common fault is to chose ducting which is undersized for the fan capacity, and this increases the noise of the system through the higher air velocity and also increases the friction in the system, which makes the fans work harder or require them to be larger than necessary.
Duct sizes should generally be kept as large as possible – a small difference in diameter makes a huge difference to the amount of restriction, as the back pressure from friction losses is inversely proportional to the power of 4.

Duct sizing
Let us take the example of an inline fan in the ceiling: If we assume that the peak pressure of the axial fans in a heat and light fan units is around 60Pa and a peak flow rate of 70l/s in free air with a more or less straight to circular curve between the points (see below). (or maybe from the manufacturers data) We also make some other assumptions: The exhaust grille (and intake if it has one) is 10-20Pa pressure loss.
The correctly sized ducting will be 1.5-3.5pa per metre
Each bend of flexiduct is equivalent to 1.5 -2 metres extra duct length
We have 2 bends, an exhaust grille, and an intake grille, as it is part of the fan and hence part of its quoted performance.
The duct length is 3m.
Our pressure loss is 15pa for each grille + ((3m+2*1.5m per bend)*2 Pa per metre) = 30+ 12 =42Pa.
We know look up our fan curve for 42Pa static pressure and read off the amount of air being moved from the bottom axis.
At the very minimum, the duct size should be at least a large as the size of the fan, i.e. a 150mm fan uses 150mm ducting.

Noise Ratings
Most Sound levels are rated in decibels, measured at 3 meters. Be aware that some fans give noise ratings in SOUND POWER LEVELS, also in decibels, which is a different measure and is normally about 21 decibels higher than the Sound Pressure @3m measurement.
An important thing to remember is that they are logarithmic, which means that they do not go up in a linear (straight-line) fashion, rather it is exponential. This means that a (more or less) 10db increase is twice as loud. A 2-3 dB difference is considered to be a just noticeable to most people, although this varies.

The search for the silent/quiet extractor fan.
Many people want to upgrade their current extractor fan with a quiet version, as most cheap bathroom fans are quite noisy. There are a number of options available.
Firstly, it depends on the duct length. If its only a matter of 3-5 meters, the TD range of mixvent fans will suffice. These are a half way house between an axial fan( suitable for short duct lengths) and a centrifugal fan (suitable for long ducting lengths) and and very quiet. Examine the dB readings on the datasheet for comparison with others.
If a more powerful extraction is required, a centrifugal fan is a must. In this case, our range of Acoustic fans are as quiet as you can get. They have sound insulation inside the box to absorb noise. They are also very powerful and easily cleaned and suitable for domestic/industrial applications where sound levels are a problem.
To absorb any noise from the rushing of air, rather than the fan chassis breakout noise itself, use acoustic ducting and/or a silencer at the input/ output. These are just hollow tubes, acoustically designed to absorb the white noise you get at the end of ducting.
Using one of these solutions will normally solve any noise problem.

Click HERE to download the Governments Ventilation Building Regs.

Generally, the noise is also proportional to the amount of airflow and the fan speed, If you have problems with a noisy fan or are putting a system in:
1/ Mount the fan towards the end of the duct
2/ Use larger ducting to reduce the air velocity, and hence the noise through fittings and grilles
3/ Use a larger fan but turn it slower. This can make a big difference, as the noise varies with the square of the fan tip speed –
4/ Try using "acoustic ducting".

Air changes per hour reqired. (Guide only)

Bakery	20 - 30
Canteen	8 - 12
Commercial kitchen	30+
Workshop	8 - 10
Fume cupboard	40 - 50
Garage	6 - 8
Spray booth	20 - 50
Warehouse	3 - 6