Ducting Applications

Ducts passing through an unheated roof void should be insulated to minimise condensate formation and must include a condensation trap and means of running off condensate. Vertical ducts should be fitted with a condensation trap and a weather proof cowl of sufficient free area for the air volume. Horizontal ducts should fall away from the fan unit.

Length of Ducting

Axial fans are suitable for use with up to 2 Mts. of ducting

Centrifugal and Mixed Flow fans should be used for ducting runs longer than 2 Mts.

For maximum performance flexible ducting should be installed fully extended and try to eliminate sags and bends and those that remain should have the widest radius possible.

Pressure Loss

It is difficult to be exact as the air friction for Soil Pipe, Solid PVC Ducting, Aluminium and PVC ducting vary considerably as does the back pressure capability of different fans.

Bends and grilles will also increase performance loss

By increasing the size of duct ( i.e. using 5" duct with a 4" fan ) will reduce the performance loss

Power showers: We recommend you use the largest duct and fan you can.

Below is an example of the type of loss you could expect. We have listed details for specific fans on individual data sheets.

Power loss values will be similar for all manufacturers of fans and reflect more on the characteristics of ducting than the performance of the fan

Typical Mixed Flow Fan Output at set distances using Flexible PVC Ducting

Fan Duct dia (mm) 0 Mts. 2 Mts. 3 Mts. 4 Mts. 5 Mts. Max Duct Length
TD160 100 160 118 113 108 104 6
TD250 100 250 163 145 126 108 6
TD350 125 350 260 249 238 228 6

Typical Mixed Flow Fan Output at set distances using PVC Soil Pipe

Fan Duct dia (mm) 0 Mts. 2 Mts. 3 Mts. 4 Mts. 5 Mts. Max Duct Length
TD160 100 160 148 142 136 130 9
TD250 100 250 204 181 158 135 7.5
TD350 125 350 324 311 298 285 9


Ventilation units should be placed as high as possible in the window or wall near any local smells and steam but not directly above eye level grills, cookers or direct sources of heat in excess of +40°C. Do not site in areas containing excessive levels of grease without suitable filters.

In large rooms the extract points should be well and safely distributed to obtain even ventilation in all areas.

A short circuit of airflow will be avoided by siting units as far away as practicable from the main source of air replacement.


Site the fan so that it is accessible for regular periodic cleaning and servicing. Make sure that grilles, motor cooling ports and impellers are able to function freely in accordance with guarantee requirements.

Electrical equipment should be isolated before carrying out any maintenance or cleaning. All fans should be regularly cleaned and checked, (every month or so, dependent on usage). Bearings should be oiled where appropriate twice a year, unless stated otherwise.

Ducted Applications Through Roof Voids

Horizontal Duct

Vertical Duct