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You are here:- home > cooling index > fan index > using extractor fans to remove hot air

Using extractor fans to remove hot air from roof spacesroof extract fan

Warm air rises because it is lighter than cool air. In commercial buildings such as factories and workshops, the warm air generated by machinery and workers rises into the roof space where it is trapped. The sun heats the air in the roof space to over 60C (140F); in turn this superheated air heats both the structure of the building and the cooler air below.

The most effective way to lower the temperature inside buildings is to fit extractor fans to expel the superheated air from the roof space.

What size fan is needed?

To calculate the size of extractor fan, work out the area of the roof and multiply by 0.7. If the outside surface of the roof is dark coloured (dark colours absorb the suns heat very efficiently) add 15%:

Airflow of extractor fan (CFM) = length of roof (ft.) x width of roof (ft.) x 0.7

If the roof is dark:   Airflow of extractor fan (CFM) = Airflow of extractor fan (CFM) x 1.15

Air inlets

To allow fresh cooler air to enter the roof space, air inlets must be installed. To calculate their size in square feet, multiply the airflow of the extractor fan by 0.0033

Area of air inlets (sq.ft.) = airflow x 0.0033

To create air movement through the roof space the air inlets should be installed at the cooler North end of the building, with the extract fans at the South end.

Controlling the extractor fan

The simplest way to control the extractor fan is to install a thermostat in the roof space set at 40C (104F). Once the temperature exceeds this, the fan will start, removing the hot air.

Free early morning cooling

An often overlooked additional advantage of extractor fans is their ability to provide ‘free’ early morning cooling.

By connecting a timeswitch to the extractor fan, cold early morning air is drawn into the building. This cold air reduces the air temperature inside and cools the fabric of the building. The benefits of early morning cooling in terms of reduced temperatures can be felt well into the following morning.

The settings for the timeswitch vary from building to building. Typically it should be set to operate between 5 and 7am when the morning air is at its coldest.

Cost effective cooling

Fitting extractor fans to remove the hot air that gathers in the roof space and to provide early morning cooling, is the most cost effective way to reduce the temperature inside commercial buildings.

Activair roof extractor fans are available in 7 sizes, from 315mm to 630mm diameter (7085CFM). They are efficient, durable, high quality units, which are quick and easy to install.


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