How good is a Roof Ventilation Fan?

by Isa Stralian

The idea of a roof ventilation fan in the roof for the purpose of venting the heat out is a seemingly attractive one.

Unfortunately electric motors in roof ventilator fans burn out quickly when operating in high heat conditions.
The average roof space temperature is around 68 degrees. ..
So one question is, how long is the fan going to last?…..
High end manufacturers such as EBM Papst and Zeihl Abegg do not recommend their fans be used in conditions above 60 degrees.
Whether 240vac or 12vdc solar powered, the life expectancy is the same…short.

So let’s go back to the beginning,
You are being presented with a product, ‘a fan sucking the heat out’…… fantastic…modern day roof ventilation……
The fan ‘sucks’ the heat out of the roof, (I say ‘sucks’ as it’s the term the sales people use when invoking a sense of positive high level extraction).
Ambient air is drawn in as a consequence, ideally via the eaves, to replace that which has been removed by the fan.

And that’s not all…who is going to maintain, repair or replace the fan?

Sounds wonderful

In principle, air the ingress points, usually the eaves, are ideally furtherest away from the fan, so as to air wash the distance in between ingress and egress.
The ingress or eaves vents themselves need to have minimal resistance, the efficiency of which will affect fan performance.

So who is going to supply these low resistance eaves vents.
To begin with, apart from the labeling, eaves vents need to be minimal resistance so as to mach the performance of the fan.
The term used is ‘free air’


Let’s take an eaves vent, say 300 x 300.
The vent has screening across it’s face, resistance of which drops the eaves vent performance by 50%.
Then looking further, the vent has a baffle behind it which drops the flow efficiency by another 40% of the original, leaving 10% efficiency over all.

You started with 300 x 300mm = 900sq/cm and you’re left with a free air area of 90sq/cm
That’s a lot of eaves vents you’re going to need.
Furthermore they are in plastic so removing and replacing will cause breakage.
When you factor the dust factor for every c/m of air that goes through the eaves vent you’re going to spend weekends cleaning them.

Ventilating a skillion roof in this manner, where the distance between ceiling and roof lining being less than 300mm, the exercise would be pointless.


The prevailing wisdom would seem to be against using fans for venting the heat out of the roof, particularly during summer months as regards to operational life and efficiency of the fan.

One Alternative

Of course there is nothing to prevent you from using a fan(s) for air ingress (via eaves) into the roof space as the ambient air flowing over the motor is never going to be critical.

In all cases whatever the roof ventilator it needs to be able to cope with the flow required to ventilate the roof efficiently.
The duty of the fan needs to be proportional as a pressurized roof under these circumstances is something to avoid.

Then there is the noise to consider……

Condor Kinetic

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