Roof Ventilation Fans
The idea of a fan in the roof for the purpose of venting the heat out is a seemingly attractive one.
Note, the fan being used here is a 240v axial or centrifugal fan not an erratic solar whiz bang
There it is, ‘a fan sucking the heat out’…… fantastic…modern day roof ventilation……if only the Romans had them.
The fan sucks the heat out of the roof, (I say sucks as it’s the term the sales people use when creating a negative pressure) and it obviously draws ambient air in, as a consequence to replace that which has been removed, using the path of least resistance.
So ideally, in principle, the ingress points have to be further most distance away from the fan, so as to air wash the distance in between the two points, and the ingress or eaves vents themselves need to have minimal resistance, the efficiency of which is going to affect the fan efficiency.
In the case of a ventilating the roof space of a skillion roof, where the distance between ceiling and roof lining can be as little as 200mm having a dedicated ingress with a fan at the opposite end, there would be some benefit in both relieving the heat gain and offsetting the saturation of the ceiling insulation.
So bearing in mind that the average roof space temperature is around 68 degrees and the heat transfer via conduction of the steel roof will be proportional to the airflow of the fan.
How long is the fan going to last?…..given that manufacturers such as EBM Papst and Zeihl Abegg do not recommend their fans be used in conditions above 60 degrees
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.
Of course there is nothing to prevent you from using a fan(s) for air ingress (eaves) into the roof space as the ambient air flowing over the motor is never going to be critical and as long as the roof ventilator does not create too much resistance so as to cause the fan motor to overheat, then it could be considered an alternative in creating a positive flow in ventilating the heat out of the roof space.
In all cases the roof ventilator needs to be able to cope with the flow required to ventilate the roof efficiently and the duty of the fan needs to be proportional to the release as a pressurized roof under these circumstances is something to avoid.