How to understand roof ventilators

by Isa Stralian

How to understand roof ventilators beginning with the so called wind driven Whirly, Whizbanger, Spinner, generic rotary ventilator fabricated in aluminium, some in plastic.
Some rotate on bushes, some rotate on plastic bearings, some rotate on steel bearings…..and either way they are going to rotate easily because there is little mass (weight) to them.

The intended function of this type of roof ventilator is to create a negative pressure in the roof by drawing out a volume of air proportional to the rotating speed of the rotor (the spinning part)
I say intended because many are not able to achieve the performance expected.

In order for the rotor to be effective, the distance between the rotor and the spigot cannot be any more than 2mm, 3mm maximum.
If it’s greater then the air that would normally be drawn up from the roof by the rotor is going to short circuit and drawn in through the gap between rotor and spigot, thereby drastically reducing the potential of the rotor.

Turbine ventilator performance
turbine vent performance

So there you are, in the hardware store, you spin the rotor of the roof ventilator on display. It doesn’t occur to you that the potential of what you are looking at is in reality half, because the surface area of the other half has got pressure against it.
Air can only escape from the area on the negative, or non pressure, side.


The best rotary ventilator in Australia, circa 1970, was the Western Rotary which was made in steel, rotated on lubricated bushes and could suck like a hungry goat.
This unit was last seen some 35 years ago and replaced by the relatively inefficient, highly marketed aluminium product as seen today.

We have all manner of pressure responsive roof ventilators, made and sold under many names and referred to as a ‘passive’, ‘static’, motionless and anything that implied that it was less dynamic than the zip zap whizbanger.
Whereas in fact there ‘passive”, ‘static’ roof ventilators operate consistently better than their ‘rotating’ cousins.

Passive explained

The performance of the ‘passive’ type of roof ventilator is governed by heat and pressure and the flow rate is proportional to the ‘free air’ area of the ventilator.
The roof ventilator size may be a x c in size but the flow potential may well be 1/3 of that, and just like it’s cousin, it’s efficiency is governed by half of it’s presence.

The variations encountered by this type of roof ventilator are far greater and what makes them more attractive to the architect and homeowner alike, is their benign appearance, as opposed to something akin to a child on the roof waving it’s arms at you.


Cupola style roof ventilator
Early designs used to ventilate the roof / attic area of a building

Then last but not least you have the Cupola style, as seen on many a gracious residence.

This style of roof ventilation was the precursor to those seen today, whereas the presence of the Cupola design was regarded as the finer touch of the ‘crown’ of the residence.


Today the Condor Cupola is used as a services hub to where all exhaust ducting is terminated, as well as performing the task of venting the roof of heat and keeping the dwelling cool.

Mounted centrally and straddled across two or more planes on the roof although is more functional than ornate, as those of the past historical roofs in architecture.
This particular unit has an optional air flow controller inserted in the throat of the Cuplola.
This is to address the flow rate, or to close off completely, in house designs where the roof area is used as a heat exchanger.

In conclusion

The question also arises as what size is required to service your roof area.

Condor Kinetic

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