Invisco is now making the best gravity rooftop turbine ventilator in history – the Tornado. And we are making it in America.  Edmonds in Australia proved this concept with their Hurricane. They ship them all over the world. 

Above is a closeup of the vanes. Air flowing through the Tornado can flow right out through the vanes. And the air flow causes the head to rotate and that action actually accelerates the air flow when the blades start to pull air up. And any breeze will accelerate air flow even more. Here is data from tests showing the effect that even a light breeze has on accelerating air flow:

Model Throat                         CFM in a breeze
    6.8 MPH (Tested) 15 MPH (Estimated)
H300T  12 in 238 525
H600T 24 in 679 1500
H900T 36 in 1681 3700

To put the data above into perspective, note that the 900 would take over 1/4 horsepower to move 3700 CFM with no breeze!

Here are the models introduced in 2017. Other sizes available upon request to

Model/Throat  Vane Height Top Diameter
H300T  12 inch throat  7 in   20 in  
H600T  24 inch  10 in   32 in  
H900T  36 inch 16 in   44 in  

Here is the sealed stainless steel bearing – two are used under the turbine head and allow the shaft to be removed easily for shipment. 

Traditional rooftops like those from Greenheck, PennBarry and TwinCityFan  have flat tops that force the air to go down before it can go up. They do not take advantage of breezes. Below is the LorenCook design – they are proud to show their radiused throat top edge that improved their free air flow slightly.