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Frequently Asked Questions
What drives the airflow if there is no fan?
Temperature difference and wind.
Warm air rises and leaves through a high opening.
Cooler air enters low to replace it.
Wind over the roof adds suction.
The building itself does the work.
Nothing consumes energy at any point.
What is given up in exchange?
Control of the rate.
Airflow is produced, not chosen.
It depends on temperature difference and height.
Wind contributes unpredictably.
Still mild days give the weakest driving force.
A gravity system cannot be turned up.
Why does the inlet matter as much as the outlet?
Air leaving must be replaced.
Ridge extraction with no low-level opening does not flow.
The building simply resists.
This is the commonest reason installations disappoint.
Size intake and relief together.
Treat them as one system.
What is free area and why is it the number?
The actual opening air can pass through.
Nominal size implies far more than that.
Louvres, birdscreens and baffles all reduce it.
Manufacturers publish free area separately.
Compare on that figure, never on dimensions.
Screens can halve it.
Which buildings suit gravity ventilation?
Tall ones with a genuine heat load.
Industrial sheds and process buildings.
Height between inlet and outlet drives it.
A low building has little to work with.
Occupancy-only heat loads are marginal.
Match it to the actual heat available.
Can it be combined with powered ventilation?
Commonly, and it is often the best answer.
Gravity handles the base load for nothing.
Fans cover still days and peak conditions.
That reduces both energy and installed fan capacity.
Controls must prevent them fighting.
Design the two together, not separately.
What maintenance does it need?
Very little, but not none.
Birdscreens block with debris and nesting.
A blocked screen removes the free area entirely.
Weather baffles corrode or distort over years.
Dampers, where fitted, seize if never exercised.
Inspect annually from the roof.