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Frequently Asked Questions
What does a relief ventilator do?
Lets air out without a fan.
It is the discharge half of a passive system.
It also relieves buildings with powered supply.
Displaced air needs a defined route out.
Otherwise it finds gaps and doors.
Both cases are common.
What goes wrong in a pressurised building?
Doors become hard to close and whistle.
The supply fan works against pressure.
It then delivers less than its rating.
Air escapes through unintended gaps.
Weather and noise follow those paths.
Relief openings prevent all of it.
Why is free area so critical here?
The driving pressure is very small.
Buoyancy and wind produce little force.
So does a modest positive building pressure.
Any restriction stops the flow.
Size relief openings generously.
Compare on screened free area.
How is backdraught prevented?
By gravity-operated damper blades.
They open under outward flow.
They fall closed when flow stops.
Air passes one way and weather is resisted.
Blades must open under very little pressure.
They must also survive years on a roof.
Where should relief openings go?
High, and away from intakes.
Warm air already wants to go up.
Discharge near an intake is drawn straight back.
That short circuit wastes the whole arrangement.
Check separation distances.
Prevailing wind affects it.
Should relief be balanced against supply?
Yes, and it decides building pressure.
Too little relief pressurises the building.
Too much lets conditioned air escape freely.
Slight positive pressure is often intended.
State the intended pressure as a requirement.
Size relief to achieve it.
What maintenance do they need?
Blade operation, above all.
Blades stick with dirt and corrosion.
A stuck-closed unit pressurises the building.
A stuck-open one admits weather.
Operate and inspect annually.
Hinges and pivots are the failure points.