Deflectors | ventilation | ventilation systems
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Ventilation deflectors – how to choose the right one
A deflector is installed on top of a ventilation duct, pipe or roof opening. As wind flows around the cap, a pressure difference is created, so air is drawn out of the duct more effectively than through an open pipe. A correctly selected deflector improves natural ventilation, reduces humidity and condensation, and protects the duct from precipitation and backdraught.
■ information and selection ■ benefits and suitability ■ check before buying ■ mistakes and safety
Rotary or static – two operating principles
This category includes two types of deflectors. Both work without electricity; they differ in design and in how they respond to wind.
Rotary (wind turbine with bearings)
The vaned head is turned by the wind and actively extracts air. Steel deflectors have ball bearings – thanks to low friction the head rotates smoothly even in lighter wind. Most effective in open, windy locations: ventilation ducts of houses and garages, attics, agricultural and industrial buildings.
Static (no moving parts)
The shape of the cap itself creates draught and prevents rain and wind from entering the duct. Maintenance-free and silent; also suitable where a rotating part would be undesirable.
Quick selection by situation
Situation What to choose Where to look Ventilation duct of a house, garage or farm building Rotary deflector + base of the same diameter Galvanized steel Brown or dark roofing, appearance matters Painted steel (brown) deflector and base Painted steel Coastal areas, livestock barns, damp or aggressive environments Stainless steel deflector – long service life without painting; for Ø125 / Ø160 ducts – plastic (does not rust) Stainless steel, Plastic Large spaces: warehouse, livestock barn, workshop Large-diameter stainless steel rotary roof ventilator (up to 800 mm) Stainless steel Ø125 / Ø160 pipe outlet through the roof Plastic static deflector, with a roof penetration (vent) flashing if needed Plastic Ventilation of an attic or pitched roof space Roof aerator Plastic Deflector groups
A versatile and economical solution for most ventilation ducts. Rotary deflectors with ball bearings and roof bases.
Diameters from D100 to D680 · silver colour
Galvanized steel with a paint coating – extra protection and colour matching to the roof.
Diameters from D100 to D680 · brown colour
The most corrosion-resistant: for damp and aggressive environments and long service life. Large industrial models available.
Diameters from D140 to D680, roof models up to 800 mm
Lightweight, rust-proof static deflectors, ventilation outlets, roof penetrations and roof aerators.
Ø125 and Ø160 mm systems
Material comparison
Material Corrosion resistance When to choose Galvanized steel Good in normal environments Residential houses, garages, farm buildings – best value for money Painted steel Good + additional paint coating When the deflector must match brown or dark roofing Stainless steel Very good, even in damp and aggressive environments Coastal areas, livestock barns, industry, long maintenance-free service life Plastic Does not rust – suits aggressive environments too Ø125 / Ø160 pipe outlets, livestock barns and damp rooms, ventilation only Deflector and base – how to make up a set
The D number in the product name indicates the connection diameter in millimetres. Exact dimensions (head diameter, height, base plate size) are always given on the product page.
- The deflector fits over a duct or pipe whose diameter matches the D.
- If the duct does not protrude above the roof, a base is required – a roof flashing plate with a spigot. The base D must match the deflector D.
- Choose the same material for the deflector and the base – mixed metals corrode faster.
- Do not reduce the duct: the deflector must not have a smaller diameter than the ventilation duct.
⚠ Ventilation deflector and flue cap – not the same thing
Galvanized steel, painted steel and plastic deflectors are intended for ventilation only – to extract humid and warm air. They must not be installed on stove, fireplace or boiler flues: hot flue gases and acidic condensate will damage them, creating a risk of fire and carbon monoxide. On a flue, install only a cap whose product page clearly states that it is intended for flues, following the chimney and boiler manufacturer's instructions and fire safety requirements. Do not install additional caps on gas boiler flues without the boiler manufacturer's approval.
⚠ What to check before buying
What to check Why it matters Duct or pipe diameter Measure and compare with the D and the dimensions on the product page – the wrong diameter means leaks and vibration. Purpose: ventilation or flue The deflectors in this category are intended for ventilation. For flues – only products specifically marked for that purpose. Is a base needed If the duct ends at roof level, the deflector cannot be installed weathertight without a base. Environment Moisture, salt and fumes from livestock shorten the service life of galvanized steel – in such locations choose stainless steel. Position on the roof A rotary deflector is most effective when it is above the ridge or not in a wind shadow. ✗ Most common mistakes
Mistake Consequence Galvanized or plastic deflector on a flue Overheating, deformation, corrosion from condensate, risk of fire and carbon monoxide Deflector too small for the duct Draught is reduced; ventilation works worse than without a deflector Base not sealed The roof starts leaking, insulation gets wet Deflector in a wind shadow, below the ridge The rotating part turns weakly, minimal effect Weak fixing Strong wind can tear the cap off the duct ⚠ Safe installation
- Roof work is work at height: use fall protection equipment; do not install in strong wind, rain or when the roof is icy.
- Fix the deflector mechanically (with screws, rivets or a clamp), not with sealant alone.
- Seal the joint between the base and the roofing with a sealant resistant to UV and temperature changes.
- In winter the rotating part may ice up and stop – this is not a fault: the duct remains open for natural draught.
Frequently asked questions
Does a deflector need electricity?
No. A deflector works only from wind and the natural draught of warm air, so there are no running costs.
Does a rotary deflector work without wind?
Without wind the head does not turn or turns slowly, but the duct remains open – warm air keeps rising by natural draught. When the wind picks up, extraction increases.
How do I choose the diameter?
The D number in the product name is the connection diameter in millimetres. Choose a deflector whose D matches the duct diameter, and confirm the dimensions on the product page. A deflector of smaller diameter will reduce draught.
Can a deflector be installed on a chimney?
The deflectors in this category are intended for ventilation ducts. On a flue, install only a cap whose product page states that it is intended for flues, and follow the chimney and boiler manufacturer's instructions.
When should I choose stainless steel?
When the deflector will operate in a damp or aggressive environment (coastal areas, livestock barns, manufacturing) or when you want the longest maintenance-free service life – see stainless steel deflectors.
In short
For a ventilation duct, choose a deflector by diameter and environment: galvanized – versatile, painted – when colour matters, stainless – for damp and aggressive locations, plastic – for Ø125 / Ø160 pipe outlets and aggressive environments. Can't find the right size or not sure? Send us the duct diameter and roofing type – we will help you choose.
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E-mail: info@elsprendimai.eu
