Protective ventilation grilles for ventilation systems
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Grille guide: how to choose protective ventilation grilles
Grilles are the last part of a ventilation system that people see and the first that the air meets. Correctly chosen, they pass the required airflow without whistling, keep out birds, rodents and leaves and, outdoors, stop cold air and rain getting back in. The wrong ones “choke” the fan, collect condensation and clog up. This guide will help you choose by opening, location and airflow.
Blue – information Green – benefit to you Orange – ⚠ check Red – ⚡ safety and mistakesContentsWhat do you need? Quick selection
Situation Solution Where to look End of a round duct indoors or on the façade Round grilles with a spigot equal to the duct diameter (Ø100, Ø125, Ø150, Ø160…) Ø100 · Ø125 · all round Cooker hood or bathroom fan exhaust through a wall External wall vent with backdraft valve – while the fan is off, the valve stays closed Ø125 · all wall vents Larger exhaust duct outdoors (Ø150–400) that must not blow back External grilles with gravity shutters and spigot Ø150/160 · Ø315 · Ø400 Natural ventilation opening in a wall, storeroom, basement Square or rectangular grilles with mesh; external grilles outdoors Plastic 300×400 · Galvanised 200×200 Need to adjust or partly close the airflow manually Grilles with adjustable louvres 483×452 mm Production area, garage, warehouse – impacts possible Metal grilles: galvanised or painted steel Square galvanised steel Seaside, swimming pool, aggressive or humid environment Stainless steel, aluminium or plastic – not unpainted galvanised steel Stainless Ø160 · Aluminium Ø315 Interior where the grilles should not be visible Concealed-mount steel grilles 290×390 mm Kitchen furniture plinth, fridge or oven recess, radiator cover Narrow furniture (plinth) grilles 480×80 mm · all furniture What you gain by choosing correctly- The fan runs at its rated capacity – the grilles add no extra resistance and do not whistle.
- Birds, wasps, rodents and leaves stay out of the duct – no need to dismantle the system for cleaning.
- In winter, no cold air blows in through the exhaust opening and no condensation builds up on the wall.
- The façade and interior look tidy: grilles are chosen by colour and material.
Grille types: what each one does
With mesh
Fixed blades and mesh keep out birds, rodents and leaves. Suitable for natural ventilation and supply openings.With gravity shutters
Light blades open under the exhaust airflow and fall shut under their own weight when the fan stops.Wall vent with backdraft valve
External vent with a non-return valve and duct spigot – a compact solution for a cooker hood or bathroom fan.With adjustable louvres
You change the blade position by hand – partly closing the airflow or directing the air.Type Direction Strengths Limitations With mesh Supply and extract Simple, inexpensive, no moving parts Do not stop draughts; the mesh reduces the free area and collects dust and grease Gravity shutters Extract only (air goes outdoors) Close without electricity or an actuator, reduce cold and wind ingress Need airflow to open; mounted vertically; not fully airtight Vent with backdraft valve Extract only Compact, with Ø100–Ø160 spigot or for 55×110 mm channel Not suitable for supply or natural ventilation Adjustable louvres Indoor supply and extract Airflow can be partly closed or directed When partly closed, resistance and noise increase; not a system balancing device Material: plastic, steel, aluminium or stainless steel
Material Where it suits What to watch for Plastic Flats, houses, bathrooms, façades; the widest choice of colours and sizes Lightweight, does not rust; not suitable near hot surfaces; cheaper plastics may yellow in sunlight over time Galvanised steel Production and plant rooms, warehouses, garages Impact-resistant; at the seaside and in chemically aggressive environments the zinc coating wears away faster Painted steel Interiors where colour and durability matter Damaged paint should be touched up to prevent corrosion Aluminium Façades, furniture, larger diameters Lightweight and corrosion-resistant; softer than steel Stainless steel Seaside, food industry, swimming pools, prestige façades The most expensive but longest-lasting solution for humid and aggressive environments How to calculate grille size from airflow
1. Required free area
A (m²) = Q (m³/h) ÷ (3600 × v).
Q – airflow, v – air velocity through the grille. In residential rooms, about 1.5–2 m/s is used in practice so that the grilles do not whistle.
Example: cooker hood 150 m³/h, v = 2 m/s → A = 150 ÷ (3600 × 2) = 0.0208 m² = ~208 cm² of free area.
2. From free area to grille dimensions
The blades and mesh take up part of the opening, so the free area is only a fraction of the total grille area (often around 40–60 %). Check the exact free area in the manufacturer’s data.
Example: 208 cm² ÷ 0.5 = ~416 cm² total area → about 210 × 210 mm.
3. For a round duct – velocity in the duct
v = Q ÷ (3600 × π × D² ÷ 4).
Example: 150 m³/h through a Ø125 mm duct → 150 ÷ (3600 × 0.01227) ≈ 3.4 m/s. If the grille free area is smaller than the duct cross-section, the velocity through it is even higher – in that case choose a larger grille or a larger duct.⚠ What to check before buying
Kibirkštis: “I bought Ø125 grilles, but they won’t fit into the pipe. It turns out “Ø125” is the size of the front face, and the spigot is Ø100.”What to check Why Spigot diameter, not the front face size The first number in the name is often the outer size of the grille (e.g. Ø150), while “with D125 spigot” is the part that fits into the duct. Compare the spigot with the inner diameter of the duct Air direction: supply or extract Gravity shutters and backdraft valves open only for exhaust air. They are not suitable for supply or natural ventilation openings Fan capacity and pressure A weak bathroom fan may not open gravity shutters, and fine mesh further reduces the airflow Installation location: outdoors or indoors Outdoors – external grilles with rain-shedding blades and mesh; indoors – decorative, often removable for cleaning Wall opening dimensions The grille frame must cover the opening, and the spigot or housing must fit into it. Measure the opening, not the old grille Environment and material At the seaside, by a swimming pool or in production, choose stainless steel, aluminium or plastic ✗ Most common mistakes
Mistake Consequence Grilles chosen by duct diameter without considering the free area Whistling, reduced airflow – the cooker hood “does not extract” Gravity shutters installed on a supply opening or horizontally Shutters do not open or do not close – air does not get in or blows back Grilles with fine mesh used for cooker hood exhaust The mesh quickly clogs with grease and dust, airflow drops External vent without a fall to the outside, duct uninsulated in a cold zone Condensation runs into the wall or back into the fan Galvanised grilles at the seaside or by a swimming pool Faster corrosion, rust streaks on the façade ⚡ Natural ventilation or combustion air opening in a room with a gas boiler or fireplace covered with shuttered grilles or closed with louvres Not enough combustion air, flue gases may return to the room – risk of carbon monoxide (CO) poisoning 
⚡ Safe installation – Fazė’s advice
Before fitting grilles at a fan, switch off its power supply at the circuit breaker, not just at the wall switch. If the fan impeller can be reached by hand, the opening must be covered with a protective grille – especially in industry and where children are present. When drilling an opening in a wall, check for hidden electrical cables. In rooms with gas appliances or fuel-fired boilers, do not close combustion air openings or cover them with shutters – their size is determined by the design and the appliance manufacturer’s requirements.☝ Elsprendimai.eu recommends- For a cooker hood or bathroom fan through a wall – wall vent with backdraft valve: grille and non-return valve in one product.
- Need a more reliable way to prevent backflow in the duct? Combine the grille with a non-return damper for the duct.
- Choose the fan, ducts and grilles in the same diameter – fans and ducts and fittings are in our catalogue.
- For roof exhaust, a deflector is often more suitable than a grille – it protects against rain and improves draught.
Diagram: cooker hood outlet in the façade – wrong and right
Kibirkštis: “The cooker hood is new, but the smells stay in the kitchen. In winter it even blows cold air, and there are brown streaks on the wall.”
Fazė: “The hood is not to blame. Look at the opening outside.”✗ WRONG① Ø150 cooker hood connected to a Ø100 duct↓② Duct rises towards the outside, uninsulated↓③ Outside – an indoor grille with fine mesh, no valve↓④ Poor extraction, cold air blows in during winter, grease and condensation streaks on the façade✓ RIGHT① Duct of the same diameter as the cooker hood outlet (Ø150)↓② Duct with a slight fall to the outside, insulated in the cold zone↓③ Outside – an external wall vent with backdraft valve or gravity shutters, spigot Ø150/160↓④ The cooker hood runs at full capacity; when it stops, the opening closesWhy – explanation by diagram number- ①: reducing the duct from Ø150 to Ø100 cuts its cross-section by more than half, while air velocity and resistance increase. The fan then runs at a much lower capacity and more noisily.
- ②: warm, humid air cools in a cold duct and the vapour condenses. If the duct rises towards the outside, the condensate runs back into the cooker hood. A slight fall to the outside and insulation in the cold zone eliminate this problem.
- ③: fine mesh quickly clogs with grease, and without a valve or shutters wind and cold air enter the duct freely when the cooker hood is off.
- ④: an external wall vent with backdraft valve or gravity shutters opens under the exhaust airflow and closes when it stops, while the blades shed rain away from the opening.
⚠ The diagram is for guidance only. Design and install the ventilation system according to the design, manufacturers’ instructions and current building regulations. The electrical part must be connected by, and is the responsibility of, a qualified electrician. UAB “Elektrotechnikos sprendimai” is not liable for installation or possible errors.FAQ
Kibirkštis: What does “D150 with D125 spigot” mean?
Fazė: D150 is the diameter of the grille front face that you will see on the wall. The D125 spigot is the collar that fits into a Ø125 mm duct. When choosing, look at the spigot.
Kibirkštis: What is the difference between gravity shutters and a backdraft valve?
Fazė: The operating principle is the same – they open under the exhaust air and close when it stops. Gravity shutters are several blades in larger grilles; a backdraft valve is usually a single flap in a compact wall vent. Both are suitable for extract only.
Kibirkštis: Can gravity grilles be used for supply?
Fazė: No. The shutters open only towards the outside. For a supply or natural ventilation opening, choose grilles with mesh or adjustable louvres.
Kibirkštis: Why do new grilles whistle?
Fazė: The air velocity through too small a free area is too high. Check the calculation in the “How to calculate” section – a larger grille or a grille without fine mesh usually helps.
Kibirkštis: How do I clean grilles?
Fazė: Most indoor grilles are removable, and some steel ones are held by magnets. Remove, wash with warm water and detergent, and dry. Check external grilles at least once a year – make sure the shutters move freely and there are no nests or leaves.
Kibirkštis: What grilles should go in a kitchen furniture plinth?
Fazė: Narrow furniture grilles, e.g. 480×80 mm. They are needed when the plinth houses a built-in fridge, dishwasher or heat pump dryer – the air must have somewhere to circulate. You will find the exact required area in the appliance installation instructions.
Sources: elsprendimai.eu product card data (groups 2698–2702); STR 2.09.02:2005 “Heating, ventilation and air conditioning”; LST EN 16798-3 (ventilation for non-residential buildings); LST EN ISO 13857 (safety distances to prevent hazard zones being reached). Air velocity values are practical recommendations, not regulatory requirements. Checked 2026-10-17.
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