Why a hood on its own solves nothing

The most common mistake in a professional kitchen goes like this: buy a powerful hood, connect it to the existing duct and hope the steam disappears. A week later the kitchen is still damp, the doors swing shut by themselves, and the dining room smells of the grill. The hood is rarely the culprit.

Extract and supply are one system. If you pull 4 000 m³/h out of a room but only 500 m³/h comes back in a controlled way, the fan drags the remaining 3 500 m³/h in through doors, window gaps and neighbouring shafts. The room goes into heavy negative pressure, the hood's real capacity drops well below its rated figure, and instead of capturing steam it starts pushing it out along the edges. That is why sizing always starts with two numbers — what goes out and what comes back in.

Hood types and where each one belongs

The hood type follows the equipment layout, not personal taste. BHS carries a series for every layout.

Sizing the airflow: three methods

Three approaches are used in practice, from a quick figure in conversation to a full engineering calculation. The first two tell you the order of magnitude; the third is what goes into the drawings.

Method A — by hood area (quick estimate)

Example: a wall hood 2 000 × 900 mm → A = 1,8 m² → L ≈ 1 800 m³/h. Useful for checking whether the duct will cope at all, but it knows nothing about whether an induction range or a lava-rock grill sits underneath.

Method B — by capture velocity

Method C — thermal calculation (EN 16282-1 / VDI 2052)

This is what real design work uses. Each appliance is assigned convective heat and moisture emission coefficients, a diversity factor is applied (equipment rarely all runs at full output at once), and the thermal plume reaching the lower edge of the hood is calculated. The result is typically 15–30 % leaner than Method A, because you stop paying to move air nobody ever moves.

Ballpark figures to open the conversation with, before the equipment data sheets are on the table:

Make-up air and pressure balance

Make-up air is the part of the design that gets cut first when budgets tighten. The outcome is predictable: the hood works at half strength, the heating cannot cope in winter, and the staff stand in a cold draught.

• Controlled supply air — 80–90 % of the extract volume.
• Kitchen held slightly negative relative to the dining room and corridors (roughly −5…−10 Pa) so odours do not spread.
• In winter the supply air must be tempered to at least 16–18 °C — otherwise the staff will simply shut it off.
• A hood with integrated supply (HPPD, HPCD, HFPD and HFCD series) delivers part of the compensating air directly at the hood — that air does not need heating to room temperature, which is a direct saving on the heating bill.
• A separate supply plenum (for example Z-HASP) is the answer when the hood is already installed but the balance does not add up.

Duct velocities and diameters

Air velocity in a grease duct is a fire-safety parameter, not just a noise question. Too slow and grease settles on the walls; too fast and you get noise and pressure loss.

Installation geometry — where it usually goes wrong

• Overhang: the hood must extend beyond the appliance outline by 150–300 mm on every open side. A hood cut «exactly to the range» guarantees steam escaping along the edges.
• Height: lower edge 1,9–2,1 m above the floor and 600–1 100 mm above the cooking surface. Every extra centimetre upward costs more air.
• Cross-draughts: keep the hood away from doors, supply grilles and fans — a cross-flow destroys the thermal plume and the steam goes past.
• Duct route: each 90° bend is worth roughly 3–5 m of straight duct in resistance. Two unnecessary bends can eat a tenth of the capacity.
• Access hatches: cleaning access is mandatory — at least every 3–4 m and at every bend. Without them the annual cleaning is physically impossible.

Filters: what actually holds grease back

A filter's job is not to make the air clean — it is to protect the duct. Every gram of grease that gets past settles on the duct wall and stays there until the next cleaning. That makes filter choice a fire-safety decision.

Maintenance: a schedule that actually works

Looking after a hood is not «a firm comes once a year». The annual system clean is a legal requirement, but the daily routine is what decides whether that clean is routine work or emergency repair.

How to wash baffle filters properly

• Hot, not boiling: 60–65 °C. Above that, grease bakes onto the metal.
• An alkaline degreaser, not ordinary washing-up liquid. The professional chemistry for grills and fryers works on filters too.
• Never caustic soda on aluminium — it literally eats aluminium mesh filters. Stainless steel tolerates it.
• Upright in the rack, not stacked flat on top of each other — otherwise only the outer surface gets washed.
• Bone dry before refitting: a damp filter collects dust and grease twice as fast.
• Never run the hood without its filters while they are in the machine. One shift without filters puts more grease into the duct than a month with them.

Fire-safety requirements across Europe

Design is governed by the European standard series, while cleaning intervals and paperwork are set nationally. If you operate in more than one country, the differences matter.

Requirements that come from practice, not just from the rulebook

• A class F extinguisher beside fryers and grills — not powder, not CO₂. Cooking oils burn at 340 °C and above; water or the wrong extinguisher causes an explosive splash.
• An automatic suppression system in the hood — a wet chemical discharged through nozzles that forms a blanket over hot oil. Most insurers already require it where there is a lava-rock grill or more than two fryers. It is the fastest way to stop a hood fire becoming a building fire.
• Do not mix grease extract with anything else — the greasy kitchen extract must not join a common duct serving offices or toilets.
• The documentation file: annual cleaning records with photographs, a fire-safety instruction covering the ventilation system, and the staff training log. The inspector asks for the file first.

How BHS works with hoods

Most kitchens are not rectangular rooms with blank walls. There are load-bearing columns, sloping ceilings, shafts built over decades ago, and a cooking line that matches no catalogue dimension. That is why BHS both supplies hoods from stock and manufactures them to an individual project, sketch or drawing.

The custom design process

Custom solutions — when no catalogue size fits

Custom hoods are a category of their own at BHS. They are needed when the room has columns, load-bearing walls, an irregular layout or specific process requirements. Every project starts with measuring the space and analysing the workflow; manufacturing normally takes 3–6 weeks from order confirmation, and there is no minimum order — a single item is fine.

What is available from the catalogue

Frequently asked questions

The requirements cited here are based on Latvian Cabinet Regulation No. 238 (clauses 92–94), the Lithuanian General Fire Safety Rules (clause 72), Estonian standard EVS 812-2 and the French Arrêté of 25 June 1980 (articles GC 18 and GC 21). Sizing methodology follows the EN 16282 series and VDI 2052. The solution for any specific site is determined by its building design — this article is reference material, not a substitute for a project.