Plant Room Design for Catering Ventilation

Option A: an internal plant room

An internal plant room sits within the building envelope - often a dedicated room on the kitchen floor, a mezzanine above the servery, or a basement space. The extract fan and make-up air unit live indoors, with ductwork routed up to a discharge point at roof level. DW/172 and wider guidance both note that fans are best located inside the building wherever practical, because doing so keeps fan noise and vibration away from open air and neighbouring properties.

The trade-off is that everything the plant does - the heat it rejects, the space it occupies, the pressure it needs to overcome - now happens inside your building. Enclosed rooms full of running motors get warm quickly, in much the same way a laundry room gets so hot, so heat management and ventilation of the plant room itself become part of the design.

Pros

• Fan noise and vibration are contained within the structure, which usually makes a BS 4142 assessment far easier to pass at the nearest neighbour.
• Plant is protected from weather, so filters, bearings and controls tend to last longer and corrode less.
• Access for cleaning, filter changes and fan servicing is possible in any weather, at any hour, without roof or ladder work.
• Discharge can still be taken to roof level through ducting, keeping odour and grease away from ground-level doors and windows.

Cons

• It consumes lettable or working floor space that a busy site may not have to spare.
• Motors, ducts and lighting reject heat into an enclosed room, so the plant room needs its own cooling or ventilation to stay within sensible temperatures.
• Longer internal duct runs up to the discharge point add resistance, which can mean a larger fan and more energy use.
• Grease-laden ductwork now passes through occupied parts of the building, raising the stakes on fire stopping, fire dampers and regular cleaning access.

Option B: an external or rooftop plant room

An external plant room places the fan, and often the make-up air unit, outside - on the roof, in a plant enclosure, or in an outdoor compound at ground level. Short duct runs from the kitchen up to a rooftop fan are common on tight urban sites where indoor space is precious. It is a clean way to free up floor area, but it moves the noise source into the open air, where BS 4142 becomes the deciding factor.

BS 4142 works by comparing the rating level of the plant to the background sound level at the nearest sensitive receptor. A rating level around 10 dB above background is taken as likely to indicate a significant adverse impact; around 5 dB above, an adverse impact; and at or below background, a low impact. Rooftop fans in earshot of homes or hotel rooms therefore often need acoustic attenuators, and sometimes a full acoustic enclosure built from 50 mm or 100 mm insulated panels, to get anywhere near those thresholds.

Pros

• It frees up internal floor space, which on a constrained site can be worth more than anything else.
• Duct runs from canopy to fan can be short and direct, reducing system resistance and keeping fan sizing sensible.
• Heat rejected by the plant is released to open air rather than into an occupied room.
• Grease-bearing ductwork spends less of its length inside the building, simplifying internal fire stopping.

Cons

• Noise and vibration radiate into the open, so attenuators, anti-vibration mounts and often an acoustic enclosure are needed to satisfy BS 4142.
• Plant is exposed to weather, shortening the life of filters, bearings and controls and raising corrosion risk.
• Access for cleaning and servicing means roof or ladder work, which brings working-at-height rules, fall protection and weather delays.
• Roof loadings, weatherproofing penetrations and planning conditions can add cost and complexity before a single fan is bolted down.

Neither option removes the underlying duties. Whichever you choose, the kitchen still needs enough make-up air to avoid pulling itself into hard negative pressure - a shortfall that slams doors and starves gas appliances of combustion air. Designers typically supply somewhere around 85% of the extract volume as tempered make-up air, leaving the kitchen slightly negative so cooking odours stay put rather than drifting into dining or corridor spaces. Sites with no easy route to fresh air, such as basement and windowless kitchens, need that make-up air strategy resolved early, because it drives the size of the plant room whichever location you pick.

The numbers that shape the room

Three figures do most of the work when you are sizing and siting a catering plant room. They set the standard you are judged against, the way your fan noise is measured, and the air balance that keeps the kitchen safe and odour-tight.

Design the plant room around access from the start and the rest tends to follow. DW/172 and TR19 Grease both expect access panels at set intervals along the ductwork and clear working space around the fan, so that the whole system can be inspected, cleaned and certificated without dismantling it. A plant room squeezed to the last millimetre with no clearance is the single most common reason a system becomes hard - and eventually unsafe - to maintain.