Natural vs Mechanical ventilation systems: What's the difference?

Fire safety is one of the most important aspects of every building, new or old. Implementing the correct smoke ventilation system can be, quite literally, a question of life or death. This is why it is so crucial to understand the different types of ventilation systems, how they work, and what they’re good for.

Notably, there isn’t a one-size-fits-all solution, as the best option for your building will depend on many factors — height, travel distance and the internal layout of the building to name a few. However, by understanding your options, you’ll be able to make an informed decision on how best to ventilate your building. That being said, you should also consult a specialist to assist you with your choice, so chat with us for expert assistance.

What is natural ventilation?

As the name suggests, natural ventilation systems manipulate the innate characteristics of smoke rather than using mechanical intervention. In a nutshell, it uses apertures in the building and the natural buoyancy of the smoke to remove it from the building, leaving escape corridors and stairs free for evacuees and fire and rescue personnel.

But, how does it work? Smoke is primarily gaseous, and as gases get hotter, they expand. When they do that, they become more buoyant. We can use the high buoyancy of the smoke to remove it from the building by creating apertures in strategic locations in the building to allow it to leave. These apertures will be formed by opening vents in the building.

There are a few different methods of natural smoke ventilation. The simplest is a head-of-stair vent — a vent that’s located at a high level at the top of the staircase. When smoke is detected, the head-of-stair vent opens, allowing the smoke out. These are usually found in small, simple buildings.

Some constructions, usually those with a longer distance between apartment doors and the stair door, will require ventilation in the corridor, too. In this case, in addition to a head-of-stair vent, you may either have facade ventilation or shaft ventilation.

Facade ventilation uses vents through the front of the building out to the open air. In the case of fire, the head-of-stair vent would open along with the facade vent on the fire floor. All the vents on non-fire floors will remain closed to prevent smoke contamination between areas of the building.

Shaft ventilation has dampers opening into a large shaft running through the building. In this scenario, the head-of-stair vent would open, then shaft damper on the fire floor, and finally, the head-of-shaft vent. Similarly to facade ventilation, the dampers on non-fire floors stay closed to resist the passage of smoke and flame.

What is mechanical ventilation?

Unlike natural ventilation systems, mechanical smoke ventilation systems don’t solely depend on the physical properties of smoke. Instead, they employ mechanical elements, such as fans, to suck smoke out — whether it is its natural trajectory or not.

There are two key types of mechanical smoke ventilation systems. The most common is MSVS systems (also known as mechanical smoke extract systems). The other type of system is a Pressure Differential System (PDS), which can come in two different forms — pressurisation and depressurisation.

Pressurisation systems are found in commercial buildings such as office blocks, and depressurisation systems, while very rare, are occasionally used in basements. For the purposes of this article, we’re focussing on MSVS systems.

Similarly to natural ventilation, shafts, vents and windows are still used. The triggering of the system will open the head-of-stair vent and the shaft dampers on the floor of fire origin. Simultaneously, all the other dampers will be sealed shut to prevent the smoke from seeping through. However, it is at this point that the two systems diverge, and the mechanical aspect of this ventilation process kicks in.

On the roof of the building sits a mechanical extract pod, forcefully drawing the smoke up and out of the building. It’s important that systems like these have a robust supply of ‘make-up’ or ‘inlet’ air. Without sufficient air, the extract zone will depressurise trapping occupants inside. Often, the stair door is used to provide this make-up air. The strength of this system is that it doesn’t have to rely on external weather conditions being favourable to effectively clear smoke.

How to choose a smoke ventilation system

Both ventilation systems have their advantages. For example, natural systems usually require less maintenance, and are more cost-effective for achieving compliance. Mechanical systems, despite being more costly, allow building designers to extend corridors, do not rely on favourable weather conditions to be effective, and take up far less space in the building.

Which system you choose depends on the characteristics of your building, with some of the key elements being travel distances, building heights and space. Guidance on making system selections can be found in Approved Document B (natural smoke ventilation systems), The Smoke Control Association Guide, and BS 9991 (natural and mechanical).

Building height

Natural smoke ventilation systems are better-suited for low-rise buildings under 18m in height, as these buildings are lower risk and don’t fall under prohibition from BS 9999 in relation to facade ventilation. MSVS systems are usually used for taller buildings, as they’re typically higher risk and also space tends to be at a premium.

Travel distance

Extended corridors would typically mean the building requires an MSVS system, as natural smoke ventilation systems and pressure differential systems can’t effectively protect long corridors. The definition of extended does vary on the building height, the number of staircases and whether or not sprinkler systems are fitted to apartments in line with BS 9991. The distances are measured from the furthest apartment door to the door to the staircase or protected lobby.

Shaft size

One of the key benefits of a mechanical smoke ventilation system is that they free up valuable floor space owing to their reduced shaft sizes. So, if you’re tight for space, a mechanical system will likely work best. Shaft-based natural ventilation systems require 1.5m2 (cross-sectional area) shafts, compared to circa 0.6m2-1m2 for MSVS systems.

Based on these factors, you may already know which system you require. However, it is still important to consult with a specialist to ensure your smoke control systems are compliant and effective.