Colt Smoke Control: Car Park Ventilation FAQ

Some frequently asked questions we receive regarding car park ventilation.

What is approved document B, and is it the same as NFPA92-B?

Approved Document B (fire safety) volume 2: buildings other than dwellinghouses is part of a series of documents published by the UK Government that provide guidance on ways to meet the building regulations.

NFPA92-B is a guidance document for Smoke Management Systems in Malls, Atria, and Large Spaces. This is part of the NFPA 92 series of standards, for Smoke Control Systems. The NFPA is the National Fire Protection Association, based in the USA.

What does CFD Stand for?

CFD stands for Computational Fluid Dynamics. Colt use in-house CFD and other design tools to simulate airflows and heat transfer within buildings. Find out more.

What software do you use for CFD? Have you used IES VE CFD software?

We predominantly use FDS for our CFD simulations. This software is commonly used within our industry for analysis of smoke ventilation systems, and is recognised by the UK approving authorities. We have no plans currently to use IES VE CFD software.

Is it essential to create a FDS model in case of smoke clearance design?

We would strongly recommend the use of a CFD (FDS) model to help justify the design of a car park smoke clearance system. It is not essential, as it is possible to prove the design in other ways. For example a hot smoke test once the system has been installed. However, it can be very costly to rectify any issues that arise post installation, should the system not provide adequate results during the test.

When you say CFD is required for mechanically ventilated systems does that include partially mechanically ventilated below ground car park systems

As with the above question, we would strongly recommend it. Although there are methods of proving the mechanically assisted system design without the use of a CFD model. It is usually quicker and involves less risk to pre-approve the system design using CFD analysis.

Is there any approved software by building control?

We cannot speak on behalf of Building Control, but in our experience, they regularly check and give approval to our design reports. The reports present the CFD results from our FDS simulations.

How can we estimate the extract rate?

For a Smoke Clearance system this a simple calculation based on the total Volume of the car park and the required Air Change rate (ACH). For example, a car park with volume of 7200m3, will require 20 m3/s of extract to achieve 10 ACH:

For a Smoke Control system, it is a calculated value based on the design fire chosen. We would not recommend estimating for this type of system.

Are the impulse fans inverter controlled or dual wound?

We typically use inverter control, but dual wound options are also available for the impulse fans.

How can we determine the delay time for jet fans activation after a fire?

For a Smoke Clearance system within a car park, the delay is generally determined by the required time for safe evacuation. This will usually be calculated by the fire engineering consultant for the specific development and will be available within the fire strategy document they produce. It is typically in the range of two to three minutes.

There is no delay for a Smoke Control system within a car park.

Running a car park ventilation system in a resident’s car park 24/7 seems very wasteful of energy, what experience do you have of fume vent systems being off for long periods?

As a minimum we provide Fume ventilation systems that run 24/7, however we can and do provide systems with more advanced controls. With design approval from building control, more sensitive monitoring of pollutants can be used to further reduce or even turn off the ventilation system during periods of inactivity. As can be expected, this more advanced control has a higher initial investment, but the energy savings could outweigh these additional costs, especially on larger car parks.

We have another webinar which explains ways to design energy efficient ventilation system available to watch here.

Traditionally car park MCCs were form 4 type 2, now though I see many form 2 panels. This seems a retrograde step in as far a maintenance goes. Why do we have a reduced standard going on?

The majority of car park ventilation systems are providing smoke clearance, as such they are not classed as life safety systems and do not require a Form 4 panel. A higher form factor may be specified for a given project, but as a minimum the requirement is currently Form 2.

Which system (Smoke Clearance or Smoke Control) would be more efficient if we are dealing with huge car park (approx.: 200.000 sq. m) with no exhaust shaft in the middle of the car park?

This all depends on the car park layout, however if we assume that this is a enclosed single level basement car park, with a floor to ceiling height of 2.4m, a smoke clearance system would need to achieve approximately 1350m3/s of extract to meet the 10 Air Change per Hour requirement of Approved Document B. As such, it would be more efficient to invest in a fire engineered smoke control system.

What is the criteria to decide the zones for any smoke extraction systems?

Smoke Control Systems for Car Parks are typically broken into zones no larger than 2000m2. The layout of the Car Park, the number and location of available smoke extract points, and the number and location of replacement air vents all have an effect on deciding the size and layout of the zones.

How does moving the bulk air for the zone differ from designing to an air change rate (ACH – air changes per hour)?

The principles are very similar, the main differences include the volume of air that is being moved and the requirements relating to controlling the movement of smoke. In a smoke clearance system, we simply need to provide an extract rate based on the volume of the car park. In a Smoke Control system, we need to limit the travel of smoke within the car park. This requires an engineered solution, including careful impulse fan coordination and calculated extract rates, based on a predetermined design fire size.