Dear All,

We are working on a project where we want to make the building naturally

ventilated as per ASHRAE 62.1-2007

The prescriptive requirements for natural ventilation are as follows:

- Naturally ventilated spaces must be permanently open to and within 25

ft (8 m) of operable wall or roof openings to the outdoors, the openable

area of which is a minimum of 4% of the net occupiable floor area being

naturally ventilated. (The net occupiable floor area is measured from the

inside surfaces of the walls, excluding permanently enclosed obstructions

such as shafts and columns.

- Where interior spaces without direct openings to the outdoors are

ventilated through adjoining rooms, the opening between rooms must be

permanently unobstructed and have a free area of not less than 8% of the

area of the interior room nor less than 25 ft? (2.3 m?).

We have square floor plate of 1000 square meter. As per ASHRAE 62.1 only

the perimeter area with a depth of 8 meter can be considered as naturally

ventilated space. But we have designed the windows and positioned the

windows in order to encourage cross ventilation and we can show sufficient

distribution of air by doing CFD simulations. Are the CFD simulations

enough to prove that this building is compliant with ASHRAE 62.1 as a

naturally ventilated building?

Thanks for your time.

Hema.

Hema,

It?s been a while since I documented a natural ventilation project for LEED (I?m assuming you are doing this for LEED?), but in the past (LEED-NC v2.2 era), I?ve used annual natural ventilation simulation results (TAS and IES Virtual Environment are a couple of tools that work) to show that both peak and typical natural ventilation air flow rates are SIGNIFICANTLY higher than the requirements if the building used a mechanical ventilation scheme. My impression of CFD is that it is more of a snap-shot in time rather than giving you results across a time-period, so maybe you could demonstrate flow on a peak day and maybe a ?stagnant? day to help present your design. Otherwise I?d recommend presenting results from an annual (or natural-ventilation-season) simulation.

Maybe people who have submitted nat vent designs for review in the more recent past can provide more insight.

Sincerely,

Nathan Miller, PE, LEED AP BD+C ? Mechanical Engineer/Senior Energy Analyst

RUSHING | D 206-788-4577 | O 206-285-7100

www.rushingco.com

Hi Hema,

For the last natural ventilation design I had reviewed, Nathan?s suggestions are sensible ? using an annual simulation and bulk airflow modelling within a software such as IES or TAS. Our reviewer requested a further 3-4 steady state CFD simulations using boundary conditions calculated in the bulk airflow model at typical times (including velocities/temperatures at openings calculated based on the same annual climate data used for the energy modelling). These CFD simulations were taken at typical times of the year to show that both mean age of the air wasn?t too high (i.e. that there were no occupied areas that the fresh air was not reaching), and that the temperatures in the spaces as a result of a natural ventilation strategy were not too cold.

Essentially you need to prove that you will achieve the fresh air rates required, without sacrificing comfort and use the CFD to prove the fresh air distribution is acceptable. I wouldn?t say you need to show significantly higher ventilation rates ? in winter this would be a significant energy penalty, and you would expect to meet the minimum standard ? however, due to the un-predictable nature of the wind, we took the approach of proving that the minimum ventilation in the occupied period always exceeded the requirements of a DOAS mechanical system. This was acceptable to our reviewer.

Unfortunately, 62.1-2007 is not a good tool for passive design ? It?s very much a worst case set of criteria.

Hope that helps,

Ed

Edwin Wealend

Principal Engineer

Cundall Engineering Design Consulting (Shanghai) co., Ltd

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www.cundall.com.cn

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