Ventilation Energy and Environmental Technology
                                     from VEETECH Ltd. 
             Updated 18th July 2013

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A Tutorial on Calculating Ventilation Rates under conditions of Natural, Mechanical Extract, Mechanical Supply and Mechanical Balanced Systems using the Free On-Line PHPAIDA Calculation Tool

[See Disclaimer and Important Notes]

Martin Liddament


PHPAIDA may be accessed at the link below: 

The purpose of this tutorial is to highlight the powerful solutions that can be achieved. This algorithm is intended as an early stage tool for sizing natural ventilation openings and basic mechanical systems. It can also be used in the development of control systems to adjust ventilation openings according to weather conditions.


Starting Conditions

For simplicity consider a building such as a dwelling in which the inside air is well mixed (e.g. open plan space or internal doors open). Assume that good air quality is needed and that there are 4 occupants. Based on a ventilation rate of 15 Litres/s (L/s) for each occupant the required ventilation rate is 60 L/s.

Assume the following building dimensions and weather conditions:

Urban Location
·        Building Height = 8 m
Building Volume = 240 M3
Outdoor Temperature = 0 ºC
·        Indoor temperature = 20 ºC
Windspeed from a nearby met station = 3 m/s


Sizing Openings for Natural Ventilation

  By entering the above data into PHPAIDA the opening areas of the 9 available flow paths can be varied to obtain a ventilation rate of approximately 60 L/s. There is no unique solution but the following is an example:

Flow Path Height (m) Opening
Area (cm2 )
1 - 4 1.5 50
5 - 8 5.0 100
9 8.0 550

The opening areas were quickly derived in three manual iterations and gave a total ventilation flow rate of  59.6 L/s combined with a fairly uniformly inflow through openings 1 to 8 combined with extract from the chimney opening. This approach can be applied to a variety of weather conditions, for example to derive a control strategy for openings to provide a uniform ventilation rate over a wide range of conditions.


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Extract Only and Supply Only Mechanical Ventilation

  An alternative approach is to consider a mechanical extract or mechanical supply system. By keeping the same openings as above, the result using PHPAIDA is:

·        Extract at 60 L/s gives a total ventilation rate of: 85.27 L/s

·        Supply at 60 L/s gives a total ventilation rate of: 90.61 L/s

Although the weather conditions and opening areas are the same as for the natural ventilation case, it is seen that the total ventilation rate is less than the sum of the natural and mechanical rates. This is an advantage of mechanical extract or supply only systems. The natural openings are actually needed to provide the supply air for an extract system or to provide an exhaust route for a supply air system. It is possible to reduce the opening sizes so that weather has a much reduced impact. This means that a certain amount of building leakage can be accommodated. However, when the opening size is reduced, the pressure difference between the inside and outside of the building increases. This means that the mechanical system has to work harder, resulting in higher energy use. Also the fan must be capable of overcoming any increased pressure difference while providing the required airflow rate.

Under many instances, natural driving forces tend to be in the low Pascal range, thus an optimum internal pressure to overcome the influence of weather would be:

·        -10 Pa (i.e. 10 Pa below atmospheric) for extract systems;

·        +10 Pa (i.e. 10 Pa above atmospheric for supply systems.

For low rise buildings this will result in a uniform ventilation flow rate over a wide range of ‘normal’ weather conditions.

A solution can be found by adjusting the opening areas in PHPAIDA until the internal pressure is approximately 10 Pa. A suitable solution is:

Flow Path Opening
Area (cm2 )
1-8 40
9 0

Differential areas between the upper and lower openings are no longer important because flow was dominated by the mechanical ventilation rather than natural forces. Also the roof stack is not required.

  • For extract ventilation this gave an under pressure of:  -9.13 Pa.

  • For supply ventilation this gave an over pressure of ; +2.98 Pa.

In both cases the flow rate was exactly 60 L/s (i.e. flow is dominated entirely by the mechanical system.)

  Of course it is possible to reduce the opening area further for the supply approach since the 10 Pa target has not been reached. This is left as an exercise along with explaining why the pressures for the extract and supply systems are numerically different.

Balanced Ventilation

Balanced Ventilation consists of mechanically providing both Supply and Extract Ventilation to the space. Thus the design ventilation rate of 60 L/s must be applied to both the Extract and Supply inputs of the PHPAIDA data input section.

When entering the mechanical balanced data to the original natural ventilation example, the over all ventilation rate becomes:

·        Balanced at 60 L/s gives a total ventilation rate of: 119.59 L/s.

This is exactly the sum of the natural ventilation rate and the mechanical ventilation rate. Thus to avoid over ventilation the building must be completely airtight. This is the major disadvantage of balanced systems.


By understanding and experimenting with these simple examples, the potential of PHPAIDA can be quickly understood. It is not possible to guarantee any results but PHPAIDA enables basic design guidance to be obtained to assist in more advanced design tools.



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