Active compared to Passive Ventilation using CFD: IBEEE’s Story

IBEEE passive house

Rising energy prices, environmental consciousness and the associated demand for energy efficiency are key topics in the design of single and multi-family houses, for reducing their ecological footprint. One of the trends for increased efficiency in housing are passive houses, which don’t require classical building heating due to their excellent thermal insulation. These houses, however, need a complex ventilation system which often causes criticism because it doesn’t allow a natural fresh air supply.

In Europe, there are 25,000 certified passive structures (from schools and commercial buildings to homes and apartment houses) — whereas only 13 have been built in the United States so far. The leading regions in passive houses are the German-speaking countries and Scandinavia. [1]

Based in Klagenfurt, Austria, IBEEE is a holistic development and engineering service provider for electronic systems and energy concepts for buildings. The team, led by Dr. Alexander Lechner, covers a broad service and industry spectrum, focusing on renewable energy.

Passive Ventilation

Passive ventilation is a natural ventilation system that makes use of natural forces, such as wind and thermal buoyancy, to circulate air to and from an indoor space. These ventilation systems work to regulate the internal air temperature as well as bring fresh air in and send stale air out. This is largely achieved through the opening and closing of windows and vents which act as a source of air as well as an exhaust

IBEEE wanted to investigate and quantify the performance of a fanless ventilation system used in their passive house design. For this purpose, they chose the SimScale simulation platform to perform two CFD simulations for identical designs, one with the active ventilation system and the other with the passive one.

A new approach in passive ventilation is using dual outer walls that guarantee fresh air supply and heat distribution across all rooms in a house. Through skillful planning, the air hull surrounding the building can be used to control temperature and air distribution without the installation of ventilators, only based on the stack effect. Convective flow effects can help to achieve both cooling in summer and heating in winter.

CFD Analysis comparing Active and Passive Ventilation

For the CFD analysis, the CAD model of the ventilation system was uploaded and meshed on the platform. Despite the complexity of the geometry, the full-automatic mesh operation for internal flows could be used.

Within a few minutes, a high-quality computational grid was generated. With the support of the SimScale team, a working simulation of the complex system was set up. The comparative analysis for the passive system could be derived within a few minutes from the existing simulation. HVAC simulation, which is part of SimScale’s CFD simulation offering has been used to perform this analysis.

Due to the availability of on-demand high-performance computing power on the SimScale platform (up to 32 cores) , the engineers from IBEEE were able to complete the simulation, which was ready for direct evaluation in the web browser.

The physical quantities within the flow field, i.e. temperature, pressure and velocity, were visualized as desired with cuts and streamlines and subsequently saved as image files. In the end, an automatic project documentation was created.


The simulation showed that the fan could not only be replaced by the stack effect, but that this is actually more powerful than the active ventilation. The flow rate with 205 m3/h for the passive ventilation is almost 40% larger than the one of the active solution. The reason is the mixing of the air at the entrance. This leads to a homogenization of the temperature and reduces the buoyancy effects.

For IBEEE, the overall costs of this simulation project was 50 € and the working time only took 2 hours.

If you’re interested in seeing more, read the original case study on our website.

Want to learn more about this topic? Read our previous article: What is the Difference Between Active and Passive Cooling?

[1] The New York Times, No Furnaces but Heat Aplenty in ‘Passive Houses’

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