www . refrigerationandaircon . co . za RACA Journal I March 2025 39
Support usually small and , in most cases , can be ignored as the amount of infiltration due to wind effect is usually greater than the amount of infiltration caused by stack effect . In residential buildings , air is often supplied through the facade and extract air is removed from selected rooms ( often kitchen and bathrooms ) through ducts . It can also enter through leakages in the facade . It is important to ensure an efficient air flow path through the building .
MECHANICAL VENTILATION Mechanical ventilation systems use electric fans to direct the airflow in the building . Mechanical ventilation can provide a constant air change rate independently of external weather conditions , but it uses electricity and usually cannot change the ventilation rate as the need changes over the day and year .
Several variations exist . Systems with both supply and extract can be combined with a heat recovery unit , which recovers by reusing the heat of the extracted air that would otherwise be lost . Up to 90 % of the energy can be ‘ reused ’. Mechanical ventilation requires filters to be changed regularly . Dirty filters are a source of pollution pertaining to the indoor air and reduces indoor air quality . This in turn reduces the performance of the occupants of the building and increases the prevalence of SBS symptoms .
If a mechanical ventilation system with heat recovery is to perform efficiently , the building must be perfectly airtight . If it is not , a substantial part of the ventilation will come from infiltration , which bypasses the heat exchanger . So mechanical ventilation with heat recovery is often not an energy correct solution for existing buildings . Mechanical ventilation systems can be central . Central systems have one central unit , with supply and extract fans . If the system has heat recovery , the heat recovery unit is included in the central unit . Ventilation ducts are installed from the unit to most rooms of the house .
Other ventilation systems do not use ducts ; instead , small units are installed in individual rooms of a house . Such a system has the advantage of not requiring space for ducts . Examples would be a bathroom or kitchen extraction systems . the air , and natural driving forces provide most of the airflow with fans used for assistance .
In reality , the need for ventilation changes constantly and the ventilation rate should be increased or decreased dependent on the demand . For example , how many people are present and what are their activity levels ?
Deon , I have investigated ventilation contextualised to your question . There are many more aspects to be considered . For example : ventilation can be used to cool an overheated environment , such as laundramats or a boiler room . The amount of people and activity levels in a space are critical to ventilation rate calculations , like a gym . There are of course regulations which stipulate minimum rates as per application , including hospital theatres , bars and restaurants .
Deon , I hope that this helps with your understanding around ventilation and solves your dilemma . RACA
Grant Laidlaw
REFERENCES :
1 . ACRA 2 . ( Bluyssen ) 3 . ASHRAE 4 . World Health Organisation
HYBRID VENTILATION Hybrid ventilation is a system that combines natural and mechanical ventilation and as such is a relevant solution for new buildings , especially if roof windows are available to facilitate stack effect . Several variations of hybrid ventilation systems exist . Hybrid ventilation can be used to optimise the indoor environment while reducing energy costs . This allows for open windows to be used which is appreciated by most users . It combines the best of both worlds : good winter energy performance with mechanical heat recovery ventilation , and good summer performance with natural ventilation .
The principle of fan-assisted natural ventilation is mainly used in larger commercial buildings where the natural driving forces are inadequate in some periods . A fan is therefore used for assistance .
The principle of stack- and wind-assisted mechanical ventilation is also used mainly in larger commercial buildings , where the ventilation system is designed with ducts to transport
www . refrigerationandaircon . co . za RACA Journal I March 2025 39