C.T.E.HS 3.Conditions for a good design of the ventilation system

C.T.E. HS 3. Indoor air quality

The Technical Building Code in its Basic Document HS-3 establishes the essential conditions to guarantee the quality of indoor air in the rooms. First, it requires the use of hybrid or mechanical ventilation systems, discarding natural ventilation due to the low level of energy efficiency it provides, and because its operation is intermittent. Generally, it is limited by the external conditions of pressure and temperature. It also specifies the direction of air circulation indoors. The air renewal must be done from dry rooms (living room and bedrooms) to humid rooms (kitchens and bathrooms).

Conditions in the design of a ventilation system

A ventilation system must meet a series of characteristics that ensure its correct functioning, including: the noise level of the installation, energy savings, the thermal comfort level provided in each room, and the maintenance that the installation will require. The design of the ducts is also another very important factor that will condition its operation. Air is affected by resistance when passing through ducts and connection pieces such as elbows or changes in section. The fan must necessarily overcome this resistance or pressure drop that should be minimized through the design of the installation. For this, the following aspects must be taken into account:

• The minimum number of bends should be used, optimizing the path of the ducts
• 90-degree bends should be avoided, with 45-degree turns being preferable
• The bends should be rigid and have a smooth interior
• In general, the use of rigid tubes should be favored as their pressure drop is lower than in flexible tubes
• Sharp reductions or increases in the section of the ducts should be avoided
• Branching from a main duct should be avoided
• Instead, it is preferable to use two individual sections that connect directly to the extractor or to the intake points

Ventilation systems in high-rise buildings

When a ventilation system is going to be installed in a building of a certain height, there are several factors that must be considered before it becomes operational: its correct location, the diameters of the ducts, water tightness, and the acoustic insulation that prevents noise from transmitting to rooms. If the extractor is individual, that is, serving a single area or room, its outlet will occur directly at the roof level. If it is collective, that is, serving an entire floor or a very large area, the extraction pipe will be connected to a collective vertical pipe that must be calculated at low pressure. In this case, it is advisable to use anti-backflow devices and fire stops. Systems of dampening should also be employed to avoid the transmission of noise produced by vibrations.