Ventilation system design in a home

Ventilation systems in a home: hybrid and mechanical ventilation

Homes, in compliance with the Technical Building Code, must have a ventilation system, either mechanical or hybrid. A hybrid ventilation system renovates the air naturally when the outdoor temperature and pressure conditions allow. When they don’t, it uses mechanical fans. On the other hand, mechanical ventilation systems are ventilation systems in which air renewal occurs through the operation of electromechanical devices.

Extraction ducts for mechanical ventilation

The design of a duct is primarily based on three principles that condition both its design and sizing. The duct network must be projected in such a way that the entire airflow is transported to the extraction and supply points. The three fundamental principles are:

1. Air properties: vary depending on temperature and pressure. The elementary properties used for duct design are air density and its viscosity.
2. Pressure losses: within the duct, there is a drop in air pressure due to friction, also known as pressure loss. These occur within the duct itself and also at obstacles that the air encounters along its path, such as bends or changes in section.
3. Equivalent duct diameter: ducts in the ventilation system can have different types of cross-sections: oval, circular, or rectangular. The equivalent diameter is used to determine the cross-section of the circular duct, equivalent to the one being studied.

Each extraction duct will have next to the last extraction outlet a mechanical extractor. Several ducts can share a single extractor, except when ventilating a vehicle parking area. As long as the ducts are located adjacent to a habitable space, the nominal cross-section of each segment of the extraction duct will be at least equal to that obtained by the formula:

S ≤ 2.5 x qv

where qv is the airflow in the duct segment (l/s)

In vertical ducts, the following specifications must also be met:

1. The cross-section must be constant in each segment of the duct between two consecutive points where air is supplied or extracted
2. The finish of the ducts must make it difficult for the walls to get dirty and at the same time they must be practicable
3. To avoid condensation when the dew point temperature is reached, the ducts must be thermally insulated
4. The ducts must be air-tight to maintain the sizing pressure

In the case where ventilation ducts are located in a roof, the cross-section must be at least equal to that obtained by the formula:

S ≥ 1.5 x qv

where qv is the airflow in the duct segment (l/s)

By using this formula, the extraction of flows in each area is kept constant regardless of the building height.