Correctly sizing a Controlled Mechanical Ventilation system means determining how much air needs to be renewed within indoor spaces and choosing a system capable of providing the required airflow under actual operating conditions.
Let’s take a look at the parameters to consider when sizing a Controlled Mechanical Ventilation system, how to make an initial estimate of the required airflow rate and which aspects should instead be assessed by a professional.
What does sizing a Controlled Mechanical Ventilation system mean?
Sizing a Controlled Mechanical Ventilation system does not simply mean choosing the unit with the highest airflow capacity. It means determining how much air renewal the home requires and how that airflow should be correctly distributed throughout the different rooms.
A properly designed system should introduce fresh air into the rooms that need it and extract stale or more humid air, while keeping the system correctly balanced.
Sizing directly affects several aspects, including indoor air quality, humidity control, comfort, energy consumption and noise levels. For this reason, the choice of unit should always be part of an overall assessment of the home and its ventilation system.
To explore the available solutions, take a look at IRSAP Controlled Mechanical Ventilation systems.
What factors affect Controlled Mechanical Ventilation sizing?
Knowing the floor area of the home is not enough to estimate the required airflow. The calculation should take several factors into account at the same time, including indoor volume, number of occupants, room use and building characteristics.
Net volume of the home
Volume is one of the main starting points and can be estimated by multiplying the internal floor area by the average ceiling height:
Volume (m³) = floor area (m²) × average height (m)
For example, a 100 m² home with an average ceiling height of 2.70 m has an approximate volume of 270 m³.
This figure can be used to estimate how much air needs to be renewed over the course of an hour, but it is not sufficient on its own to determine the final system design.
Number of occupants
The number of people who regularly use the rooms also affects ventilation requirements. Breathing and normal everyday activities generate CO₂, water vapour and other indoor pollutants.
For the same indoor volume, a home permanently occupied by several people may therefore have different air renewal requirements from a home occupied by just one person or used only occasionally.
Room use
The purpose of each room affects airflow distribution. Fresh air is generally supplied to main living spaces such as living rooms and bedrooms, while air is extracted from areas more prone to humidity and odours, such as kitchens, bathrooms and laundry rooms. We will look at how this distribution works in more detail below.
Building insulation level
In modern buildings, thermal insulation is not the only relevant factor: building airtightness also plays an important role. High-performance windows and more airtight building envelopes reduce the uncontrolled air infiltration typically found in older buildings.
For this reason, air renewal needs to be managed appropriately rather than relying exclusively on occasionally opening the windows.
How to calculate the airflow rate of a Controlled Mechanical Ventilation system
To obtain an initial estimate of the required airflow rate, you can start with the volume of the home and the number of air changes required per hour.
In residential settings, approximately 0.5 air changes per hour is often used as an indicative reference value. However, this is not a universal figure that can be applied automatically: the actual airflow rate should be determined according to applicable technical standards, room characteristics and the system design.
The basic formula is:
Airflow rate (m³/h) = home volume (m³) × air changes per hour
Practical calculation example
Consider a home with the following characteristics:
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floor area: 100 m²;
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average ceiling height: 2.70 m;
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volume: 100 × 2.70 = 270 m³;
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assumed air change rate for the estimate: 0.5 air changes per hour.
The indicative airflow rate will therefore be:
270 × 0.5 = 135 m³/h
As an initial approximation, the system should therefore be capable of providing an airflow rate of approximately 135 m³/h under the expected operating conditions.
This result provides an initial indication of the airflow rate required by the home.
How is the airflow distributed across different rooms?
Once the total airflow rate has been determined, it is necessary to establish how much air should be supplied to or extracted from each room. This distribution enables the Controlled Mechanical Ventilation system to renew the air effectively throughout the home.
Supply air rooms
Fresh air is generally supplied to the rooms where people spend the most time, such as:
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living rooms;
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bedrooms;
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home offices.
The airflow allocated to each room should be determined according to its characteristics and how it is used.
Extract air rooms
Air is generally extracted from areas where humidity and odours are most likely to be produced, including:
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bathrooms;
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kitchens;
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laundry rooms;
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other utility rooms or particularly humid areas.
This allows air to move through the home along a controlled path before being discharged outside.
Why is system balancing essential?
In a balanced Controlled Mechanical Ventilation system, the total supply airflow and total extract airflow must be correctly balanced according to the system design.
It is therefore not enough for the unit to have an adequate nominal airflow capacity. Air terminals and the distribution network must also be correctly adjusted so that each room receives or extracts the intended amount of air.
Balancing is particularly important in dual-flow systems.
Does sizing vary from one home to another?
Yes. Two homes with the same floor area may require different configurations. For example, an apartment with one bathroom will have different requirements from a house of the same size with two bathrooms and a laundry room. Similarly, a home occupied by a large family will require different considerations from one occupied by only one or two people.
What happens if a Controlled Mechanical Ventilation system is incorrectly sized?
Incorrect sizing can compromise system performance. The consequences vary depending on whether the system is undersized for the home's requirements or unnecessarily oversized.
Undersized system
If the Controlled Mechanical Ventilation system cannot provide the required airflow rate, air renewal may be insufficient. As a result, CO₂, humidity and indoor pollutant concentrations may increase.
Inadequate humidity control can also encourage condensation and, where favourable conditions exist, contribute to mould growth.
Oversized system
Choosing a Controlled Mechanical Ventilation unit that is significantly more powerful than necessary does not automatically provide an advantage. An oversized unit may involve a higher initial investment and, if it is not correctly selected and adjusted, may also result in higher energy consumption or noise levels.
The key is therefore to choose a system capable of delivering the design airflow rates within an appropriate operating range, while also taking the characteristics of the air distribution network into account.
Can you size a Controlled Mechanical Ventilation system yourself?
A calculation based on indoor volume and hourly air changes can be useful for obtaining an initial estimate and understanding the approximate airflow rate required. However, it is not sufficient to design an entire system independently.
Final sizing must also take into account:
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length and layout of the ductwork;
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duct diameter;
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pressure losses;
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position and airflow rate of the air terminals;
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available fan pressure;
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noise levels;
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airflow adjustment;
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balance between supply and extract air.
For example, a unit rated for a certain nominal airflow will not necessarily provide that same airflow once connected to a duct network that creates resistance to air movement.
A DIY calculation can therefore be useful to get an initial idea of your requirements, while final system selection and sizing should be entrusted to a professional.
FAQ
How do you calculate the airflow rate of a Controlled Mechanical Ventilation system?
An initial estimate can be obtained by multiplying the volume of the home by the required number of air changes per hour. The result should then be checked against the characteristics of the building, individual rooms and applicable technical requirements.
How many air changes per hour does a home need?
Approximately 0.5 air changes per hour is often used as an indicative reference value, but this is not a rule that applies to every home. The actual value should be determined according to the system design and applicable requirements.
Does a larger home always need a more powerful Controlled Mechanical Ventilation system?
In general, a larger volume means more air needs to be renewed, but floor area is not the only factor. The number of occupants, room layout and use, and building characteristics all affect system sizing.
How do you size a decentralised Controlled Mechanical Ventilation system?
With a decentralised Controlled Mechanical Ventilation system, sizing is based on the individual room or rooms served by each unit, rather than on a centralised network serving the entire home. The required airflow rate and the specific characteristics of each room must still be assessed.
Can the system sizing be changed after installation?
In some cases, it is possible to adjust airflow rates and system balancing after installation. However, if the unit, ductwork or other components are fundamentally unsuitable for the home's requirements, more extensive work may be necessary. This is why correct sizing during the design phase is essential.
