Calculating the Correct Size of an MVHR System • Brookvent

The correct sizing of an MVHR (Mechanical Ventilation with Heat Recovery) system is crucial to ensure optimal performance, compliance with regulations, and the delivery of high-quality indoor air. This blog aims to highlight the significance of selecting the appropriate MVHR unit and ducting system, as well as provide guidance on how to size an MVHR unit effectively.

Importance of Correct Selection:

When it comes to MVHR installations, it is essential not to rely on a one-size-fits-all approach. Simply picking up any MVHR unit from a local builder’s merchant without proper consideration can lead to significant issues. Untrained installers often make this mistake, resulting in inefficient systems that fail to meet the required standards.

Airflow Calculation:

To ensure the correct unit is selected, it is imperative to perform a comprehensive airflow calculation. In both the UK and Ireland, Part F of the building regulations mandates a general ventilation rate of 0.3 litres per second per square meter of floor area. Failure to achieve these ventilation rates can have severe consequences, including noncompliance, noise problems, reduced efficiency, shortened lifespan, ineffective humidity control leading to mold and dampness, and poor indoor air quality. Undersized units are a common cause of such issues.

Oversizing Concerns:

While undersized units pose numerous problems, oversizing can also be wasteful from a financial perspective. Although oversized units can achieve the required airflows, investing in an appropriately sized unit is a more efficient and cost-effective option.

Noise Issues:

Undersized units operating near capacity often result in excessive noise levels. Occupants may choose to turn off the unit to mitigate the noise. However, in an airtight property, switching off the MVHR unit can quickly lead to the emergence of mold and condensation issues.

Selecting the right-sized MVHR unit is paramount to the success of the installation. It ensures efficient operation, optimal performance, and a healthier indoor environment for the occupants.

How to Size an MVHR Unit:

Sizing an MVHR unit correctly requires the expertise of a suitably qualified individual. In Ireland, the system designer should be considered a “Competent Person” who can demonstrate their competence through professional qualifications or City & Guilds certification.

Determining the Required Airflow:

The first and most crucial step in sizing an MVHR unit is to establish the required airflow. As mentioned earlier, a general/trickle ventilation rate of 0.3 liters per second per square meter is necessary for all properties. In Ireland, an additional requirement demands a unit capacity of 25% above the general/trickle ventilation rate.

Reading MVHR Unit Performance Data:

To make an informed selection, it is vital to examine the performance information provided for each MVHR unit. The performance data includes details about the unit’s ability to move air under specific pressures. A well-designed residential ventilation ducting system typically maintains a duct pressure of approximately 150 Pascal (PA). Although the actual pressure may vary depending on factors like size, complexity, and installation quality, this rule of thumb allows for correct unit sizing. Below is an example of the Brookvent Aircycle 3.1.

Taking the Brookvent Aircycle 3.1 as an example, if we consider a static pressure of 150 PA, we can achieve an approximate airflow of 100 liters per second. However, it is crucial not to specify this unit for a property requiring 100 liters per second in general ventilation. Running an MVHR unit at maximum capacity can lead to noise issues and system failure. To avoid such problems, Irish building regulations recommend specifying units with at least 25% capacity above the general ventilation rate.

Capping the Unit Size:

By limiting the size of the property in which the Aircycle 3.1 unit is installed to 69 liters per second or a house of 230 square meters, several benefits can be ensured. Firstly, it allows for the achievement of required airflows even if the duct pressure exceeds 150 PA slightly. Secondly, running the unit at a medium speed with efficient heat recovery reduces noise levels and prevents premature fan burnout. Acoustic data provided by manufacturers also aids in determining suitable unit installations.

Compliance and Commissioning:

Compliance with Part F of the building regulations is essential for a successful MVHR system. Designers must be well-versed in the requirements outlined in Part F, which include not only the necessary airflows for each room but also the maximum permitted noise levels. The noise thresholds specified in Part F are 30 decibels (dB) for habitable rooms, 35 dB for wet rooms, and 35 dB for breakout noise. Even with the correct choice of an MVHR unit, poor ducting design or substandard installation can compromise the system’s performance and compliance. Therefore, a comprehensive evaluation of the MVHR system is crucial, considering all interdependent elements, designs, and specifications. The commissioning of an MVHR system must be performed by a competent person. Even a well-designed unit and ducting system require proper balancing to ensure the correct airflow in each room. Commissioning engineers must be familiar with the unit controller and provide the customer with a commissioning document that verifies the achieved airflows and demonstrates the correct specification of the installed unit. For more information check out the GOV.UK Statutory guidance

The correct sizing of an MVHR system is vital for optimal performance, compliance with regulations, and maintaining a healthy indoor environment. Understanding the required airflow, reading performance data, and adhering to building regulations are essential steps in the sizing process. By selecting the appropriate MVHR unit and ensuring proper commissioning, compliance with Part F can be achieved, providing occupants with comfort, efficiency, and improved indoor air quality.

If you have any further questions or need assistance, please feel free to contact Brookvent at hello@brookvent.co.uk.

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