01738 479 004Installing an MVHR (Mechanical Ventilation with Heat Recovery) system in a building is a smart choice for improving indoor air quality and energy efficiency. However, it's crucial to ensure that the MVHR system is properly sized to cater to the building's demands. An undersized system may result in inadequate ventilation and compromised air quality, while an oversized system may lead to excessive energy consumption and unnecessary costs. In this blog, we will guide you through the process of calculating the correct size of an MVHR system, taking into account factors such as building size, occupancy, and airflow requirements.
Assessing Building Size and Ventilation Needs
The first step in determining the correct size of an MVHR system is to assess the size of the building and its ventilation needs. Factors to consider include:
Total Floor Area
Measure the total floor area of the building, including all rooms, hallways, and communal spaces. This will provide an estimate of the volume of air that needs to be ventilated.
Ceiling Height
Higher ceilings may require greater airflow to ensure adequate ventilation, as there is more available space for air to circulate.
Room Usage and Occupancy
Different rooms within the building may have different occupancy levels and ventilation requirements. For example, rooms that are frequently used, such as living rooms or kitchens, may require higher ventilation rates compared to bedrooms or storage areas.
Building Type
Consider the type of building being ventilated. Residential buildings typically have different ventilation requirements compared to commercial or industrial buildings. Each building type may have specific regulations or standards to meet.
Calculating Ventilation Rates
Once the building size and ventilation needs have been assessed, the next step is to calculate the ventilation rates required for each space. Ventilation rates are typically expressed in air changes per hour (ACH) or liters per second (l/s) and indicate how many times the air within a space should be exchanged within a given period. Some common ventilation rate recommendations include:
Bedrooms: 0.5 to 1 ACH or 1 to 2 l/s per person
Living Rooms: 0.5 to 1 ACH or 4 to 8 l/s per person
Kitchens: 10 to 12 ACH or 15 to 20 l/s per square meter of floor area
These are general guidelines, and specific requirements may vary depending on local regulations and building standards. It is recommended to consult these regulations to ensure compliance.
Factors Affecting Ventilation Rate Calculations
The ventilation rate calculations should take into account several factors that may affect the actual airflow requirements:
Indoor Air Quality (IAQ) Standards
Higher IAQ standards may require higher ventilation rates to achieve a certain level of air quality.
Activities and Occupancy
Certain activities, such as cooking or exercising, may generate more pollutants, requiring increased ventilation rates during these times.
External Air Quality
In areas with high levels of outdoor pollutants or allergens, additional ventilation may be necessary to maintain good indoor air quality.
Seasonal Variations
In colder seasons, when windows and doors are more likely to be closed, higher ventilation rates may be needed to ensure sufficient fresh air exchange.
Sizing the Heat Recovery Unit
Once the ventilation rates have been determined, it's essential to size the heat recovery unit (HRU) of the MVHR system appropriately. The HRU is responsible for recovering and transferring heat from the outgoing stale air to the incoming fresh air, ensuring energy efficiency.
Sensible Heat Recovery Efficiency
The sensible heat recovery efficiency (SRE) of an HRU determines how effectively it can recover and transfer heat. It is expressed as a percentage, representing the proportion of heat transferred from the outgoing air to the incoming air. Higher SRE values indicate more efficient heat recovery.
Airflow Rates
To size the HRU, consider the total airflow rates required for both the supply and extract air. This will depend on the ventilation rates determined earlier in the calculations.
Heat Recovery Performance Curves
Consult the manufacturer's specifications and performance curves to ensure that the selected HRU can handle the calculated airflow rates and provide the desired heat recovery efficiency. Choosing an HRU that matches the calculated requirements will maximize energy efficiency and ensure optimal performance.
Professional Assistance and Local Regulations
Calculating the correct size of an MVHR system can be complex, especially when considering multiple factors and local regulations. It is advisable to seek professional assistance from an experienced HVAC engineer or consultant who understands the principles of MVHR and can guide you through the process. These professionals can conduct detailed calculations based on the specific building requirements and local regulations, ensuring compliance and optimal system performance.
Conclusion
Properly sizing an MVHR system is crucial for maintaining indoor air quality and energy efficiency in a building. By assessing the building size, ventilation needs, airflow requirements, and heat recovery efficiency, you can calculate the correct size of an MVHR system and select an appropriate heat recovery unit. Remember that the calculations should comply with local regulations and standards. Seeking guidance from HVAC professionals will ensure accurate calculations and optimal system performance. With the correct size of an MVHR system in place, you can enjoy the benefits of improved air quality, energy efficiency, and a comfortable indoor environment.