Constant speed vs Constant Volume

13th November 2018

An important consideration when choosing an MVHR system is deciding between Constant Volume fans and Constant Speed fans.

Constant speed fans will run at a predefined speed and do not adjust according to system pressure. The airflow rate will reduce as the system pressure increases. However, the system will use a constant amount of electrical power.

Constant Volume fans will maintain the commissioned airflow regardless of the system pressure (within limits). The fan speeds are automatically adjusted according to the system pressure. This can is done by monitoring the performance of the motor driving the fan. When there is a noticeable variation in airflow – caused by resistance in the system – the speed of the motor(s) is adjusted to maintain a constant volume of air.

The Brink Flair 325 has a new unique method for providing accurate control of airflow. This is achieved using an anemometer fitted to the discharge tube of each fan and enables a fast and very precise response to airflow requirements.

The use of constant volume fans helps ensure the desired performance levels remain consistent over time. This feature simplifies the procedure which ensures that the supply air and extract air rates are exactly balanced.

Many factors can affect the system pressure – blocked filters, change in wind pressure, blocked external vents and increased resistance in the ductwork.

When an MVHR system is running at an imbalance due to one of the factors above, it will result in positive or negative pressure in the dwelling. This will cause inefficiencies as some of the air will pass through the leaks in the building fabric rather than via the heat exchanger.

Having a balanced system not only reduces heat losses but will also prevent structural damage – especially in older buildings. There is a possibility that warmer, humid, indoor air can penetrate the structure of a building and, on reaching a colder external surface, can cause condensation leading to damage.