ENERGY MANAGEMENT Fan speed control in vav systems:

a more energy efficient approach By Ian Thomas, Product Technical Manager – Air Flow Controls, TROX UK

Eecve control of fan speed is key to system eciency, and the latest generaon of Variable Air Volume (VAV) systems have made significant improvements in environmental performance possible.

approach based on fan speed optimisation using damper blade positioning.

H Traditional control methods

The fan speed control methodology most frequently applied in today’s projects is based on constant duct pressure control, with pressure transducers utilised to measure duct pressure and control fan speed. If demand reduces, the damper closes to reduce the volume flow. This causes system pressure to rise, following the fan curve. In constant duct pressure systems, the controller addresses this by reducing the fan speed and reducing the system pressure to drop back to the set point (275 Pa). See Figure 1. However, at a lower volume flow (to meet the building’s reduced demand), the set point of 275 Pa which is being maintained by the controller is no longer appropriate in terms of the characteristic fan curve. A set point of 125 Pa would be more appropriate, to follow the curve.

owever, by rethinking how to optimise fan speed in VAV systems, it is possible to achieve an additional 45% reduction in energy consumption. This article describes a proven

There are disadvantages associated with this method of controlling fan speed. Firstly, as constant duct pressure control forces a pressure drop to return system pressure to the set point of 275 Pa, the VAV units have to create the pressure that is not needed, involving a degree of energy wastage. Secondly, by creating a deviation from the characteristic curve (maintaining a set point of 275 Pa, rather than the 125 Pa that would better reflect the curve), constant duct pressure control forces the system to operate in its least effective working area (damper position below 40% open). This impacts on the acoustic performance of the system, elevating operational noise, and requiring higher energy consumption to meet the same load. See Figure 2.

Alternative approach

With the latest generation VAV technology, it is possible to rethink fan speed control using damper blade positioning to avoid the problems inherent in constant duct pressure systems. VAV units are now readily available with BMS protocols including BACnet ms/tp, Modbus RTU, KNX and LON. However most projects today install VAV controllers with analogue 1-10v communication, and fit a local digital BMS controller to control the VAV via analogue signals. This limits the amount of data that can be communicated.

Alternative best practice is to use a digital VAV controller, making all data available digitally over the BMS network. As digital control technology provides the opportunity for increased levels of data management in comparison to analogue technology, the position of the damper blade itself can be used to indicate the appropriate fan speed to meet demand. This eliminates the need for the fan speed to be controlled by a pressure transducer, meaning that duct pressure does not have to be measured and maintained at an unnecessarily high set point. By contrast, as demand falls, both fan speed and pressure can fall together, following the characteristic curves of the devices, as shown in Figure 3.

Benefits of alternative approach

Firstly, enabling the fan speed and pressure to reduce uniformly along the device’s characteristic curve harnesses the potential of the fan laws

(power is proportional to rotary speed cubed: P  N3), reducing energy costs throughout the equipment’s lifecycle. Installations employing this approach to date indicate that fan energy consumption can be reduced by around 45%.


Secondly, the damper blade position will be between 40% and 100% open at this lower pressure point, facilitating better control of the VAV units and improving acoustic performance. Lastly, controlling fan speed in this way removes complexity during commissioning, avoiding the difficulties associated with the positioning of pressure transducers in the duct. This approach also provides a more stable control circuit on which to design and operate the system.

Control technology

It is necessary of course to have controls capable of monitoring and acting upon the damper blade positioning data. One option is to use the central Building Management System (BMS) to fulfil this function. However, this can involve complex (and therefore expensive) bespoke programming. A less complex and more cost-effective option is to harness the digital control capability already resident in the VAV systems themselves. A number of standard TROX products and solutions, for example, have integral control technology able to facilitate fan speed optimisation without the need for bespoke BMS programming. TROX’s X- CUBE air handling unit, for example, incorporates the option for fan speed optimisation via damper blade positioning as standard. Connecting to a centralised BMS (via Modbus TCP, Ethernet or BACnet), it facilitates room-by-room air management and adjusts fan speeds automatically in line with changing requirements. TROX’s X-AIRCONTROL system is another example. X-AIRCONTROL’s main focus is the individual, demand-based control of air volume flow rate and circulation on either a room-by- room, or zone-by-zone basis. Fan speed is controlled based on the damper blade position of the VAV terminal units for reduced energy consumption, improved acoustic performance and simplified installation and commissioning. To conclude, fan speed control strategies based on constant duct pressure control are no longer a necessary evil with the latest generation of digital VAV controllers. Utilising damper blade positioning to indicate the appropriate fan speed delivers significant benefits by enabling both fan speed and pressure to fall together, following the characteristic curves of the devices, when demand reduces. This avoids energy wastage associated with maintaining a higher set point, ensures that devices within the system are operated within the more efficient section of their operating range, improves acoustic performance, and provides a more stable control circuit.

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