Constant Volume Fans: Detecting HVAC System Failures

The undetected failure of fan coil units is a long standing problem in heating and ventilation. Fan coil units like the types found in h&v applications tend to be unmonitored and system failures can often go unnoticed to the point where a refit or refurbishment is required or where air simply ceases to flow from the ductwork.

The oldest format for monitoring the FCU is to use differential pressure switches. An FCU would place a differential pressure switch on either side of the fan inlet and outlet and monitor the pressure difference between the two. Tubing from each sensor is connected to a mechanical switch and when a change in pressure is detected, the switch will open or close to raise an alarm. The use of pressure differential switches has its limitations, these can be summarised in the following points:

  • They are easily clogged by air born debris, requiring regular maintenance to ensure reliable operation.
  • The switch it self is mechanical and prone to corrosion, this will lead to ceasing and non-operation.
  • The switches can be time consuming to calibrate.

A more elegant solution is to monitor the current draw of the fans within a fan coil unit. A fan failure depending on its nature will often result in a drawn current spike or of course zero current. This requires the installation of additional components in the control circuit.

A part solution has been offered for a number of years utilising the tacho pulse output generated from the hall affect sensors found within many AC or EC motors. This is a limited solution however.  At failure the fan when used in a multi-fan system, will start to free wheel backwards, due to the pressure differential on downstream side of the plenum forcing air upstream past the fan. A tacho pulse will therefore still be generated and fan failure is not picked up (this limitation is applicable to multi-fan systems). Only one fan manufacture offers a solution to this problem that will help to keep fan systems running at maximum efficiency, for longer.

Constant Volume Industrial Fans

With the on board alarm now available in Ecofit EC motors, it is possible for Building Management system (BMS) to constantly monitor the air handling systems that are on the network. Allowing for the identification of the faulty air handling unit very quickly, no inspection of multiple assets is required to locate the fault. If required individual fans can be monitored depending on the requirements of the BMS specification.

This has the significant advantages when looking to resolve faulty FCU’s. Down time of the FCU is minimised through rapid identification of faulty assets, the labour required to resolve the issues is significantly reduced because faults are quickly identified, productivity of the work space is kept high because disruption to the work environment is reduced and because air quality is so vital to productivity the FCU is operational again very quickly.

The European fan manufacture Ecofit have developed a double inlet, forward curved centrifugal fan that not only features a highly efficient EC motor, surpassing ERP 2013 and 2015, but also has a built-in failure monitoring system, unique to their EC motor, to detect failures of individual fans or fans within a system. Along with all the other well know benefits of using EC motors over traditional AC motors.

The failures are monitored via a normally closed contact within the motor which will open when the fan speed drops below 200RPM but only when the fan is receiving a run command, so as the motors RPM falls below 200 the monitoring circuit is opened. This switch is controlled internally on the motors PCB by an opto-coupler. An opto-coupler is used due to its high resistance to inference from Electromagnetic radiation. In essence this device is used because it isolates the alarm circuitry from electromagnetic radiation that could cause false readings. The on board alarm system is such that multiple fans can be wired in series and powered either using the fans own 0-10v output or an external input from the BMS system, 48V 5mA max. The flexibility of EC allows for the customisation of the RMP “trigger” point and whether the system operates normally closed or normally open. This circuity within the fans printed circuit board will generate an alarm signal which the Building Management System will detect and respond accordingly.

For both the end user and the fan coil manufacture the on board alarm of the Ecofit EC motor hold many advantage. For the end user, reduced down times for maintenance, expensive building assets are running at optimum efficiency for longer and productivity of the work space is maintained. For the fan coil manufacture no additional components are required to monitor tacho outputs before integration with the building management system, meaning reduced costs, and quicker construction times and less complex assembly procedures.


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