What Is Electronic Feedback In HVAC Systems?

In discussions about electronic actuation of dampers and of control valves in HVAC systems, we often hear that a particular actuator has feedback capability or that a particular control scheme utilizes feedback.  But what does that mean and more importantly why is it beneficial?

The term “feedback” typically refers to a signal (usually a 0-10 VDC or 4 to 20 MA) that can be used to determine the exact position of a valve or damper.  This information can be very useful to facility managers that utilize a Building Automation System (BAS) to control and monitor building operations. 

The feedback signal is usually available as an output from the actuator to either monitor the actuator position or as a signal for part of a control sequence.  As such, it can alert facility personnel (via the BAS) of problems such as jam in the damper linkage that might otherwise go undetected for days or weeks.   Large facilities, such as hospitals or college campuses may have hundreds of valves and actuators, so troubleshooting these malfunctions can be a time consuming process of elimination in which facility workers must physically verify the valve or damper position at each piece of equipment. With feedback, the controls operator knows exactly where the failed operation is without leaving his or her desktop.

If a proportional actuator or three-point actuator with a 2-10 feedback is used, the signal can even be used to measure flow through the corresponding valve.  This value can also be displayed by a BAS, helping the user see when and where energy is being consumed, which permits further energy optimization.

Both pneumatic and electronic valves can be wired for feedback signals, although electronic actuators with position feedback provide more precise information.  The compressed air in pneumatic actuators creates the opportunity for more variance, while electronic feedback provides a repeatable, geared interface between the actual actuator position and its feedback monitoring system.

Belimo takes the advantages of feedback a step further with its Characterized Control Valves (CCVs) and Multi-Function Technology® (MFT). For most ball-type valves, flow typically starts at about 15º and reaches maximum at 85º.  However, with MFT the actuator is electronically adapted to the precise opening and closing points of the CCV so that the full 2-10V control resolution is used.  The result is a linear energy output and much more stable control.  The application of MFT with electronic feedback capability has been used successfully by a number of buildings, from commercial to schools.