HVAC controllers are used to monitor and control the performance of heating, ventilation and air conditioning systems (HVAC) in buildings. They monitor indoor environmental factors such as humidity and temperature, and control heating and cooling to achieve and maintain desired levels. HVAC controllers receive inputs from thermostats and humidity sensors, and send output to HVAC chillers and other HVAC systems. Most industrial thermostats are timed, programmable, and/or remote-controlled. The user interface for an HVAC controller may consist of a digital front panel with a menu or keypad, or include analog components such as knobs and switches. Computer-programmable, web-enabled, and Ethernet-ready HVAC controllers are commonly available. Some HVAC controls include a programmable logic controller (PLC). Others feature human machine interface (HMI) software or provide supervisory data acquisition and control (SCADA) functionality. An HVAC controller may also provide rate indication, data logging, and totalizing capabilities.
HVAC controllers use several different control techniques. Examples include limit control, linear control, PID control, feedforward control, fuzzy logic, and advanced or non-linear controls. Limit control establishes set points or limits that, when reached cause the HVAC controller to send a signal to stop or start a process variable. Linear control matches a variable input signal with a correspondingly variable control signal. Proportional, integral, and derivative (PID) control requires real-time system feedback. Feedforward control provides direct-control compensation from the reference signal. Fuzzy logic is a type of HVAC control in which variables can have imprecise values (as in partial truth) rather than a binary status (completely true or completely false). Advanced or nonlinear controls for HVAC controllers use algorithms such as adaptive gain and neural networking.
Parameters for HVAC controllers include form factor and mounting style, number of inputs, number of outputs, input types, and output types. Some HVAC controls have a printed circuit board (PCB) form factor. Others are designed for mounting in a rack, on a wall, or with a DIN rail. Stand-alone HVAC controllers are benchtop or floor-standing units with a full-casing or cabinet and an integral interface. HVAC equipment suppliers specify the number of inputs as the total number of signals sent to the HVAC controller. The number of outputs is the total number of signals used to control, compensate or correct the heating or cooling process. Input types for HVAC controllers include: direct current (DC) voltages, current loops, analog signals from resistors or potentiometers, frequency inputs, and switch or relay inputs. Output types include analog voltages, current loops, switch or relay outputs, and pulses or frequencies. Some HVAC controllers can also receive inputs or send outputs in serial, parallel, Ethernet, or industrial fieldbus formats.