American Society of Heating, Refrigeration and Air Conditioning Engineers
The Building Automation and Control networking communication protocol: ANSI/ASHRAE Standard 135-1995. BACNet is an open protocol
Building management system. A building-wide network which allows communication with and control of items of HVAC plant. May also include other systems such as lighting
The period when plant is operated at full capacity, normally in the preheat period before occupancy
A common communications link which carries messages between devices (nodes) which are connected to the bus. The term field bus refers to low level communications
A control system consisting of a slave controller controlling a system variable and a master controller controlling the set point of the slave. Also known as reset control.
A contact not connected to any voltage source. Also called a voltage free contact
Normally implies weather compensation - a control device whose function is to vary flow temperature based on outside air temperature
Term used to describe type of control, for example, compensator, night set back, et cetra.
A preset variable used in a control algorithm, e.g. width of the proportional band
(1) The range of values of the controlled variable within which a floating controller produces no change in control action, also called neutral zone (2) The range of values of the controlled variable in which HVAC plant is off, e.g. the interval between heating and cooling operation (3) The range between upper and lower switching values for a twoposition controller. Better termed the differential
The time required for a response in the controlled variable to be observed following a step change in input to the controller
See Deadband (2)
Stored value of a control parameter or other system variable to which the system reverts in the absence of an instruction
Time taken for the proportional term to match the derivative term of a PID controller when the error changes linearly with time
A control method in which the control output signal is proportional to the rate of change of the controlled variable. Normally used in conjunction with other modes, e.g. PID
The desired value of the controlled variable. Not necessarily the same as the set point
The difference between the set point and the actual value of the controlled variable
The difference between the higher and lower operating levels of an on/off controller
A system where data processing is carried out at local controllers or outstations as well as at the central station
A control method which alternates or cycles the sequence of plant use to ensure standby plant is brought into use.
Controlled mixing of recirculated and outdoor air to give economy in heating or cooling costs
Controlled mixing of recirculated and outside air based on their enthalpies. Only used in association with humidity control of supply air
Air which is discharged to outside
Air which is extracted from an occupied zone
Also called closed loop control, where a measure of the controlled variable is fed back to the controller
See open loop control
A control mode where the output signal changes at a steady rate when the input signal falls outside the deadband
(1) The use of outside air in an economiser control system to minimise or avoid the use of chiller plant (2) The provision of chilled water from a cooling tower or dry cooler, without the use of chiller plant
Another term for the supervisor of a BMS
A control state where the system does not settle to a steady value but oscillates about the set point
The time it takes for the integral term of the control output equation to match the offset caused by the proportional term on a step change in error
A control mode where the output signal changes at a rate which is proportional to the deviation of the input from the set point. Usually found combined with proportional control
The difference between the set point and the steady state value of the controlled variable of a proportional or other controller
The reduction in set point of a heating system during unoccupied hours
A control system where the controller does not receive information on the results of its actions cf. feedback control
A protocol available for use by anyone. It may be protected by copyright and licence fees may be charged for its use. Changes in an open protocol may be made at any time by the original author
A control system employing an optimum start algorithm
A control system or algorithm which starts plant operation at the latest time possible to achieved specified conditions at the start of the occupancy period
A control system or algorithm which stops plant operation at the earliest possible time such that internal conditions will not deteriorate beyond preset limits by the end of the occupancy period
A device connected to sensors and actuators which can perform local control and other functions. It can exchange information with the central station
Control incorporating proportional, integral and derivative actions
The range of input values corresponding to the full range of output variation of a proportional controller. May be expressed in actual units (e.g. temperature) or as a percentage of the full input range of the controller
Control action where the control output is proportional to the deviation of the deviation of the input from the set point
Reciprocal of proportional band
Air which has been extracted from a zone and which is mixed with outdoor air to make up the supply a i r
Integral control action
Non-standard term for cascade control
Inverse of integral action time
The smallest change in input signal that can be discriminated in a device
Air extracted from a zone and returned to the main air handling plant. Not necessarily the same as recirculated air. Extract air is preferred term
A controller which controls the operation of multiple boilers or chillers, both for modulation and duty cycling
The value to which a controller is set to achieve the desired value of the con trolled variable
The user interface for a BMS. Also called head end
Air supplied by a central air handling unit to the occupied zones
Another term for PID control
See proportional band
The time required for a quantity to reach 63% of its new final equilibrium value following a change or disturbance.
Device which controls and/or monitors a single piece of equipment
The control of heating medium temperature as a function of external temperature
An excessive or out-of-range control action produced by an integral action controller following start up or other period when there has been a deviation from the set point
A part of a building which is controlled independently of other zones