BEMS Terminology

Below is a list of some of the BEMS Terminology.


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

Boost period

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

Cascade control

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.

Clean Contact

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

Control Function

Term used to describe type of control, for example, compensator, night set back, et cetra.

Control parameter

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

Dead time

The time required for a response in the controlled variable to be observed following a step change in input to the controller

Dead zone

See Deadband (2)

Default value

Stored value of a control parameter or other system variable to which the system reverts in the absence of an instruction

Derivative action time

Time taken for the proportional term to match the derivative term of a PID controller when the error changes linearly with time

Derivative control

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

Desired value

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

Distributed intelligence

A system where data processing is carried out at local controllers or outstations as well as at the central station

Duty cycling

A control method which alternates or cycles the sequence of plant use to ensure standby plant is brought into use.

Economiser control

Controlled mixing of recirculated and outdoor air to give economy in heating or cooling costs

Enthalpy control

Controlled mixing of recirculated and outside air based on their enthalpies. Only used in association with humidity control of supply air

Exhaust air

Air which is discharged to outside

Extract air

Air which is extracted from an occupied zone

Feedback control

Also called closed loop control, where a measure of the controlled variable is fed back to the controller

Feedforward control

See open loop control

Floating action

A control mode where the output signal changes at a steady rate when the input signal falls outside the deadband

Free cooling

(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

Integral action time

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

Integral control

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

Load error

The difference between the set point and the steady state value of the controlled variable of a proportional or other controller

Night set back

The reduction in set point of a heating system during unoccupied hours

Open loop control

A control system where the controller does not receive information on the results of its actions cf. feedback control

Open protocol

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

Optimum start

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

Optimum stop

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

Proportional band

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

Proportional control

Control action where the control output is proportional to the deviation of the deviation of the input from the set point

Proportional gain

Reciprocal of proportional band

Recirculated air

Air which has been extracted from a zone and which is mixed with outdoor air to make up the supply a i r

Reset action

Integral control action

Reset control

Non-standard term for cascade control

Reset rate

Inverse of integral action time


The smallest change in input signal that can be discriminated in a device

Return air

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

Set point

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

Supply air

Air supplied by a central air handling unit to the occupied zones

Three term control

Another term for PID control

Throttling range

See proportional band

Time constant

The time required for a quantity to reach 63% of its new final equilibrium value following a change or disturbance.

Unitary controller

Device which controls and/or monitors a single piece of equipment

Weather compensation

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