Book - School of Science and Technology

1 2 3Building regulations 623(a) Straight pattern(c) Angle pattern with remote sensor1 2 3(b) Angle pattern(d) Single entry horizontal patternFigure 22.2 Thermostatic control valves for radiators, etc.non-linear characteristics. Electronic systems provide accurate control and, being freefrom mechanical parts, are now very reliable. With the reduction in the cost of microprocessors,software logic, or `intelligence' is now being built into some field mountedsensors themselves. Where software logic is built into the controller the system may bedescribed as direct digital control (DDC).For large installations, electric/electronic systems of control are by far the most common,and may be applied to perform any number of desired functions in a great variety ofways. The simplest electrical system comprises an on±off thermostat connected to anelectrically operated valve or damper such that, when the thermostat calls for heat, thevalve or damper is opened and, likewise when satisfied, the valve or damper is closed.More sophisticatedly, modulating controls enable, for instance, a mixing valve to be soadjusted automatically that it finds some mid-position and so supplies the desired flowtemperatureto the system. For example, for a quick heat-up in the morning, the controlsystem may be so arranged as to call for full heat for the first hour or so according toexternal weather, after which the water temperature will be reduced proportionately to theexternal temperature.The wiring to electrical controls and to the circuit boards for electronic controls,together with the associated switches, indicator lights and displays can become complexand depending on the manufacturers range it is usual to centralise these components in apre-wired control panel. This panel may also accommodate remote temperature andhumidity indicators and recorders and other instruments if required; motor starters mayalso be installed into such panels. It is essential that all components within and on the faceof control panels are permanently and accurately labelled, and that all items required tobe read for normal operation, can be read without needing to open the panel. With thedecreasing cost of electronic processing power, many controls manufacturers are movingto intelligent controllers which can be connected by a data bus, whilst being located locally