Automatic controls in buildingsHaving appropriate automatic control systems, and keeping them correctly programmed, is one of the best ways of ensuring economy in the use of utilities within buildings. Automatic controls can and should be applied to the following building services:
Control of heating, ventilation and air conditioningThe purpose of an HVAC system is to provide:
Control of temperature is the prevalent focus of concern in most HVAC systems, since failure to control temperature has the most direct impact on occupants. How the temperature is controlled depends on the type of heating system used:
Control of humidity: this is only provided in mechanically-ventilated systems. Relative humidity is best controlled from measurements sensed in the extract air duct from the conditioned space. This ensures that a representative sample of room air is used.
When the relative humidity in the space falls below the lower control threshold, the two simple strategies are either to introduce water vapour by means of a humidifying spray in the air supply duct, or, if there is the facility for recirculating the extract air, to increase the proportion of air recirculated (within appropriate limits). The latter option, even if available, may not be acceptable if the recirculated air would introduce unwanted heat, and therefore increase the demand for cooling. See remarks below on enthalpy control.
When the relative humidity in the space exceeds the upper threshold level, the simple control options would be: either to reduce the proportion of recirculated air (if applicable), or to actively dehumidify the air supply.
Active dehumidification entails first chilling the air to reduce its moisture content (surplus moisture condenses out) and then reheating the air to the required supply temperature.
Enthalpy control is a complex strategy which optimises the heating, chilling, and recirculation of air in order to achieve the desired internal temperature and humidity for a given outside air condition. For example, it may be necessary to dehumidify the air supply when it is cold outside. The outside air will have low moisture content, and reducing the amount of recirculation will enable this dry air to dilute the humid supply air - but at the cost of increased heat requirement. The alternative is to chill and reheat the humid air supply as described above. The enthalpy control system chooses a balance setting which minimises the energy requirement.
Time control For any building which is not continuously occupied, there are savings to be had from ensuring that service is only provided when needed. For example in mechanically-ventilated buildings, the supply of fresh air could be minimised, when the building is unoccupied, by switching off supply and extract fans. If there are recirculation fans these should also be shut off, but may need to be brought back into service ahead of the start of occupation, in order to ensure effective preheating (or precooling) of the conditioned spaces. As for the supply of heating (or cooling) the time-control regime can be either
Zone controlThis can be provided on hot-water-circulating systems by means of electrically-actuated isolating valves. It tends to be a relatively costly option because of the need to break into pipework to fit the valves, plus the cost of the control wiring. However, in large buildings with multiple pumped circuits, it may be easier because circulating pumps can be idividually controlled. The control logic should be arranged such that the boiler plant shuts off when no circuit pumps are running (including the DHW circuit if served from the same set of boilers). Beware, however, the potential for reverse flows in the idle circuits, caused by the return legs of active circuits being at slightly elevated pressures.
Internal control of HVAC plant
There are certain aspects of control of heating and air conditioning which have no impact upon the delivered service, but which can result in significant economies in operation.
For example, many heating systems in commercial premises use multiple heating boilers. To keep all the boilers live and in service all the time is uneconomical because of the high proportion of standing losses which will result. It is desirable to automatically shut down and isolate all surplus boiler capacity (similar considerations incidentally apply to chillers, fan-assisted cooling towers, and any other multiple-unit plant). Indeed there are likely to be long periods on many days when the building’s required internal conditions are satisfied, and no additional heating is required. There should be no need to have boilers idling under these circumstances; their scheduling control should be programmed to stand them all down when no zones are calling for heat.
A further opportunity may present itself in the control of air supply and extract fans where the air supply and extract rate is variable. If the air is supplied by fans running at fixed speed, with supply volumes regulated by dampers, it may be economical to fit a variable-speed drives. This is because the throttling effect of a partly-closed damper introduces energy losses which can be avoided if the dampers are left fully open and air supply rates varied by slowing down or speeding up the fans.
Oxygen-trim control is a specialised technique designed to ensure that combustion efficiency is maintained at optimum levels under all operating conditions.
Frost protection: special consideration should be given to frost-protection strategies, which can be a classic cause of hidden loss. One of the least economical frost-protection strategies is to use the outside air temperature as a trigger. A well-insulated building should remain well above the outside air temperature for a long period after the heating has been turned off. It is preferable to use the internal space temperature as the trigger for frost protection. This has the dual advantage that (a) the use of heat will be postponed until actually necessary and (b) internal sensing will regulate the application of heat to the minimum necessary to maintain the building safely above freezing point.
‘Two-stage’ frost protection is sometimes used on central-boiler heating systems. This initiates the heating pumps only (without the boilers) when internal temperatures drop to a certain level. Active heating is only called for if the space temperature drops to a critical level.
Domestic Hot Water Control: time control of DHW is usually on a fixed seven-day pattern. Where DHW is provided from a central boiler installation which also serves space heating using optimum-start control, the DHW’s timing must be separate from the heating’s time control and the boiler control will need to sense whether either the heating or DHW require heat. If this is not done, then either:
Lighting ControlsAlthough it is possible to give general advice on lighting control, it is strongly recommended that reputable suppliers or advisers are consulted on the design of new control schemes. This is because of the risk of very visible failure and consequent wasted investment.
The broad recommendations for various indoor lighting scenarios are as follows:
In large spaces it may sometimes be beneficial to have two or more separately-controlled lighting circuits using distinct control strategies. For instance in a large open plan office, the main space could be on semi-automatic control while circulation areas are on fully-automatic presence detection and perimeter areas on daylight sensing.
Two overlapping circuits with the same control strategy may also be beneficial. This might for instance occur in an area with discharge lighting, where a staggered “off” signal would give one set of lamps time to cool off ready to restrike quickly once the area has been put into total darkness by the second circuit shutting down.
Nuisance switching - whether ‘on’ or ‘off’ must be a serious concern because it is the feature which people most remember. Appropriate technology (e.g. the right choice of sensors), adequate technical features (e.g. passing-cloud delays) and proper commissioning (e.g. careful setting of lux-level thresholds) are all important.
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