Automatic controls in buildings

Having 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:
  • Heating, ventilating and air conditioning systems;
  • Lighting installations
  • Domestic hot water systems
In some buildings there may already be a computer-based building management system (BMS) programmed to carry out some or all of the functions described in this article. In other cases, “stand-alone” controllers fulfil the stipulated requirements to varying degrees. This article applies whether stand-alone or BMS controls are used, and concentrates on desirable general principles.

Control of heating, ventilation and air conditioning

The purpose of an HVAC system is to provide:
  1. Comfortable conditions for the occupants of the building
  2. Protection of the building fabric, and installed equipment, from extremes of temperature (and possibly humidity) and the risk of condensation or freezing.
The ideal control system would ensure that service is provided to the standard required, but only at the times required, and only in the parts of the building where it is necessary. However, there is a fourth objective -- one which does not impinge on users -- and that is to manage the HVAC system ‘behind the scenes’ in such a way as to optimise its technical efficiency and minimise internal energy losses.

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:

Heating using central boilers and radiators (or finned-tube natural convectors, i.e. without fans) It is possible to use ‘compensated’ control whereby the circulating water temperature is modulated according to the weather, reducing heat output when the weather is mild and increasing it when it is cold. On some older installations this may be the only form of space temperature control. This type of control 'rations' heat output and does not, for example, increase the amount of heat delivered if windows are left open.
Central boilers with fan-assisted room convectors The circulating water temperature must be held constant. Heat output is usually regulated by individual room thermostats switching on and off the heater fans. Heaters with integral mechanical thermostats may not be well-controlled. It may be beneficial to upgrade to wall-mounted electronic thermostats, which can measure the room temperature at a more representative position and have a narrower "switching band". This avoids wide temperature swings and allows minimum temperature conditions to be maintained with less excursion to unnecessarily high temperatures.

Some users are tempted to modulate the circulating-water temperature so as to reduce heat losses from distribution pipework, and unnecessary heating by natural convection from heaters themselves. Unfortunately this does not work because when the convector fans run at low circulation temperatures, the resulting air flow feels like a cold draught.

Heating from central boilers, via heater batteries in central air handling units and air ducted direct to conditioned space. In this configuration, circulating water temperature is again held constant. Air temperature may be sensed either (a) Within the room(s) supplied or (b) in the room extract ductwork. The automatic control regulates the supply of hot circulating water to the heater battery.
Independent direct-fired space heaters Usually controlled by a local room-air thermostat, or a temperature sensor connected to a BMS. If the heater is a radiant type, a “black bulb” sensor should be used, since radiant heat achieves comfort at air temperatures lower than otherwise required.
Independent air cooling Usually controlled by a local temperature sensor or thermostat.
Cooling from central chillers with chilled water feeding convector batteries in the supply ductwork: ducted air supply from central air handling units direct to conditioned space The automatic control regulates the supply of chilled water to the battery, based on an air temperature measurement from a sensor located (a) within the supplied room space, or (b) in the air extract ductwork
Tempered air supply via ducts, with terminal heating/cooling Heating and cooling batteries in the supply air handling unit provide air at a constant temperature, controlled by a sensor in the supply ductwork downstream of them. Each room-air supply terminal is fitted with both heating and cooling coils. A local temperature sensor regulates their operation so as to increase or reduce the final air supply temperature.

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

  1. Fixed on/off times. Ideally this should be a regime which allows each day of the week to have its own pattern, with the option of temporary time extension and holiday omission.
  2. Optimum-start control (OSC): this provides a variable plant starting time depending on the estimated preheat or precool time required. The control system continuously monitors conditions during the ‘off’ phase and automatically postpones start-up to the latest time consistent with achieving desired conditions at the start of occupation. Most modern OSCs are self-adaptive and ‘learn’ the characteristics of the building to which they are fitted. Optimum-start regimes are more economical than fixed-start controls in theory, but must be correctly set up and appropriately programmed or they risk consuming more energy than necessary. There are various factors which can cause the self-adaptive procedures to increase preheat/precool times without limit. Users and maintainers sometimes misunderstand the OSC principle and program them wrongly by setting the start time to the old plant start time instead of the start-of-occupation time. This results in a building which is heated prematurely in the early hours of the morning. Note that such problems can occur with both stand-alone OSCs and BMS installations.

Zone control

This 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:

  1. There may be times when DHW service is delayed because the space heating requires only a short preheat period; or
  2. Space-heating preheat times will be excessive in order to guarantee a supply of DHW.
On central-storage DHW systems there may be savings from time-control of the DHW recirculation pumps as it is not necessary to keep the pipework hot outside occupied hours.

Lighting Controls

Although 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:

SituationControl options
No daylight available Time switching, occupancy sensing, and more localised manual switching should all be considered.
Daylight available, low occupation Because the lights are only rarely needed, daylight-sensing is unlikely to be worth paying extra for. Occupancy linking should be considered, perhaps using a “manual on, auto off” strategy.
Daylight available one or two occupants More localised switching should be beneficial, perhaps using a “manual on, auto off” strategy if occupation is irregular and intermittent. Daylight sensing may be warranted if the space is continuously occupied. Timed “off” control may be worthwhile if working hours are regular. (see below)
Daylight available, multiple occupation Timed “off” control is likely to be worth considering under all circumstances. This is a regime whereby a signalling pulse is sent out at key times (during lunch, after close of business, etc.) to turn off lights at work stations. A local reset switch (often a pull-cord) is provided at each work station to allow those still at work to restore their local lighting.

Photoelectric (PE) daylight linking may be worthwhile for spaces which are fully occupied during working hours, and which therefore need continuous lighting. PE control can be restricted to perimeter zones in deep-plan spaces.

Corridors Fully automatic on/off control using presence detectors.

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.

V.V. 20/11/05

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