Energy Efficiency E-modules - Guidance Efficient Operation of HVAC Controls in the Public Sector
Efficient Operation of HVAC Controls in the Public Sector 2 Contents 1 Introduction 3 2 Learning Objectives and Outcomes 3 2.1 Learning Objectives 3 2.2 Learning Outcomes 3 3 HVAC Controls Overview 4 3.1 The Scottish Building Standards 4 3.2 Types of Control Systems and Features 5 3.2.1 Time clocks 5 3.2.2 Seven day multi-event programmable controller 5 3.2.3 Optimised Start 5 3.2.4 Weather Compensation 6 3.2.5 Additional Control Features 6 4 Surveying Building Controls 7 4.1 Energy Data 7 4.2 Setpoints and Outputs 7 4.3 The Design of the Control System 8 4.4 User Display and Physical Control Components 8 4.5 User Engagement 9 5 Efficient Operation of HVAC Controls in the Public Sector 10 5.1 Maintenance 10 5.2 Changes to the Control System 10 5.3 Designing the Control System 10 5.3.1 Zone Control 10 5.3.2 Simultaneous heating and cooling 10 5.3.3 Internal Sensors 11 5.3.4 Occupancy control 11 5.3.5 Building Energy Management Systems 11 5.3.6 Other Design Considerations 11 5.4 Commissioning of Building Controls 11 5.5 Additional Features 12 5.5.1 Variable Speed Controls 12 5.5.2 Boiler Sequencing Control 12 6 Useful Links and References 13
Efficient Operation of HVAC Controls in the Public Sector 3 1 Introduction This guidance follows the format of the e-module Efficient Operation of HVAC Controls in the Public Sector and provides further details on the subjects covered in the module. Please note that module users working in a healthcare environment should always refer to the relevant Scottish Health Technical Memorandum (SHTM) prior to considering installation of the measures suggested in the module. The advice given in the SHTM may conflict with the advice given in this module, as it has been developed for the wider public sector. The relevant SHTM can be found on the Health Facilities Scotland website. 2 Learning Objectives and Outcomes 2.1 Learning Objectives The learning objectives for this module are to: Understand the different types of HVAC controls and their applications; and Understand typical issues which can lead to poor control of HVAC systems. 2.2 Learning Outcomes The learning outcomes for this project are to: Describe the basic principles explaining how different HVAC control systems work; Identity where different control systems are/should be used in Scottish public sector sites and buildings; Describe the main HVAC control technologies, including their typical capability and the advantages and disadvantages of each technology; Be able to identify basic HVAC control errors or malfunctions; Be able to carry out basic HVAC control alterations to meet the requirements of public sector buildings; Be able to prioritise the opportunities for improving control systems in public sector buildings; and Evaluate the key aspects in relation to HVAC control projects when building a business case.
Efficient Operation of HVAC Controls in the Public Sector 4 3 HVAC Controls Overview The HVAC controls, like any other aspect of building control, should ensure that a building operates safely and efficiently. Any control system operates essentially in three basic parts with: A sensor measuring some variable e.g. temperature; A controller which develops an output signal in response to the measured quantity; and A controlled device which is attempting to achieve some control set point. It is helpful to understand the main elements and features of a modern control system as part of being an effective energy manager is being able to spot when something has gone wrong with a control system. To consider how the building efficiency can be maximised, it is important to realise that alterations to, and failures in, building controls can have impacts on building user safety and comfort. Building control is a complex and specialist field so where there is uncertainty about what action to take in relation to a controls problem, it is always best to seek specialist guidance. Those control functions related to safety must always take precedence. One further point to consider is the potential for building user control. There is an ample body of evidence to suggest that building users experience a great improvement in physical and mental wellbeing where some level of user control is available. Where this is present, staff can frequently make do with or tolerate less than perfect environmental conditions, whereas if this is absent it can lead to continuous complaints and frequent readjustment of control with little perceived benefit. 3.1 The Scottish Building Standards The Scottish Building Standards sets out the minimum standards of control package for different types of HVAC systems. However it is important to realise that things can easily go wrong with control systems even when they are well designed and commissioned. Table 3.1 outlines the selected control requirements for boiler heating systems, taken from Section 6.6.4 of the Energy Technical Handbook, Scottish Building Standard 2013. Table 3.1 Control requirements Boiler plant output and controls Less than 100kW (Package A) 100-500kW (Package B) Minimum controls Timing and temperature demand control which should be zonespecific where the building floor area is greater than 150m² Weather compensation except where a constant temperature supply is required Controls package A above plus: Optimal start/stop control is required with night set-back or frost protection outside occupied periods Boiler with two stage high/low firing facility or multiple boilers should be installed to provide efficient part-load performance For multiple boilers, sequence control should be provided and boilers, by design or application, should have limited heat loss from non-firing modules, for example by using isolation valves or dampers Individual boilers, by design or application, should have limited heat loss from non-firing boiler modules. For boilers that do not have low
Efficient Operation of HVAC Controls in the Public Sector 5 Boiler plant output and controls Minimum controls Greater than 500kW individual boilers (Package C) standing losses it may be necessary to install isolation valves or dampers Controls package A and B above plus: The burner controls should be fully modulating for gas-fired boilers and multi-stage oil-fired boilers 3.2 Types of Controllers and Features Control systems for HVAC systems can vary widely, from simple time clocks that are found on domestic type boilers, through to more advanced, programmable, seven day, multievent controllers. The next level of sophistication is a full Building Management System (BMS) that can use many different variables and inputs to control advanced multi-zone and multi-building HVAC systems. The complexity of controls is not always a good thing. However appealing a sophisticated approach might be, it is worth remembering that a simple system is likely to be easier and cheaper to maintain, repair and understand. This in turn might provide a lower risk route to maintaining good energy efficiency over a long period. Not surprisingly, simple HVAC systems tend to require a lesser degree of automation and complexity in their controls. 3.2.1 Time Clocks Controller Simple time clocks can take many forms. These include boiler mounted digital clocks, mechanical clocks and wall mounted digital clocks. These controllers tend to be associated with single items like boilers or ventilation fans and are generally only able to accommodate daily programmes. This means that the time settings will be repeated daily or weekly depending on manufacture. While the most basic controls will only switch the boiler or fan on and off in response to time settings, more advanced versions are able to control based on time and temperature, usually maintaining a temperature set-point during times when the controls are on. 3.2.2 Seven Day Multi-event Programmable Controller A seven day multi-event programmable controller is a more advanced type of controller. As the name suggests, this type incorporates multiple events over a period of seven days. For example, this type of controller would be appropriate for a small office where heating is required on weekdays but not at weekends, or where time schedules vary from day to day. As with the previous examples, this type of controller can also incorporate temperature setpoint controls, although they are generally still only capable of serving a single system or zone. Often, with these and other types of controllers, the temperature set-point is determined by a remote thermostat. These can be either fixed or adjustable. As well as heating systems, similar systems are used to control air conditioning systems. 3.2.3 Optimised Start Feature An optimised start feature automatically varies the system start time depending on the external temperature. For example, during warmer weather the system will activate the boilers a little later in the morning ensuring that the building achieves a set temperature by
Efficient Operation of HVAC Controls in the Public Sector 6 the time it is in use but minimising the time that the boiler is operating. This approach can be very useful where the occupancy is not continuous (e.g. offices and schools). An optimum start controller operates by learning the optimum operating conditions and so will take a period to bed in. During this period reactive adjustments to the control set points will detract from the ability of the controller to properly adapt. Optimised start (and in some cases optimised stop) functions can typically save 10% of overall heating consumption. 3.2.4 Weather Compensation Feature Weather Compensation varies the boiler flow temperature depending on the external air temperature. This is often achieved by a mixing valve placed near the boiler to control the maintained temperature and reduce the distribution losses from the system. In condensing boilers it can be possible to directly compensate or adjust the maintained boiler temperature. This has an added advantage as it allows the boiler to operate in condensing mode to achieve maximum heat recovery from the flue. The addition of weather compensation can often offer paybacks within a two year period. 3.2.5 Additional Features In larger buildings there may be additional features and a greater degree of complexity to include multiple boilers and several controllable heating zones. For wet systems there may be other local control features (e.g. thermostatic valves) as well as a number of items of plant to control. These might include Air Handling Units (AHUs), chillers and local air conditioning. 3.2.6 Building Management System Where there are multiple items of plant and multiple conditioned zones, it is likely that a BMS is likely to be needed to allow the plant to be controlled efficiently and economically. These incorporate central and local controllers capable of communication and programming. A BMS can include other functions like data logging of temperatures and energy monitoring.
Efficient Operation of HVAC Controls in the Public Sector 7 4 Surveying Building Controls It is important to assess the performance of controls in existing buildings. The following need to be considered when surveying building controls: 4.1 Energy Data As with any other deficiency in energy management, poor HVAC control will most likely manifest itself in a building s energy consumption. A good starting point is therefore to look for poor performance in relation to other similar buildings and published guides and for unexplained increases in consumption over time. Poorly controlled heating systems will have a weak correlation with actual heating requirement as expressed in degree days. For this reason a degree day analysis should be undertaken. Half hourly electrical (and in some cases fossil fuel data) may be available for buildings. It is often useful to compare equipment start times with that denoted in the BMS. A control problem can lead a plant to operate well beyond its core hours of operation. This can often be quickly identified using half hourly energy data. 4.2 Setpoints and Outputs The next area to check is the actual setpoints and outputs of the control system. In some cases the controls may be relatively intuitive; however in others it may be necessary to spend some time understanding the display functions of a particular manufacturer s boiler controller. Hidden problems can become quickly apparent once the operation of a particular controller or BMS is well understood. Check that the current setpoints and time schedules are appropriate for the way in which the building is being used. Over time, changes to work patterns can lead to different heating or cooling requirements. Unexpectedly high or low supply temperatures may indicate a broken heating control valve or potentially a faulty sensor. Either of these is likely to have a negative impact on the energy efficiency of the system or the comfort of the occupants. Often it is found that the controls are set up with a warm up time allowance added by the operator. However, many modern controllers will already aim to have the building at the right temperature during the time schedule. It may therefore be possible to reduce the operating hours of the system, further increasing efficiency. Some BMS systems include visual displays of the heating equipment. These can be useful in interpreting what is actually happening in the system at any given moment. For example, heating coils in an Air Handling Unit (AHU) activating during summer months could be indicative of a problem with their control. Data logging functions can include room temperature and supply air temperature, and how these outputs are changing over time. For example where a room is being heated or cooled outside of its expected hours of occupancy, this will usually be apparent in the overnight temperature. Depending on the nature of the systems this may also indicate other factors such as faults in the building fabric or frost protection system.
Efficient Operation of HVAC Controls in the Public Sector 8 Table 4.1 outlines the common faults in a BMS system. Table 4.1 Control requirements Fault Poor or excessive fabric and frost protection Passing heating and cooling batteries in AHU Simultaneous heating and cooling Comments This can be picked up on plots of internal temperature in individual spaces or boiler flow temperature Half hourly data of gas or electricity can show equipment activating very early or through weekends even in mild weather Possible problems may be the setpoints, the strategy or potentially the external or internal sensors Passing valves on heating or cooling batteries do not properly close and therefore are continually activated at low level. This is wasteful of the energy consumed at the central boilers or chillers and can also lead to simultaneous heating and cooling This can clearly be picked up on AHU visual displays where for example cool external air is warmed by an AHU battery (check before and after temperature) despite the battery output being at 0%. The valve or actuator will need to be repaired or replaced There are a range of potential solutions to this to include: Application of a dead band within which neither heating or cooling is provided Summer and winter changeover where heating and cooling central plant or circuits are deactivated Careful adjustment of ventilation supply air temperature to minimise the chance of reheating and re-cooling at terminal units 4.3 The Design of the Control System As well as the individual controller and control features it is worth considering how well designed and assembled the control systems are. A common problem in many buildings is simultaneous heating and cooling where one system tries to heat air only for another to cool it at a later stage. This problem may not be apparent in terms of occupant comfort but can only come from observation e.g. the control of heating and cooling batteries. It is worth considering whether the building controls match the guidance provided in the Building Standard Technical Standards for new and refurbished buildings. Many building controls are developed in a piecemeal fashion. As a result a large complex building might be served by many separate control systems. In these circumstances there will probably be the opportunity for improvement. 4.4 User Display and Physical Control Components Another activity that should be undertaken is to look at the physical control components and the control displays. While a BMS can be set up correctly, if the control valves it governs are broken then the desired level of control will never be achieved. Similarly physical observation of equipment as it operates over time can also be informative. For example, if the boilers are switching duty every two minutes, this could be an indication that the sequencing control has not been properly set up within the BMS. It is also worthwhile to look at the power and control panel settings. It is common to find equipment set to manual control which should be under automatic control.
Efficient Operation of HVAC Controls in the Public Sector 9 4.5 User Engagement It is important to engage with the people who can have an impact on the building controls e.g. building users. Points to discuss are how they perceive the level of comfort in the building and how they use the local controls (including blinds and windows) available to them. Also consider if the occupants are contributing to energy wastage e.g. by switching off the Thermostatic Radiator Valves (TRV) in unoccupied rooms. It is good practice to become familiar with the features of individual controllers or BMS. Achieving this familiarity before the controls survey is undertaken will help to gain maximum benefit from this exercise. This will also help to identify instances where the input of a controls engineer is required. This might be to investigate a potential fault or to reprogram a control system (especially for complex systems). This specialist input might come from a term controls contractor who is responsible for the maintenance of the building controls, or there may be a controls specialist within the organisation. It may also be appropriate for key staff to undergo training on a particular control system type, particularly where this system is replicated or applied throughout the estate.
Efficient Operation of HVAC Controls in the Public Sector 10 5 Efficient Operation of HVAC Controls in the Public Sector 5.1 Maintenance A good controls strategy across an estate will greatly increase the energy efficiency of the building stock. The first point in any controls strategy is good maintenance. Controls tend to suffer disproportionately where maintenance is ignored or cut back. A gradual degradation in performance usually occurs which will affect occupant comfort, usability and efficiency. Regular checks and reviews of controls and control systems are strongly recommended. The need for maintenance should be considered when a control system is being selected. The maintenance of any system represents an ongoing cost and this cost rises as the complexity of the system increases. As a result, the complexity of the controls system should match the actual requirements. This approach will also reduce the costs of repair associated with component failure. 5.2 Changes to the Control System Even when overseeing a relatively small building stock, it is likely that there will be frequent changes and improvements to the buildings. Many of these will present the opportunity or requirement for changes to controls. It is therefore important to regularly review setpoint, timer control, temperature and other settings so that they match current and not past need. While your organisation might have a core specification for new control systems, it is worth considering if this is resulting in well performing systems. When reviewing a building control system, the following should be considered: Schematic diagram of the HVAC plant; Description of plant; Control strategy; Specification clauses; BMS setpoints list; Summary of plant operation; and Control flowchart. 5.3 Designing the Control System 5.3.1 Zone Control Zone control might be considered as part of a HVAC strategy for a number of reasons including: Changing occupancy requirements; Different temperature requirements; The growing instances of multiple tenants; Different energy performance on different floors (particularly where top floors are poorly insulated); Building orientation that creates differences in heat gains and losses; More than one building on the heating circuit; and The obvious opportunities to create separate circuits in the heating system. 5.3.2 Simultaneous heating and cooling Simultaneous heating and cooling is a common fault and one which results in a needless waste of energy. This should be avoided or minimised in the control design. For instance some systems may incorporate a summer and winter time switchover setting for which the heating or cooling provisions are deactivated respectively.
Efficient Operation of HVAC Controls in the Public Sector 11 5.3.3 Internal Sensors Poor placement of internal and external sensors can cause considerable problems for controls, both in terms of comfort and efficiency. Internal sensors should be placed in areas that are deemed representative of the actual space temperature and away from other heat sources. Thermostats for optimum start should be located in the coldest part of the building. External sensors should be place away from direct sunlight. 5.3.4 Occupancy control Occupancy control is another important feature of any design. Occupants should be given some control over their environment. However, while occupancy control should be part of the design of the control system, it should be limited so as not interfere with the overall control strategy and efficiency of the building. 5.3.5 Building Energy Management Systems Building Energy Management Systems (BEMS) are types of BMS or parts of a wider BMS strategy. These offer the facility to gather and monitor energy data and can be a useful tool especially in buildings with substantial and complex HVAC requirements. BEMS can lead to energy savings of 10% to 20% compared to a system based on a collection of local controllers. 5.3.6 Other Design Considerations Complexity in a control system is not necessarily a good thing. For example, too many heating zones or overly sophisticated strategies may discourage the building users from using the controls effectively. Simple systems well matched to the building requirements will offer reduced maintenance costs, lower incidence of failure and lower capital cost. The user interface should be simple, intuitive and easily interpreted, even by users who are unaware of the control strategy. The building HVAC controls do not operate in isolation. They are part of the overall building services providing a safe and comfortable working environment. Therefore the efficiency of the controls will depend on other aspects of the HVAC design. For example if the boiler equipment is oversized, no matter how well the control system is designed, it will be difficult to ensure it operates efficiently. Components for BMS systems can quickly become obsolete depending on a number of factors including whether the manufacturer or vendor can continue to support the system with spare parts. It is important to consider these points at the outset of any procurement process, as it can be difficult to undertake repairs and maintenance on a system without the support of the original manufacture. 5.4 Commissioning of Building Controls Another point often overlooked or left partially complete is the commissioning of building controls. In specifying HVAC controls it is important to refer to or stipulate adherence to the Chartered Institute of Building Services Engineers Commissioning Code C. Many buildings never achieve their full energy saving potential through their controls due to poor commissioning.
Efficient Operation of HVAC Controls in the Public Sector 12 5.5 Additional Features 5.5.1 Variable Speed Controls A common energy saving measure included in many control systems for ventilation and air based heating systems is the use of variable speed control. This is usually applied via an inverter drive to motors serving fans and pumps. The relationship between fan power and fan flow is cubic, so any reduction in fan flow results in a proportionately far greater reduction in power demand. The fan efficiency laws are summarised in the equation, where P is Power in kw and Q is airflow in m³/sec: Inverter or variable speed drives (VSDs) provide a control solution to take advantage of this effect. When fans are oversized and the actual demand for ventilation is low or when the building use changes, there may be potential to reduce fan speed. In some cases it may be best to apply a fixed reduction (i.e. VSD at fixed output). In others the requirement for ventilation may vary a great deal through each day in which case a more sophisticated strategy could be employed to include occupancy sensing control through a PIR or CO 2 sensor. With this latter approach, the energy consumed by fans and by any heating providing temperature control of the supplied air can be greatly reduced. 5.5.2 Boiler Sequencing Control For multi boiler systems, one of the more common potential failings is poor sequencing control, so this can often be a good improvement for existing buildings. Boilers or assemblies of boilers often operate inefficiently when they deal with low thermal loads. This is either because of the heat losses through the boiler casing or the process of purging any combustion gas in the heat exchanger when they fire. Where this happens continuously, the losses through the flue can be substantial. Sequencing controls ensure that the boilers are brought on and off load in the most efficient manner. Sequencing controllers can come as stand-alone units which can bolt on to existing systems and modern BMS systems often have some level of sequencing control (although it may not be correctly set up). Larger sequencing systems may include isolation valves for each boiler to allow these to be hydraulically separated when offline and prevent casing losses. For existing buildings, boilers frequently coming on and off load may be an indication that the sequencing control could be improved.
Efficient Operation of HVAC Controls in the Public Sector 13 6 Useful Links and References Title Source Link Zero Waste Scotland Resource Efficient Scotland www.zerowastescotland.org.uk www.resourceefficientscotland.com Heating control technology guide (CTG065) The Carbon Trust www.carbontrust.com/media/10361/ctg065_heating_control.pdf How to implement heating zone controls (CTL148) How to implement boiler sequence controls (CTL144) How to implement air quality sensors (CTL059) How to implement thermostatic radiator valves (CTL040) How to implement optimum start control (CTL035) Control strategies for Low Carbon Buildings CIBSE Guide H: Building Control Systems The Carbon Trust The Carbon Trust The Carbon Trust The Carbon Trust The Carbon Trust University of Exeter The Chartered Institute of Building www.carbontrust.com/media/147171/j8055_ctl148_how_to_implement _heating_zone_controls_aw.pdf www.carbontrust.com/media/147155/j8051_ctl144_how_to_implement _boiler_sequence_control_aw.pdf www.carbontrust.com/media/147151/j7972_ctl059_implement_air_qual ity_aw interactive.pdf www.carbontrust.com/media/147135/j7961_ctl040_thermostatic_radiat or_valves_aw.pdf www.carbontrust.com/media/131445/ctl035_how_to_implement_optim um_start_control.pdf http://emps.exeter.ac.uk/media/universityofexeter/emps/research/cee/ lchsmodule1notes/control_strategies.pdf www.cibse.org
Efficient Operation of HVAC Controls in the Public Sector 14 Services Engineers CIBSE Commissioning Code C The Chartered Institute of Building Services Engineers www.cibse.org Building Standards Handbook 2013- Mon Domestic Section 6.6.4 Efficiency of MVAC equipment Scottish Government www.scotland.gov.uk/resource/0043/00435261.pdf
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