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Energy Efficiency E-module - Guidance. Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector

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Energy Efficiency E-module - Guidance Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 2 Contents
Energy Efficiency E-module - Guidance Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 2 Contents 1 Introduction 3 2 Learning Objectives and Outcomes Learning Objectives Learning Outcomes 3 3 Overview and Principles of LTHW Heating Systems Conventional Boilers Condensing Boilers Wall Hung Boilers Modular Boilers Boiler Efficiencies System Components How to Survey a Boiler House 10 4 Energy Saving Opportunities Boiler Replacement Boiler Compliance - Ventilation for Gas Systems Boiler Compliance - Gas Safety Regulations Improving an Existing Boiler House Insulation Improvements Hydraulic Layout Pump Upgrades Combustion Efficiency Checks Heating System Maintenance - Poor Practice 15 5 Building the Business Case Before Considering a Boiler Upgrade Project Information Gathering Business Case - Case Study 17 6 Useful Links and References 19 Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 3 1 Introduction This guidance follows the format of the associated e-module, Efficient Operation of Low Temperature Hot Water (LTHW) Boilers in the Public Sector. It 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 from this module are to: Understand the different LTHW boiler types and their applications; Understand the main principles of how LTHW boilers and distribution systems operate; and Understand the different measures which can be implemented to improve boiler house efficiency. 2.2 Learning Outcomes The learning outcomes from this module are for the reader to be able to: Understand the basic principles regarding how different boiler types work; Understand where the opportunities for improving LTHW heating systems exist in Scottish public sector sites and buildings; Describe the main boiler technologies including their typical efficiency range, and the advantages and disadvantages of each technology; Carry out an audit of a boiler house and identify opportunities for making improvements; Identify when a boiler should be replaced and what technology could be applied; Prioritise the opportunities for improving LTHW systems in public sector buildings; and Understand the key aspects in relation to LTHW boiler projects when building a business case. Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 4 3 Overview and Principles of LTHW Heating Systems Table 3.1 provides a brief overview of some of the different LTHW boilers that are currently in operation. Table 3.1 An Overview of LTHW Boilers Boiler Type Conventional Condensing (Floor Standing) Condensing (Wall Hung) Modular (condensing/ condensing and conventional) Floor Standing, atmospheric or pressure jet burners, large in size. Boilers of this type are likely to be 15 years or older Smaller than conventional boilers. Have extra heat exchanger surfaces. Installed since 2005 Smaller than floor standing units. Have extra heat exchanger surfaces. Installed since Typically used for domestic applications Several boilers linked to give more flexible and efficient output to service larger heat loads Description Typical Seasonal Efficiency 45-70% (depending on condition) 85-90% (depending on heat emitters) 85-90% (depending on heat emitters) Depends on boiler type 3.1 Conventional Boilers This section introduces the basic principles of LTHW heating systems and the different boiler types available. A LTHW boiler works by burning a fuel and then using the heat energy from this combustion to heat water which is pumped around the heating distribution system, typically to radiators, air handling units or fan convectors. A basic cross section of a conventional gas boiler layout is shown in Error! Reference source not found.. Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 5 Figure Conventional Boiler When the boiler starts up, the burner fan first blows cold air through the combustion chamber to purge any residual gases from the previous combustion cycle. Once this process is complete, the burner then ignites and burns fuel in the combustion chamber. The hot gases from this combustion process then pass over the boiler heat exchanger, heating the water before passing out of the boiler through the vent. On the wet side of the system, when the boiler fires, the heating circulating pump switches on to ensure that there is a flow of water going through the boiler heat exchanger. The heated water is then circulated round the heating distribution system. This process is described in more detail in the following section. Low temperature hot water boilers generally heat water up to a maximum flow temperature of 90 C, with systems typically designed based on flow and return temperatures of 82 C and 71 C respectively. Most boilers have an integral temperature sensor which controls the burner to meet the target flow temperature. In smaller, simpler boilers this can be achieved by switching the burner on and off. Larger, more complex boilers can have two stage burners which allow them to alternate between low fire, high fire and off. Others have fully modulating burners which can alter the amount of fuel burnt to meet variations in the heat load. Fully modulating burners tend to have the highest overall operating efficiency, whilst single stage on/off burners are least efficient. A typical conventional boiler can achieve seasonal efficiencies of around 70-75%. This capacity to modify boiler output to meet heat demand is a feature of gas and oil fired systems. As shown in the biomass e-learning module, this ability is not shared by boilers burning solid fuels. These require different control strategies as a result. Since 1997, conventional, non-condensing boilers that meet the minimum efficiency requirements of the boiler efficiency regulations are usually marketed as high efficiency boilers. These boilers tend to have higher seasonal efficiencies of around 82%. Conventional boilers can come in a variety of shapes and sizes. They tend to be floor standing and draw their combustion air from the space around the boiler making the provision of sufficient ventilation essential. Burners tend to be either: atmospheric, relying on convection; or forced draft. The latter use a fan to draw combustion air into the burner. Forced draft burners allow for closer control of the flow of combustion air into the burner. This reduces flue gas volumes and thus the size of the flue required. 3.2 Condensing Boilers Condensing boilers differ from conventional boilers in that they have a secondary heat exchanger on the boiler flue through which the heating return water passes prior to entering Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 6 the main boiler heat exchanger. This increases the efficiency of the boiler as more heat is extracted from the combustion gases prior to them being exhausted to the surrounding atmosphere. Where the system can operate with return temperatures below 50 C, higher levels of efficiency can be achieved. This low water temperature causes the moisture in the flue gases to condense thus releasing the latent heat contained in the vapour. This is why these boilers are known as condensing boilers. Figure 3.2 shows a basic cross section of a condensing gas boiler layout Figure Condensing Boiler Condensing boilers are best applied to systems where weather compensation can be applied, ensuring that return temperatures can be minimised and that the boiler can operate in condensing mode for as much time as possible. For more information on weather compensation control, see the accompanying e-module on controls. However, it is worth noting here that this can usually only be applied to systems which exclusively serve heating circuits, with domestic hot water provided separately. LTHW boiler systems which also serve domestic hot water tanks usually require constant temperature supplies which negate the system s ability to compensate at all. Condensing boilers tend to have fully modulating burners allowing them to vary output to meet a fluctuating load. Condensing boilers can come in many forms including floor standing, wall hung, and modular. Modular and wall hung boilers tend to have much lower water content than conventional boilers. 3.3 Wall Hung Boilers Wall hung boilers tend to be smaller than floor standing boilers. These boilers can also be flued horizontally through walls, provided a balanced flue is used and the flue terminal is sufficiently far away from any nearby openings such as windows or ventilation louvres. Balanced or concentric flues have two orifices. The inner tube transports flue gases to atmosphere and the external tube supplies air to the boiler. When this arrangement is used the ventilation requirements for the boiler room or cupboard are reduced as the combustion air is provided via the flue. The same effect can also be achieved by using separate flues, although these cannot be installed horizontally and must terminate at least 1 metre above the roof level of the building in question. Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 7 Wall hung boilers are generally available in sizes ranging from 30 kw up to 120 kw. Larger boilers are floor standing. Additionally, wall hung boilers tend to have a low water content and therefore a high hydraulic resistance. This is an important consideration when replacing old high water content boilers with wall hung boilers as additional pumping capability may be required. 3.4 Modular Boilers Modular boiler systems are arrangements where several boilers are linked together to meet the heating demand of a building. Typically, these boiler systems are made up of between 2 to 12 identical modules, although sometimes a combination of condensing and noncondensing boilers will be used. Modular boilers have a number of benefits. Like wall hung boilers, they have lower water content, taking up less physical space than a conventional boiler. In addition, modular boilers can offer an attractive solution to effectively and efficiently meeting the varying heat demand of a large commercial building by allowing modules to be sequenced to operate at maximum efficiency for as much of the time as possible. For example, condensing boilers operate most efficiently at part load, whereas noncondensing boilers are generally most efficient at peak load. Consider a building with a peak heating demand of 500 kw but a summer base load of only 100 kw and it is evident that a modular boiler can help improve seasonal efficiency. If a single 500 kw non-condensing boiler was to be installed, it would only operate at optimum efficiency during the coldest months of the year while the rest of the time it would be operating at part load with reduced efficiency. If on the other hand a boiler with five separate 100 kw modules was installed, the boilers could be sequenced to ensure that modules operate at peak load (and thus peak efficiency) for a much greater time period. For condensing boilers, where optimum efficiency is at part load, similar benefits can be achieved by ensuring that the boilers are sequenced to operate at part load for as much time as possible. There can also be benefits to using a combination of condensing and noncondensing boilers to meet varying loads as efficiently as possible, especially in applications with a high demand for hot water. As with wall hung boilers, the low water content of modular systems leads to high hydraulic resistance and additional pumping capability may be required as a result. 3.5 Boiler Efficiencies Table 3.1 shows typically quoted values for boiler efficiencies. Table 3.1 reported boiler efficiencies Reported Efficiency Efficiency Type Boiler 1 102% Net Efficiency Boiler 2 94% Gross Efficiency System 87% Seasonal Efficiency Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 8 This shows that there is an important point to be considered regarding boiler efficiency and how they can be compared It is not uncommon to see boiler literature advertising boiler efficiency of 102%, whereas other figures may suggest a peak efficiency of 94%. This is because the first figure refers to net efficiency, which assumes that the energy contained in the water vapour in the combustion gases is recovered. The second value is gross efficiency and assumes that the energy is not recovered. Another efficiency term often used in reference to boilers or heating systems is seasonal efficiency. This is a weighted average of the efficiencies of the boiler at 15%, 30% and 100% output. The overall system seasonal efficiency will also be influenced by the type of heat emitters used. For example, a condensing boiler serving only an underfloor heating system and operating at relatively low temperatures will have a higher seasonal efficiency than the same boiler serving a system including fixed temperature heat emitters with a flow temperature of 82 C. It is important to be aware of the various ways of reporting boiler efficiency when evaluating which boiler to install and to undertake comparisons on a like-for-like basis. 3.6 System Components How the boiler is connected to the rest of the heating system and the key components of that system must be considered. Error! Reference source not found. is an energy efficient schematic boiler house with multiple condensing boilers, connected in parallel. Figure Boiler House Schematic From the diagram, it can be noted that the boilers are connected and can be isolated in such a way as to allow any one of them to be taken offline or even removed without the system having to be shut down. Within the boiler flow pipework there will typically be temperature and pressure gauges or sensors as part of the control and safety systems. Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 9 The system shown has a single circuit, though in practice there can be any number of circuits depending on the heating load being serviced. All flow circuits should be prior to any return connections, ensuring that each circuit gets boiler temperature water. In systems where there are a mixture of variable and constant temperature circuits the flow temperature will be varied on the variable temperature circuits, using mixing valves. Another component of a system of this type will be a dirt separator and deaerator. This will be located at the hottest part of the system. These devices remove circulating air, microbubbles and any small particles of dirt in the system. This is important as it ensures that air and dirt are removed from the system to prevent corrosion, improve heat transfer, and prevent unnecessary deterioration in other system components such as boilers and pumps. All modern boiler installations include a common header which acts as a barrier between primary and secondary circuits, allowing for optimum control and hydraulic performance. The benefits of this component are explored in more detail later in this document. The pressurisation unit and expansion vessel are also important components of the system. The pressurisation unit automatically maintains pressure in sealed systems by pumping in mains cold water when the system pressure drops below a predetermined level. As the name suggests, the expansion vessel accommodates expansion of the fluid in the system as it heats up. It consists of a tank with a rubber diaphragm separating the fluid from a charge of nitrogen gas. As the system heats up, the diaphragm expands to accommodate the increased volume. Some older systems will have a feed and expansion tank at high level which performs both functions for systems operating at atmospheric pressure. The primary pumpset, usually comprises a twin-head pump with automatic changeover, to give redundancy to the system. Finally on the water side, a number of balancing or commissioning valves are present, to allow the system to be set up with adequate flow through each boiler. Other important items are the condensate drain (often via a tundish) for condensing boilers, and the various gas isolation valves, allowing for individual isolation of each appliance and isolation of the gas as it enters the boiler house. This is also a requirement under the gas safety regulations. It is also good practice to include a strainer to capture any pipeline debris such as scale or rust before it reaches the pumps and boilers. It is important that this component is located correctly to protect these expensive plant items. Table 3.2 contains a list of LTHW boiler system components. Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector 10 Table Boiler system components for LTHW systems Component Purpose Impact on Efficiency Isolation Valves Balancing and conditioning Valves Mixing Valves Temperature Gauge/Sensor Pressure Gauge/Sensor Dirt Separator and deaerator Common Header Secondary flow and return circuits To isolate boilers, components and for gas safety To maintain adequate flow through boiler Mixing temperatures To measure the temperature within the boiler and heat circuit To measure the pressure within the boiler and heat circuit Removes dirt and air from the system Acts as a barrier between primary and secondary circuits Secondary circuits providing different areas of the building Allows removal/isolation of boilers, components etc. for maintenance or replacement but the system can still operate in most cases Ensures the water quantity is met whilst avoiding the entry of large quantities of potentially damaging cold water Ensures each circuit and emitter gets the correct temperature If the temperature reads below/above the range set by the manufacturer this could indicate a fault within the system or a component If the pressure reads below/above the range set by the manufacturer this could indicate a fault within the system or a component. The pressure within a system affects the quantity and the temperature of the water. Both low and high pressure can be dangerous Improves heat transfer, prevents corrosion and damage to pipeline and components Allows for optimum control and hydraulic performance Ensures each circuit gets boiler temperature water Pressurisation Unit and Expansion Vessel Strainer Pump Set Condensate Drain Maintains pressure and contains the expansion of system as it heats up Removes pipeline debris including scale, rust etc. Twin set of pumps Removes condensate from condensing boiler The pressure within a system affects the quantity and the temperature of the water. Both low and high pressure can be dangerous Prevents damage to components The pumps allow the water to flow through the entire hot water and heating system. Two pumps allows for redundancy Essential for boiler operation 3.7 How to Survey a Boiler House There are key items to look out for when surveying a boiler house. A good exercise when surveying a boiler house is to try and sketch out a schematic (as shown in Figure 3.3) to show how the system is laid out. Step-by-step guide to surveying a boiler house: Identify the boiler type, capacity and age; Identify the flow and return pipes note arrangement of pipework and pumps; Identify condensate drain if appropriate; Identify expansion vessel and pressurisation unit; Identify header and number circuits; Identify ancillary pipework e.g. dirt separators, deaerators and location of components; Identif


Jul 23, 2017


Jul 23, 2017
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