CPD PROGRAMME Professional development Direct-fired storage water heaters for hot water generation This CPD article considers the application of direct-fired storage water heaters for supplying domestic, sanitary, potable hot water The CIBSE Journal CPD Programme Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers. Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a longterm commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK). CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSEs requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institutions guidance. Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at www.cibsejournal.com/cpd while the information they contain remains current. You can also complete the questionnaire online, and receive your results by return email. cPd QuESTiOnnAiRE To take part in this months CPD, simply read through the module here, then follow the link at the bottom of this section. Alternatively, visit www.cibsejournal.com/cpd you will receive notification by email of successful completion, which can then be used to validate your CPD records. More recently, installed indirect systems would have a condensing boiler as the heat source. This relies on return primary water temperatures below 55C to ensure condensing (and, so, high-efficiency operation), but during periods of low DHW consumption and particularly when there is no space-heating load this may not be possible. A direct gas-fired storage water heater can replace the calorifier from the system in Figure 1 and remove the need for that part of the primary pipe loop. The principle of direct-fired storage water heaters High-efficiency, gas-fired storage water heaters are designed to heat water by using energy directly from the combustion process, and from latent heat collected by condensing water vapour in the combustion gases. The heat exchanger is located directly within the storage cylinder and, therefore, heat is transferred directly to the stored potable domestic water. The performance of building envelopes improves incrementally as the demands for low energy and low carbon building operation become more stringently defined in national and international codes, standards and guidelines. As the absolute carbon impact of a buildings fabric is reduced, the proportion of total energy consumed in a building by the generation of domestic (potable) hot water (DHW) becomes ever greater. It is estimated that providing DHW consumes about 3% of the total EU gross energy consumption (which, to provide context, is roughly the annual energy consumption of Sweden)1 and approximately 24% of global residential and 12% of global commercial buildings energy consumption.2 The energy consumed in generating hot water is tied to the simple relationship that: heat energy to water = m x Cp x , where m = mass water (kg), Cp = water specific heat capacity (kJkg-1K-1), and = water temperature increase (K). However, the gross amount of energy required to deliver that heat to the water will depend on the system configuration, as well as operational conditions and maintenance procedures. In the UK, the tradition has been to install a boiler that indirectly supplies heat through a separate, indirectly heated calorifier (a domestic hot water storage vessel with internal heat exchanger) for the production of DHW, as shown in Figure 1. Irrespective of the boiler type that supplies the primary heating, this system will experience losses from the boiler, the primary circuit pipework (the pipework that carries water that passes through the boiler) and the storage calorifier. Potentially, when there is little demand for DHW and no demand on the heating circuits the boiler can experience short cycling periods that will increase the proportion of standing losses, reduce firing efficiencies and increase nitrogen oxide (NOx) and carbon monoxide (CO) emissions.3 There are various high-efficiency, condensing, direct-fired storage water heaters available. For example, singleburner models are commonly available that deliver a high rate of heat exchange to the water this is known as the recovery rate, since it relates to how swiftly the hot water is replaced, or recovered, after a period of hot-water use. Single-burner models have a long internal flue, with heat baffles within the flue that provide an extended surface area for the water vapour in the flue gases to condense, as shown in Figure 2. Hot-water heaters with multiple burners are typically equipped with two or four modulating modular burners as shown in Figure 3 with external stainless-steel heat exchangers located outside of the storage cylinder. This storage cylinder would usually have a capacity of 200 to 300 litres. Because of thermal stratification, the lowesttemperature water is drawn into the heat exchangers from the base of the vessel, allowing the unit to act in condensing mode for longer periods. This lowtemperature feed water ensures the burners remain at full output for longer periods, producing short recovery times. Boiler 1 Calorifier Boiler 2 Boiler 3 The multiple heat exchangers can work independently, providing built-in system redundancy and greater opportunity for modulating control. Figure 1: A simplified sketch of the arrangement that has traditionally been applied for providing both heating and domestic hot water Air Flue Gas Hot Sizing direct-fired storage water heaters It is important to size the direct-fired water heater properly, to suit the specific application within the building. Most manufacturers have sizing calculation tools such as that shown in Figure 4 readily available to aid selection. These should be based on hot-water requirements as enumerated by CIBSE Guide G Public health and plumbing engineering.4 It is important to consider the profile of hot-water use across a whole day and, in many cases, different daily profiles, depending on the assumed building use. Once the daily usage is determined, the more critical peak demand can be assessed. Traditionally, hot water peak storage has been based on a two-hour recovery period. When calculating hot water storage volumes, an availability factor should be applied, assuming stratification of 80% unless otherwise stated or known. This implies that 80% of the storage capacity will provide usable hot water.4 It is important to consider extraordinary circumstances. For example, a health club offering an open day Hot Cold Cold Figure 2: A representation of the heat exchange path in a single-burner, directfired condensing water heater that encourages more people than usual to turn up and use the showers could if not properly considered result in the building running out of hot water. Sizing packages would normally allow such diversity to be included. Typically, water heaters will be selected to provide the buildings peak demand. For example, a particular four-burner, 300-litre storage, high-efficiency, gas-fired storage water heater can readily supply hot water (at a 50K temperature rise) at a rate of up to 1,920 litres per hour. So, considering a typical CIBSE Guide G recommendation of a two-hour storage recovery period, such a direct-fired, gas-fired water heater is able to produce 3,840 litres of hot water over the two hours. Many installations specify multiple water heaters to be installed to allow for maintenance or breakdown. Those models that have multiple burners may effectively have built-in redundancy that is controlled by the units software, allowing water to be provided by remaining burners when one fails. Take, for example, an annexe to a business hotel, with 45-person occupancy, equipped with showers and wash-hand basins in each room. Assume each person has an hourly hot-water allowance of 25 litres for a shower and 2.5 litres for hand washing (reasonably generous hot-water allowances, based on the mixed water allowances in Table 2.12 from CIBSE Guide G). 45 people at 27.5 litres of water per person = 1,238 litres per hour. So power required = energy/time = m x Cp x /(3,600 seconds) = 1,238 x 4.18 x 50 = 72 kW 3,600 This is rather simple, and takes no account of diversity of use or occupation this requires some detailed knowledge of the particular application. So, in the above, Flue Air Gas Figure 4: Example of manufacturers sizing tool (Source: Andrews Water Heaters) Figure 3: A representation of heat exchange in a multiple-burner, directfired condensing water heater simplified example, the consultant would select a direct-fired storage water heater with a water heating power of 72kW to serve the annexe building. installation of direct-fired storage water heaters Direct-fired storage water heaters can be installed in many different locations within commercial premises. Manufacturers supply many different models that offer the designer of the hotwater services flexibility of installation. water treatment Water treatment for the cold water supply is an important consideration, especially in areas of hard water. Water quality is variable according to geographical location for example, in England, water is typically harder towards the south and east, and softer towards the north and west. But there are local variations, depending on the physical source of the water. Hard water contains dissolved minerals mainly calcium, magnesium and associated anions bicarbonate, sulphate and chloride. When hard water is heated, bicarbonate decomposes and calcium carbonate is deposited into the water-heater tank and associated pipework. Mineral deposits can dramatically reduce the efficiency of appliances and, eventually, cause the unit to fail. For example, a water-heater tank with a limescale coating of one-millimetre thick can equate to approximately 7% loss in efficiency.6 Water treatment should, of course, be properly considered with all types of hot water services, including continuous-flow water heaters and water heated indirectly using calorifiers. Models that are equipped with roomsealed flues and that have long flue runs can be installed within smaller, dedicated plantrooms often close to the point of use. This lowers the amount of energy required to supply domestic hot water over a long pipe run, and reduces heat losses from distribution pipework. This also helps lessen the length of any dead legs in the system, and more readily allows for compliance with the Health and Safety Executives Legionnaires disease Technical guidance 5, which suggests the distribution pipework should be designed to enable the water to reach all outlets at 50C within one minute of opening an outlet. The direct-fired storage water heater has been shown to have a low incidence of colonisation by legionella bacteria.5 When considering the installation of gas-fired storage water heaters, careful thought should be given to: Flue location Provision of gas pipework Space requirements for maintenance Type of cold water supply tank-fed system or unvented system (direct from a mains supply). Hot Cold Solar thermal CHP Heat pumps Single coil calorifier Water heater Figure 5: Integration of domestic hot water pre-heating from a low carbon energy source manufacturers heat generator seasonalefficiency figure or, in the case of directfired water heaters, the manufacturers thermal-efficiency figures. For example, if a direct-fired, multipleburner, gas-fired condensing water heater is equipped with fully automatic ignition controls (installed as standard) with a gross thermal efficiency of 98%, selected by the use of the manufacturers selection tool: Water heater thermal efficiency 98% Fully automatic ignition heating efficiency credit 0.5% Sized in accordance with manufacturers selection-tool heating efficiency credit 2% Total to be entered into SBEM NCM software 100.5% direct-fired storage water heaters and low carbon technologies Renewable or low carbon technologies can be readily integrated into a scheme using direct-fired storage water heaters to provide pre-heat feed water, as shown in Figure 5. The additional pre-heat cylinder will be sized to maximise the use of the low carbon heat source economically. This cylinder should be located as close as practicably possible to the gas-fired storage water heater to reduce losses in transmission. As an example of the benefit of heating and hot water services (not for new buildings); 1% for the use of direct-fired water heaters with integral combustion circuit shut-off devices; 0.5% for fully automatic ignition controls; and 2% for the use of manufacturers water-heater sizing guides and/or technical helplines. To test compliance with the Energy Performance of Buildings Directive (EPBD) by employing the SBEM National Calculation Method (NCM) software, these credits are used in conjunction with the application, a well-designed and applied solar thermal system in the UK may well be able to satisfy around 30% to 40% of the annual hot water load this is known as the solar fraction. It is important that the solar cylinder is sized correctly to maximise the amount of pre-heated water, meeting the needs of the water system and so reducing the building energy usage and carbon emissions. It is equally important to size the solar collector array to match the size of the solar cylinder, mitigating the risk of stagnation at times of low hotwater demand. Tim Dwyer and Jonathan Tedstone, 2015. water storage tanks and packages of REfERENCES: 6 Andrews Water Heaters, internal 1 Commission delegated Regulation Required storage water heater efficiencies Legislative requirements will often set minimum performance requirements for DHW systems. For example, the England Non-Domestic Building Services Compliance Guide 2013 7 requires a minimal thermal efficiency of 90% for natural gas-fired storage water heaters larger than 30kW, and 73% for smaller units (LPG system efficiencies are 92% and 74%, respectively). In England and Wales, when assessing the expected performance for compliance purposes, an additional 2% heat efficiency credit is given for decentralisation of the research document. (EU) No 812/2013, 18 February 2013 7 Non-domestic Building Services energy labelling of water heaters, hot Compliance Guide (England), NBS, 2013. water heater and solar devices. 2 rge-Vorsatza d., et al, Heating and cooling energy trends and drivers in buildings, Renewable and Sustainable Energy Reviews, Volume 41, January 2015, pp8598. 3 CIBSE AM14 Non-domestic hot water heating systems, CIBSE, 2010, section 5, p3. 4 CIBSE Guide G Public health and plumbing engineering, CIBSE, 2014. 5 HSG274: Legionnaires disease Technical guidance, UK Health and Safety Executive, 2014 Module 74 - March 2015 Fill in this months questionnaire online. You will receive notification by email of successful completion, which can then be used to validate your CPD records in accordance with your institutions guidance. CLICK HERE To FILL IN THE QuESTIoNNAIRE "