Hot Water - Plumbing & Gasfitting - PDF

Chapter 7 Hot water T o remain healthy, people need hot water to keep their bodies and living areas clean. To do this, a regular supply of hot water is required to break down and dissolve oil and dirt. Some s...

Chapter 7 Hot water T o remain healthy, people need hot water to keep their bodies and living areas clean. To do this, a regular supply of hot water is required to break down and dissolve oil and dirt. Some substances, such as fatty foods, require a temperature of at least 60°C to dissolve them. Water heating process, an example The Law The Building Act 2004 is the legislation that governs the building industry in New Zealand. The Act aims to improve control of, and encourage better practices in, building design and construction. The objectives of the New Zealand Building Code correspond to the purposes of the Building Act. The Code defines how a building must perform, and consists of a number of clauses, including clause G12. Plumbing and Gasfitting Essential Skills 183 Book 1.indb 183 19/07/12 4:17 PM  chapter 7: Hot water Structure of the Building Code (Adapted from the DBH website) Means of compliance The Building Code is made up of a number of clauses including clause G12. This clause contains compliance documents for the supply of hot water. G12 AS1, for example, includes an acceptable solution for hot water supply installations. G12 AS1 also links to reference AS/NZS 3500 Part 4 as a verification method for meeting the Building Code requirements. Any work on hot water services that you do must conform to the Building Code. The following list is a summary of Clause G12 AS1 ‘performance criteria’ (mandatory requirements) for hot water services which must: be delivered at a temperature that avoids the likelihood of scalding have adequate flow rates to sanitary fixtures and sanitary appliances avoid the likelihood of leakage allow a reasonable access to components likely to need maintenance, including a means of isolation be capable of being controlled to prevent the growth of legionella bacteria have vessels used for producing or storing hot water to be provided with safety devices that relieve excessive pressure during both normal and abnormal conditions and limit temperatures to avoid the likelihood of flash steam production in the event of rupture. The methods of hot water supply covered in this text book will be of best trade practice and the acceptable solutions of the NZBC. It is not intended to cover alternative designs which are at certifying level. 184 Plumbing and Gasfitting Essential Skills Book 1.indb 184 19/07/12 4:17 PM  chapter 7: Hot water Building Code requirements To obtain a building consent for a water heating installation, the installation must be shown to comply with the relevant sections of the Building Code. Relevant clauses of the Building Code for water heating systems are: B1 — Structure B2 — Durability E2 — External Moisture G12 — Water Supplies H1 — Energy Efficiency. Clause B1 — Structure This clause of the Building Code ensures that the support structure of the building is capable of supporting the loads placed on it for the life of the building. With regard to installing a water heating system the structure must be able to support the load. Clause B2 — Durability This clause sets out the requirements in terms of the durability of the product that is to be installed in a structure. Water heating system components, in general, are expected to last at least 15 years. Some components requiring maintenance or replacement may have an expected life less than 15 years provided this is clearly identified in the owner’s manual. Installers should ensure that information supporting the durability of a system is addressed in the application for a Building Consent. Clause E2 — External Moisture This clause sets out the requirements for protecting the structure’s weatherproofing envelope to ensure that external moisture does not leak into the building. This clause also covers penetrations of the roof cladding and acceptable means of waterproofing any penetrations. Clause G12 — Water Supplies Clause G12 sets out the requirements of the Building Code in terms of protecting the purity of the potable cold and hot water and also the safety of these systems. G12 includes an acceptable solution G12/AS1 which brings together all the various parts of the Building Code into a single place. The acceptable solution sets out a means of compliance to the Code. However if G12/AS1 is used to demonstrate compliance, it needs to be used in its entirety and work kept within the scope of the Acceptable Solution. Clause H1 — Energy efficiency Clause H1 covers all aspects of a building structure in terms of energy efficiency. With regard to water heating systems, it sets out the requirements in terms of insulation of hot water storage vessels and pipes. This clause also refers to NZS 4305 which deals with domestic type electrical and gas systems having a storage water heater capacity up to 700 litres. Larger systems and their associated piping and controls are not controlled by the NZBC. Clause H1 includes means of minimising pipe runs to conserve water and energy. Plumbing and Gasfitting Essential Skills 185 Book 1.indb 185 19/07/12 4:17 PM  chapter 7: Hot water Hot water science Water becomes different in many ways when heated: Though practically incompressible, it changes volume when heated Its boiling point changes It dissolves and expels gases Some solids are dissolved by it It may be corrosive It flows differently. Liquid water can be compressed a small amount but only by terrific pressures. The point to consider here is that if water is forced into a container, or it expands, no standard piping or cylinders can restrain it. Water expands when it freezes, and when it is heated. The damaging effect of water’s expansion due to freezing is often seen as burst pipes. When water is heated it becomes less dense and thus occupies a larger space. Heated water will form layers, hottest at the top of a vessel and coolest at its base. This is known as stratification. Water is at its maximum density at 4°C. The expansion of water from 4°C upwards is not constant, being greater at the higher temperature. For practical purposes, the expansion between 4°C and 100°C is approximately 4% of the original volume. On average the cold incoming water is at 10°C and the hot water temperature 65°C. Within this range the water will expand 2–3% of its original volume. Water expanding when it is heated is normally taken for granted and is usually only seen as a dribble from an expansion valve. The greater the heat input, the greater the pressure generated. Pascal’s Law states that pressure applied anywhere to a body of confined or enclosed fluid is transmitted with undiminished force in every direction. Where there is no allowance for heated water expanding, it may explode. The force of an explosion depends on the strength of the material used in the container and how the vessel was constructed. Boiling point of water The relationship of temperature and pressure in boiling water is so important every plumber should be familiar with it. The Building Code requires people be safeguarded from: injury caused by hot water explosion and the likelihood of flash steam production. Any water heating system has to be installed correctly to avoid the possibility of excess temperature and steam production. 186 Plumbing and Gasfitting Essential Skills Book 1.indb 186 19/07/12 4:17 PM  chapter 7: Hot water Avoiding the creation of steam In an open container, for example, an electric jug, water will boil to become steam at 100°C at sea level. But if the water boils under pressure, in, say a pressure cooker, its boiling point will rise significantly. The boiling point will now occur at a higher temperature. And because water uses a lot of heat energy to change from liquid into steam, it will create violent forces if this high temperature water under high pressure is allowed to turn to steam. As it turns to steam the heated water will expand explosively as a gas takes up more space than its liquid form. Steam inside a water storage vessel can also cause the vessel to collapse. If steam is allowed to form inside a storage vessel and allowed to condense back to water, it can create a vacuum and thus collapse the cylinder. Vent blocked with ice Reduced pressure in cylinder Hot water Cold flowing feed Atmospheric pressure Cylinder collapse Types of water heaters Refer to the Basic plumbing services skills text book pages 308–313 for a comparison of the basic types of water heaters.Water heaters can be for a single point use or for use as a central storage heater. The design of a building, its occupancy rate and its use, for example if it is a bach or an apartment, will determine whether a central storage system or a localised system would be best. In some houses a long pipe run may supply hot water to only one hot tap. This is referred to as a dead leg. This is uneconomical, as a significant amount of hot water is required to displace the cold water, and there is too much heat loss from the pipework. Clause H1 of the Building Code defines minimum and maximum flows to draw off points. To avoid this, a single point electric or gas heater could be installed. Plumbing and Gasfitting Essential Skills 187 Book 1.indb 187 19/07/12 4:17 PM  chapter 7: Hot water A choice of water heater will be from the following list: instantaneous water heaters storage water heaters with high, medium or low pressure heat exchange (coil) storage water heaters boiling water heaters. Water capacity Determining the capacity of the water supply system is the hardest factor to ascertain because of many variables. Average water use is in the order of 40-60 litres per person per day, and this factor can be used to size hot water supply. However, there are variables for determining hot water capacity which include the: number of people to be served number of appliances using hot water, and in particular showers size of the dwelling winter cold water temperatures standard of living of the users habits of the users use of spa baths. In general, a water heater should be sized according to the accommodation potential of the house and not necessarily the number of users at present. The system should be designed to accommodate ‘peak load’. This occurs when the maximum number of hot water users are making the maximum demand on the hot water system. Peak demand can be calculated using flow rate tables from the Building Code or similar, or using the manufacturer’s information, for example, Rinnai determine peak flow for their systems as 30–40 l/hr. A water heater sized for a couple in a four bedroom home would be out of proportion to the potential of the home. Always consider an appliance that uses hot water such as a washing machine, as another person, and allow a further person for a second bathroom. Types of energy sources The choice of water heater will be decided in conjunction with the type of energy source to be used. Energy sources include fuels which can be burnt. To achieve the greatest convenience and best running costs, a combination of one or more types of the following energy sources can be used: electricity, including single phase immersion elements and three phase electrode boilers solid fuel, including wood, coal and pellets gas, either LPG or Natural gas steam, including geothermal oil solar, either as solar gain from collectors or heat pumps. 188 Plumbing and Gasfitting Essential Skills Book 1.indb 188 19/07/12 4:17 PM  chapter 7: Hot water The choice of energy source will depend on factors such current and predicted fuel prices, availability of fuel, and fuel efficiency technology. For example, gas water heaters have moved from approximately 80% to 95% efficient in recent years. Energy sources can be either controlled or uncontrolled. A controlled heat source can be regulated to stop continual heat input, for example, an electric cylinder has a thermostat to regulate temperature and often, an over temperature cut out device should the thermostat fail, thus terminating the available heat source. An uncontrolled heat source has the potential to continuously heat water up to boiling point. For example, a wet-back or solar heat source used to heat storage water heater is uncontrollable because the heat source cannot be turned off. The water temperature will continue to rise as long as the energy source is available to heat the water. Water heater construction Hot water cylinder construction — G12 requirements The building code has requirements for water heater construction and the effects of temperature and pressure on water heater construction. Unit standards for installing, commissioning and maintaining hot water supply systems require some knowledge of relevant regulations, standards and codes. For example AS/ NZS 3500.4:2003 Plumbing and drainage — Heated water services, Clause G10 Piped Services, and Clause G12 Water. Water heater construction must also comply with sandards and regulations. The NZBC clause G12 AS1 contains a list of those standards and regulations in Table 5. Some examples of these standards and regulations follow: Low pressure electric copper storage water heater must comply with NZS 4602. This standard applies to the design, materials, construction, thermal insulation and performance of thermostatically controlled heaters of 6.5 litre to 450 litre capacity designed for pressures up to 120 kPa (12m head). If the copper water heaters above are installed in a vertical position and open-vented they must comply with G12 and to NZS 4603, if valve vented comply with NZS 4607. Electric storage water heaters, other than those covered by NZS 4602, must comply with NZS 4606, for example, mains pressure storage water heaters. Parts 1, 2 and 3 of this standard specify general constructional and performance requirements for storage water heaters of 6.5 litres to 630 litres capacity. This standard specifies the allowable heat loss, the provision of user-adjustable means of temperature control, and routine and type of test schedules. Gas-fired water heaters specifications are referenced in the Gas Regulations. Plumbing and Gasfitting Essential Skills 189 Book 1.indb 189 19/07/12 4:17 PM  chapter 7: Hot water Materials This section focuses on the construction, components and materials of water heaters. An introduction to this topic is discussed in the Basic plumbing services skills text on pages 308–309. Some cylinders are made of a wide variety of materials. Others are made of principally one material, for example copper or stainless steel. Copper cylinders are made to suit low to medium pressure, with medium pressure cylinders being made from thicker copper. Stainless steel needs to have suitable qualities of corrosion resistance and stress cracking resistance for use in hot water heaters. 2304 Duplex is commonly used in water heaters and is a low-alloy duplex stainless steel. It is high-strength and low-cost. 2205 Duplex Stainless Steel is a medium-alloy duplex stainless steel with higher corrosion resistance. Where the water supply is high in chlorine content, for example higher than 50 ppm, a stainless steel cylinder may also need a sacrificial anode. As far as is practicable, the same or similar materials should be used throughout a system. However, use of different materials can be unavoidable, for example when combining independent boilers, made of cast iron, and cylinders made of half-hard copper. Joints in copper cylinders, and between the copper and the brass bushes, are generally zinc alloy or silver-alloy brazed. Note the method of separating dissimilar metals on a tank attached cylinder shown below. Separating dissimilar metals on a tank attached cylinder using hardboard Cylinders are available with standard connections, or built to order with inlets fitted which allow either bottom or side entry for their water supply connection. Others can be purpose made with multiple inlets. It can be useful to fit a cylinder with at least two extra inlets as it makes fitting solar or a heat pump easier in the future. Purpose made cylinders can be made with one or more coils fitted for use with indirect heating. For greater detail refer to the ‘Indirect heating’ section. 190 Plumbing and Gasfitting Essential Skills Book 1.indb 190 19/07/12 4:17 PM  chapter 7: Hot water Effect of pressure on cylinder construction Cylinders should be labelled to identify the pressure at which they are designed to operate. A cylinder will be damaged if it is subject to pressure above the identified rating. There was a period in New Zealand hot water cylinder manufacture where vitreous enamelled steel cylinders were jacketed to look the same as mains pressure units. In fact, they were neither pressure tested nor rated to withstand mains pressure and the main distinguishing feature was only a small label as shown below. Check the label Low and medium pressure cylinders are labelled with a pressure rating measured in metres head for example, a label could state the cylinder’s maximum working pressure as 7.6m head. The test pressure of this cylinder is 120 kPa. Mains pressure cylinders are labelled with a pressure rating measured in kPa. Exposing hot water cylinders made of copper to excess pressure can result in dangerous outcomes. Refer to ‘Hot water system faults’ section for further details on these outcomes. Cylinders are manufactured with domed ends for extra strength. Older copper cylinders were made with corrugations on their sides. Modern copper cylinder sides are supported by the dense polyurethane insulation encasing them. Vitreous enamel lined cylinders are concave at the top, and convex at the bottom to withstand pressure. Effect of temperature on cylinder construction The building code requires the limiting of temperatures on hot water services. This is to avoid the production of steam and to protect users from excessively hot water. Refer to the section ‘Avoiding the creation of steam’ for greater detail. There are other reasons to limit the temperature of hot water systems. Hot water cylinders have limitations on the temperature of stored water according to the material they are made of. As discussed, vitreous enamelled steel cylinders have limitations on maximum temperature. This makes them unsuitable for connection to uncontrolled heat sources. Maximum temperatures will usually be achieved at night, in particular if ‘night rate’ electricity is used. Plumbing and Gasfitting Essential Skills 191 Book 1.indb 191 19/07/12 4:17 PM  chapter 7: Hot water Various components, apart from the storage vessel itself, need to be protected from excess temperature. Valves, and in particular tempering valves, have design specifications which must match the outlet temperature of the heated water. Uncontrolled heat sources, such as solar and wetbacks, place extra demand on the storage vessel and associated valves and system components. Components of water heaters There are many different manufacturers of water heaters which are made for many different applications. The components vary according to the use of each unit and according to the type of fuel used. The simplest water heater is probably a billy on a camp fire. This water heater has no real means of temperature control or heat retention and so is a very inefficient use of energy. Gas water heating appliances are heated with a gas burner. Electric water heating appliances are heated by an element, and boilers can be heated by various means, including oil and pellet burners. Electric water heating appliances, heated by an element, will generally have a lower energy rating than gas or solid fuel burners. This makes electricity slower to heat water to a usable temperature compared to gas and solid fuel burners. This is because of the limitations of electrical power supply. Older electric water heaters were fitted with elements as small as 1 kW. Currently, most new electric heaters are fitted with 2 kW or 3 kW elements. The correct rating of element should be chosen when replacing a failed existing one. A replacement 3kW electric immersion element More powerful electric elements draw larger current and this is limited by the size of the electric wiring. Normal domestic wiring is generally designed to safely carry the current of a 2 kW element. An electric instantaneous water heater can have, for example, a 27 kW element but this requires 3 phase wiring. A typical 4 m2 solar collector will transfer energy at the rate of approximately 2 kW/hr. A typical gas instantaneous water heater can be rated at 55 kW, and a pellet burner as high as 200 kW. Storage water heater elements The materials that elements are made of need to be compatible with both the cylinder material and the quality of water to be heated. When different metals are in close proximity to one another in solution, corrosion activity will take place and the least superior metal will corrode. 192 Plumbing and Gasfitting Essential Skills Book 1.indb 192 19/07/12 4:17 PM  chapter 7: Hot water The table below indicates that nickel plated elements are suitable for copper low pressure cylinders, and that tin plated elements are suitable for glass lined steel mains pressure cylinders. Incaloy provides better protection from corrosion in hard water areas. Catalogue Appliance Appliance Type Wattage Element description Hermetic Page No Manufacturer at Part No 230/240V 1.6 Low pressure Copper low 1000W Nickel plated copper 1122H water heater pressure hot Foldback ‘O’ ring/ manufacturers water cylinders gasket 1.25˝ BSP boss 1.6 Low pressure Copper low 1500W Nickel plated copper 1124H water heater pressure hot Foldback ‘O’ ring/ manufacturers water cylinders gasket 1.25˝ BSP boss 1.6 Low pressure Copper low 2000W Nickel plated copper 1126H water heater pressure hot Foldback ‘O’ ring/ manufacturers water cylinders gasket 1.25” BSP boss 1.6 Low pressure Copper low 3000W Nickel plated copper 1129H water heater pressure hot Foldback ‘O’ ring/ manufacturers water cylinders gasket 1.25˝ BSP boss 1.6 Low pressure Low pressure hot 2000W Incoloy tube gasket 1138H water heater water cylinders, 1.2˝ BSP boss manufacturers hardwater areas 1.6 Low pressure Low pressure hot 3000W Incoloy tube gasket 1140H water heater water cylinders, 1.2˝ BSP boss manufacturers hardwater areas 1.1 Copper Steriliser Steriliser 2000W 1.25˝ BSP steriliser 1160H Morrison without thermostat Industrial 1.1 Mercer Steriliser Steriliser 2000W 1.25˝ BSP steriliser 1162H Element without thermostat 1.7 Southcorp NZ Glass lined 1500W Tin plated copper 1164H (Rheem) mains pressure foldback ‘O’ ring 1.25˝ cylinders BSP boss 1.7 Southcorp NZ Glass lined 2000W Tin plated copper 1166H (Rheem) mains pressure foldback ‘O’ ring 1.25˝ cylinders BSP boss 1.7 Southcorp NZ Glass lined 3000W Tin plated copper 1169H (Rheem) mains pressure foldback ‘O’ ring 1.25˝ cylinders BSP boss 1.7 Low Pressure Copper low 1500W Low Watts Density 1180H Hot Water pressure hot Nickel plated copper Manufacturers water cylinders foldback ‘O’ ring 1.25˝ BSP boss 1.7 Low Pressure Copper low 2000W Low Watts Density 1181H Hot Water pressure hot Nickel plated copper Manufacturers water cylinders foldback ‘O’ ring 1.25˝ BSP boss Selecting an appropriate element for an appliance Plumbing and Gasfitting Essential Skills 193 Book 1.indb 193 19/07/12 4:17 PM  chapter 7: Hot water Tin plated elements This type of element is manufactured especially for glass-lined water cylinders. In a glass-lined cylinder steel, tin and magnesium are close together in water, the latter being the solution. Use of other types of plated element will cause the sacrificial anode to dissolve too quickly, thus shortening the life of the water heater unnecessarily and making void any warranty claims that may arise. Nickel plated elements In water heaters with copper cylinders, it is necessary to use a nickel plated element. While there is no sacrificial anode in this type of water heater, there are two different metals present in the solution, these being the copper of the cylinder and the nickel plate on the element. This is the correct design relationship for this type of water heater and using other types of plated elements will cause unnecessary corrosion of the element. In areas having hard water conditions, an incoloy element is sometimes used. Incoloy is an alloy with a very high nickel content. Low watts density and incoloy This combination element is the ultimate solution to problems found in areas where there is high lime and mineral content in the water. The reduced watts per square cm reduces the likelihood of element failure due to excessive lime build up which is one of the most common causes of failure in hard water areas. Quick recovery Gas water heaters act as quick recovery water heaters. This is because of the speed at which stored water can be raised in temperature. Electric water heaters can also be designed to be quick recovery heaters, for example by the addition of an extra electric element. A second element, positioned near the top of the cylinder, will provide fast heating of water at the top of the cylinder. Faster heating can also be achieved by using dual elements, an example is shown below. A dual heating element 194 Plumbing and Gasfitting Essential Skills Book 1.indb 194 19/07/12 4:17 PM  chapter 7: Hot water Quick recovery can also be achieved by enclosing the element in a special compartment. This compartment is attached to a tube which directs the heated water to the top of the cylinder ready for use. Hot water outlet Cold supply Heated water is directed to the top of the cylinder ready for use The simplest electric boiling water unit, see example shown below, shuts off the power supply to the element when boiling commences. The power is switched off when steam activates a pressure switch. This water heater is also insulated to retain heated water for future short term use, and is designed to heat a chosen quantity of water. Vent pipe Free water surface Pressure switch Element Outlet valve Inlet valve The simplest electric water heater will have some means of temperature control, for example this type has a steam activated switch Plumbing and Gasfitting Essential Skills 195 Book 1.indb 195 19/07/12 4:17 PM  chapter 7: Hot water The next simplest water heater will include a thermostat to allow selection of a preset temperature of water. The thermostat automatically switches off the power when the set temperature is reached and switches on again when hot water is drawn off, or the temperature drops through heat loss. Many thermostats have a maximum temperature setting of approximately 70°C, especially vitreous enamelled cylinders. Other applications use 10 to 88 degrees and a 0 to 110 degrees, for example in commercial applications. For more information on suitable storage temperatures refer to the section ‘Legionella control’. Older cylinders have a thermostat inserted in a pocket or sleeve, which might be brazed into the copper cylinder, or the pocket might be screwed into a threaded brazed connection. Older cylinder ready for a thermostat to be inserted in a pocket A common thermostat for water heating is a rod thermostat. Rod thermostat This consists of a rod of invar steel which expands very little. This is fixed inside a tube of brass which expands approximately 18 times as much as the invar steel. When the brass tube, which is in contact with the water, expands, it draws out the invar rod with it and breaks the electrical contact. A magnet ensures a clean snap action, as the magnet will hold the electrical make and break switch until the last minute, so preventing excessive arcing and rapid deterioration of the contact points. 196 Plumbing and Gasfitting Essential Skills Book 1.indb 196 19/07/12 4:17 PM  chapter 7: Hot water Invar rod Brass casing Side of hot water storage vessel Earth Jointing ring A.C. Electrical supply Components of a rod thermostat Newer cylinders have a clip on thermostat, which is held against the copper or steel cylinder with metal bracket. Clip on thermostat If the storage vessel is a valve vented water heater, it will require extra safety components to ensure the energy supply is terminated before water is allowed to boil. Boiling will occur in the event of a thermostat’s failure to shut off the energy supply. One such device, in the case of early gas fired water heaters, was an energy cut-off device (ECO). More modern gas water heaters incorporate a fusible link ECO. The fusible link is integral with the gas control. It cannot be reset and the whole control unit will need replacing. Electric storage water heaters, which are not open vented, require an energy cut off device which must be manually reset if overheated. Plumbing and Gasfitting Essential Skills 197 Book 1.indb 197 19/07/12 4:17 PM  chapter 7: Hot water The sacrificial anode in the water heater is designed to corrode in preference to the steel body of the water heater and the tin plating on the element sheath. The vitreous enamel can dissolve at high temperature and some parts of the steel cylinder may not be covered by the glass enamel, for example, threaded connection ports. If a water heater is not used for several weeks, and it is fitted with a sacrificial anode, a quantity of highly flammable hydrogen gas may accumulate in the heater. The first water drawn from the system should be via a tap and not an electric appliance such as a dishwasher or washing machine. Other ignition sources should be kept from any gas which escapes the tap. Stratification, or layering, of heated water will mean that components at the top of a hot water cylinder are subject to the highest temperatures. This is the area where greatest corrosion and build up of scale occur. Storage water heaters can be fitted with one or more coils which act as heat exchangers. The heat exchangers can be arranged for optimal stratification of the stored water. These can be designed to heat the coldest water in the cylinder by solar heating. More complex water heaters will include more complex electronic control of temperature and energy supply, and have a larger number of components, including moving parts. For example a heat pump water heater can include a fan and compressor. Insulation of newer water heaters must comply with minimum energy performance standards (MEPS). Originally, electric storage water heaters were poorly insulated with wool flock, or recycled clothing material. This insulation was superceded by polyurethane which has a low K (heat transference) value. Currently MEPS requires the thickness of insulation to be at least 50 mm. Gas fired water heaters are insulated with non-flammable fibreglass. Safe trays — purpose and materials Clause E3 of the NZBC, ‘Internal moisture’, includes the requirement of preventing water from penetrating another household unit which is within the same building, for example, in a multi-level apartment building, or a two-storey two flat dwelling. One important solution in preventing such water damage is the use of a safe tray. Safe trays can be fitted to supply tanks or any other water storage vessel, including hot water cylinders. Clause G12 AS1 outlines the requirements for water tank installation, including safe trays. These requirements include a minimum diameter of 40 mm for overflow pipes. A method of combining the tank overflow and the tray overflow is shown below. The material that a safe tray is made of will be dictated by the materials of the storage vessel or supply tank, and the energy source. Old concrete supply tanks, which eventually crack and become porous, are often rested on a concrete safe tray. The concrete safe tray shown below is well designed but superseded by lighter materials. 198 Plumbing and Gasfitting Essential Skills Book 1.indb 198 19/07/12 4:17 PM  chapter 7: Hot water Good safe tray design — but materials superseded Note the use of water proof fabric shown over. This is necessary where the materials of the safe tray and storage vessel are electrochemically incompatible, for example a copper supply tank on a galvanised steel tray. A plastic safe tray overcomes corrosion problems but this material is not suitable directly below a gas-fired storage water heater. Ventilation lid Working Roof space At least Unions 25mm Tank overflow Hinged flap Stop-tap Waterproof fabric Cold-water Tank outlet High-pressure Supply-pipe Tray Ceiling Tray overflow Wall Tank overflow and the tray overflow combined Whether the safe tray is below a supply tank or storage water heater, it will need adequate support. Clause G12 AS1 gives minimum requirements for supporting volumes of water up to 150 litres, for example, 20 mm sheet flooring over a load bearing wall. Buildings can require cold water storage as a reserve in case of the water supply failing. Hospitals, for example, require a minimum of 50 litres per person of cold water as a reserve in case of the water supply failing. Plumbing and Gasfitting Essential Skills 199 Book 1.indb 199 19/07/12 4:17 PM  chapter 7: Hot water The diagram below gives a solution to the problem of adequate structural support for a storage tank. Supply tank Support platform Partition walls Adequate structural support for a storage tank As well as adequate structural support, Clause B1 of the NZBC requires storage vessels to have adequate seismic restraint. (For further information refer to the section ‘ Provision for seismic movement’). Water supply to water heaters Water supply to water heaters must achieve minimum flow rates from those heaters. Care must be taken to match the expected hot water demand with the available supply. If the water supply pressure is marginal, it may be best to use a system fed by a supply tank. Water flow rates from hot water services are generally determined by the available water supply pressure, the size of the heating element, pipework size and the size of the storage vessel. For example, an instantaneous gas water heater can provide heated water at a constant temperature indefinitely. It heats the water to the required temperature as fast as the heated water is used. A storage vessel heated by an undersized electric element will not deliver sufficient heated water to meet the building code requirements. Water supply to water heaters must not exceed the working pressure of storage vessels. G12 AS1 specifies four means of supply to protect such storage vessels. Supply of cold water to a storage water heater can be via: water tank pressure reducing valve pressure limiting valve mains pressure supply. 200 Plumbing and Gasfitting Essential Skills Book 1.indb 200 19/07/12 4:17 PM  chapter 7: Hot water Minimum flow rates to sanitary fixtures must also be achieved and these are specified in G12/AS1, Table 3 below. It can be seen that the flow rates are measured WAT E R S U P P L I E S at specific Acceptable Solution G12/AS1 minimum temperatures. Table 3: Acceptable Flow Rates to Sanitary Fixtures Paragraph 5.3.1 Sanitary fixture Flow rate and temperature How measured l/s and °C Bath 0.3 at 45°C Mix hot and cold water to achieve 45°C Sink 0.2 at 60°C* (hot) and Flow rates required at both hot and cold taps 0.2 (cold) but not simultaneously Laundry tub 0.2 at 60°C* (hot) and Flow rates required at both hot and cold taps 0.2 (cold) but not simultaneously Basin 0.1 at 45°C Mix hot and cold water to achieve 45°C Shower 0.1 at 42°C Mix hot and cold water to achieve 42°C * The temperatures in this table relate to the temperature of the water used by people in the daily use of the fixture. Note: The flow rates required by Table 3 shall be capable of being delivered simultaneously to the kitchen sink and one other fixture. Acceptable flow rates Table 4: Tempering Valve and Nominal Pipe Diameters Amend 5 As wellParagraphs as needing anand 5.3.1 adequate 6.12.1 water supply, the supply will need to be controlled so as Feb 2004 not to exceed the maximum design pressures Low pressure of the cylinder, Low associated valves and and medium end-use tap ware. (i.e. header tank pressure unvented Mains pressure supply or low (valve vented) and pressure) open vented Most mixer taps require a mains pressure supply to operate effectively. Generally these Pressure of water at 20 – 30 30 – 120 over 300 fixtures tempering will valve deliver (kPa) more than the minimum flow rates required by the NZBC. In some cases Metres headthe (m)flow through these fixtures 2–3 is so high it needs >3 – to 12 be reduced by fitting over 30 flow restrictors at the outlet. This is done to achieve a measure of energy efficiency and water Minimum tempering valve size 25 mm 20 mm 15 mm Pipes to tempering valve conservation. Suitable tapware25should mm be selected to20match mm the hot water20 mm system. (see Note 3) (15 mm optional) (see Note 1) Pressure Pipes to shower measurement 20 mm 20 mm 20 mm (see Note 4) (see Note 5) Some valve manufacturers produce valves which incorporate a pressure (15gauge. Usually a mm optional) separate gauge is necessary to measure pressure, which can be connected to a threaded (see Note 1) Pipesoutlet, such as(see to sink/laundry a hose Note tap. 2) Pressure20 mmis regarded as low, 20 up mmto 7.6 m head, 15medium mm pressure Pipes to bath (see Note 2) 20 mm 20 mm 15 mm is between 7.6 m head and 12 m head and high if the pressure is above 12 m head. Water Pipes to basins (see Note 2) 15 mm 15 mm 10 mm supply pressure varies according to the effective head of the supply. In the case of a supply Notes: 1. Iftank in by supplied theseparate ceiling, pipethe fromavailable pressure storage water heater to will beoutlet. a single approximately 2.5 kPa and can be as low as 1.0 2. This tablekPa. This is based pressurepipe on maximum is determined by the distance between a shower rose and the lengths of 20 metres. 3. 2water levellength m maximum in a from supply watertank sitting heater outlet toon the ceiling tempering valve. joists. 4. 15 mm if dedicated line to shower. 5. 10 mm if dedicated line to shower. Valves and valve trains 6. Table 3 pipe sizes have been calculated to deliver water simultaneously to the kitchen sink and one other fixture. Valves and valve trains (a combination of valves), control water. Pressure ratings of valves are generally measured in metres head for low pressure systems, and kPa for mains 5.4 pressure systems. Maintenance Clause G12 of the Building COMMENT: facilities Code indicates some typical valve train layouts. Additional isolating valves may be provided for the 5.4.1 The water supply system shall be maintenance of storage water heaters, valves and provided with an isolating valve where a components. supply pipe enters the building or at each Dwelling unit within a Multi-unit dwelling. 5.4.3 Provision shall be made for draining storage water heaters in accordance with 5.4.2 Where the water supply pipe serves a Figure 7. Plumbing and Detached Gasfitting dwelling, the Essential Skills required isolating valve 201 by Paragraph 5.4.1 may be located at the property boundary. 26 Book 1.indb 201 25 February 2004 D E PA R T M E N T O F B U I L D I N G A N D H O U S I N G 19/07/12 4:17 PM  chapter 7: Hot water Mains pressure water heaters are typically rated from 850 kPa to a maximum of 1400 kPA depending on the cylinder material. Low and medium pressure water heaters are designed to withstand a maximum of 7.6 and 12.0 metres head respectively. Various water heaters, for example, boiling units, can have maximum pressures from as low as 350 kPa to 1000 kPa. Water supply pressures may well exceed these water heater maximum pressures. To protect the water heaters a number of valves are required to control the water pressure. Valve controls can be divided into two categories — functional controls and safety controls. A line strainer is necessary to protect all valves downstream of itself and could be considered to be both a functional and safety device. Functional controls protect the water heater, and other components, from overpressure (being subject to excessive pressure) and over temperature (being subject to excessive temperature) during normal operation. Functional controls include pressure limiting valves, pressure reducing valves, non return devices, expansion control valves, and thermostatic control of the energy input. Temperature and pressure relief valves (TPR valves) and relief valves can operate as functional valves if no expansion valve is fitted. These valves are subject to continuous use and will eventually fail. Causes of failure include overpressure, impurities in the water supply, failure of energy controls, and mechanical wear or corrosion of the valve. Safety controls protect the water heater, and other components, from overpressure and over temperature should the functional valves not function correctly. These controls should only operate in emergency conditions. Safety controls include combined temperature and pressure relief valves, pressure relief valves, vacuum drain devices. Valve trains should be installed according to Clause G12 AS1. Valve trains are commonly known as combination valves or ‘combo sets’. The first valve in a valve train will be an isolation valve. The second type of valve required is a line strainer to protect other components of the hot water system. Both of these valves could be incorporated into a combination valve such as a three in one valve. The next valve function required is that of water system pressure control. In the case of a low or medium pressure system a pressure reducing valve (PRV) is used. In a mains pressure system a pressure limiting valve is used to control pressure. Note that a pressure control valve must be set at no more than 80% of downstream relief valve pressures. For example, if a relief valve is pressure rated at 600 kPa the preceding pressure limiting valve must be set at no more than 600 kPa x 80% = 480 kPa. If a preset 500 kPa pressure limiting valve was used in this situation, the relief valve would open prematurely. 202 Plumbing and Gasfitting Essential Skills Book 1.indb 202 19/07/12 4:17 PM  chapter 7: Hot water This would waste heated water and energy, and potentially ruin the relief valve as it would have unnecessary water flow through it. It is useful to have an adjustable pressure control valve, and vital that its pressure setting is checked by measurement. To ensure heated water is not lost from the storage vessel, or drawn water is not returned, a non-return valve is required. Without a non return valve, heated water can backflow. This will waste heated water and potentially allow heating elements to burn out if they are not immersed in water. This could happen, for example, if the water service pipe is cut and drained. This valve is commonly part of a combination valve such as a three in one valve. Back flow prevention requirements also state that drawn water is not able to be returned. Clause 3.1 G12AS/1 details these requirements. A separate non-return valve should also be fitted as required in G12 AS1 for valve vented hot water cylinders. The non return valve is to prevent hot water entering the branch cold water supply pipes should the main water supply be disconnected or fail. In particular, the valve will prevent backflow through fittings located below the base of the cylinder, for example where there is a shower in basement bathroom. Note that some manufacturers’ instructions specify that an isolating valve be fitted between the cylinder and cold water expansion valve. This is considered to be accepted trade practice and facilitates easy maintenance of the valve train. A drain valve should be fitted to the valve train to drain the system, and it can be separate from the valve train. This may be incorporated into a multi-functional valve or located away from the valve train. Low and medium pressure systems have valve configurations which differ from mains pressure systems. Low pressure and medium pressure systems usually have relief valves rather than TPR valves. Clause G12 AS1 includes reference to low pressure valve vented systems which do include a TPR valve. Relief valve drains should include an air break in these applications. The diagram on the next page shows a relief valve which acts to relieve pressure when water expands as it is heated. This relief valve is known as a Cold water Expansion Valve (CWEV). Plumbing and Gasfitting Essential Skills 203 Book 1.indb 203 19/07/12 4:17 PM  chapter 7: Hot water An example of a relief valve — a cold water expansion control valve (CWEV). Relief valves should be matched to the energy source of the system they are attached to. The valve below is rated to relieve water heated by up to 20 kW. Higher heat inputs require higher rated valves. TPR valves and some relief valves can have two features to further protect the cylinder from excess or negative pressure. They incorporate a drain relief and a vacuum break on the back of the valve. The drain relief will blow a sealing pin from the back of the valve should the TPR valve drain become blocked. The same pin can act as an integral vacuum break device by allowing air into the relief drain. TPR valves should be operated at intervals recommended by the manufacturer to ensure they function correctly. It is possible for the inlet of a TPR valve to become blocked with corrosion or lime scale and so cease to function as a safety valve. Only regular checking will highlight this problem. A water diffuser has many functions. Water supply to hot water systems is made more efficient by fitting a water diffuser in the cold inlet of the cylinder. The diffuser disperses cold water as it enters the cylinder and helps prevent discoloured water and under or over heating of the water. Without a diffuser, heated water can become discoloured from sludge or sediment being stirred up by an excessive stream of inlet water. A stream of cold water can also move through the stratification layers and allow cool water to exit hot taps. 204 Plumbing and Gasfitting Essential Skills Book 1.indb 204 19/07/12 4:17 PM  chapter 7: Hot water Overheating of the cylinder water can occur if a stream of cold water cools the thermostat probe which prompts heating of the water. This effect can contribute to ‘stacking’. Stacking is where water at the top of the cylinder ends up being overheated from short and frequent draw-offs. These draw-offs cause the energy source to heat the water in bursts. Stacking can cause high enough temperatures in the top of the cylinder to activate the TPR valve. This not only wastes heated water but can cause premature failure of the valve. General piping requirements Water heaters should be installed to comply with the NZBC, manufacturer’s instructions, and job specifications, which can include owner’s instructions. Materials The three materials listed in G12 AS1 as suitable for hot water pipe work are copper, galvanised steel and polybutylene. Galvanised steel and black iron pipework in radiator systems can be protected by the water circulating through an indirect system, as the water in radiator systems eventually becomes inert. Often rust inhibitors are added to these closed systems. Galvanised pipe should not be used as a vent. Note that the NZBC G12 AS1 includes AS/NZS 3500 Part 4 as a verification method. The list of pipes and fittings found in Appendix C also comply as acceptable solutions. Note that there are restrictions on the use of these materials covered in clause 2.4 ‘Pipes and fittings’. These restrictions relate to maximum pipe size, pressure, temperature, many restrictions on the use of plastic pipes. Some instantaneous gas water heaters generate mildly acidic condensate continuously in the combustion chamber, up to 5 litres per hour. The manufacturer’s instructions must be followed as they supersede the requirements of G12 AS1. The instructions include suitable pipe materials, drain length, suitable termination points and provision for freezing. G12 AS1 specifies the installation of valves and pipework for hot water services. Pipe sizes must meet the minimum requirements of the table shown on the next page. These sizes are designed to at least achieve minimum flow and temperature requirements of hot water services. Note that a dedicated line to a basin, up to 20 metres long, can be as small as 10 mm if the system is mains pressure. Plumbing and Gasfitting Essential Skills 205 Book 1.indb 205 19/07/12 4:17 PM  0.2 (cold) but not simultaneously Basin 0.1 at 45°C Mix hot and cold water to achieve 45°C Shower 0.1 at 42°C Mix hot and cold water to achieve 42°C * The temperatures in this table relate to the temperature of the water used by people in the daily use of the fixture. Note: chapter 7:required The flow rates Hot water by Table 3 shall be capable of being delivered simultaneously to the kitchen sink and one other fixture. Table 4: Tempering Valve and Nominal Pipe Diameters Amend 5 Paragraphs 5.3.1 and 6.12.1 Feb 2004 Low pressure Low and medium (i.e. header tank pressure unvented Mains pressure supply or low (valve vented) and pressure) open vented Pressure of water at 20 – 30 30 – 120 over 300 tempering valve (kPa) Metres head (m) 2–3 >3 – 12 over 30 Minimum tempering valve size 25 mm 20 mm 15 mm Pipes to tempering valve 25 mm 20 mm 20 mm (see Note 3) (15 mm optional) (see Note 1) Pipes to shower 20 mm 20 mm 20 mm (see Note 4) (see Note 5) (15 mm optional) (see Note 1) Pipes to sink/laundry (see Note 2) 20 mm 20 mm 15 mm Pipes to bath (see Note 2) 20 mm 20 mm 15 mm Pipes to basins (see Note 2) 15 mm 15 mm 10 mm Notes: 1. If supplied by separate pipe from storage water heater to a single outlet. 2. This table is based on maximum pipe lengths of 20 metres. 3. 2 m maximum length from water heater outlet to tempering valve. 4. 15 mm if dedicated line to shower. 5. 10 mm if dedicated line to shower. 6. Table 3 pipe sizes have been calculated to deliver water simultaneously to the kitchen sink and one other fixture. Nominal pipe diameters 5.4 Maintenance facilities COMMENT: Additional isolating valves may be provided for the 5.4.1The Theacceptable water supply flow rates to system sanitary shall be fixtures in Table 3 can also be achieved by a manifold maintenance of storage water heaters, valves and system. The manifold system provided with an isolating valve where a can easily, undercomponents. high pressure supply conditions, meet these supply pipeflow rates enters thebybuilding using fittings which result in full bore flow and when each fixture is or at each 5.4.3 Provision shall be made for draining delivered Dwelling water aindividually unit within from the manifold. Multi-unit dwelling. storage water heaters in accordance with 5.4.2 Where the water supply pipe serves a Figure 7. Manifold Detached systems dwelling, the need to comply isolating with NZBC clause H1 which states “in a household valve required unit, the 5.4.1 by Paragraph developed may be length of pipe located at therun from the water heater to the kitchen sink shall be property boundary. minimised”. Table 5 in G12 AS1 provides acceptable maximum pipe lengths. This table 26 includes a maximum length of 25 m for a 10 mm pipe. 25 February 2004 D E PA R T M E N T O F B U I L D I N G A N D H O U S I N G Heat traps Some manufacturers require a thermosiphon trap to protect cold water supply valves from excess heat. In any event it is good trade practice to fit a heat trap to the cold supply as it limits heat loss to the cold supply pipe as well. This trap should be fitted close to the cylinder. The outlet pipe of the cylinder is potentially the hot water system’s greatest heat loss as this pipe will contain the hottest water. On an open vented system, which requires a continuously rising vent pipe, the best method of heat retention on the outlet pipe is to offset the vent. 206 Plumbing and Gasfitting Essential Skills Book 1.indb 206 19/07/12 4:17 PM  chapter 7: Hot water This limits thermo-siphoning of water within the pipe (coring) which would occur more easily if the pipe was vertical. There are several methods of fitting heat traps to the hot outlet on valve vented cylinders. The simplest method is to comply with NZBC G12 AS1 which requires one metre of copper tube between the cylinder outlet and the tempering valve. In using the metre of copper tube, and fitting the tempering valve below the top of the cylinder, the hot water will not be able to thermosiphon from the cylinder outlet. Heat traps should be made deeper if there is a high circulation pressure in pipes attached to a storage vessel. For example, where a solar collector is at a significant height above a storage water heater. Water temperatures Instantaneous hot water supply systems are designed to deliver heated water which meets minimum temperature requirements and yet is at a safe temperature. For example, a domestic gas instantaneous water heater can deliver heated water at 55°C which meets the requirements of the building code. Storage hot water supply systems are also required to deliver water which is hot enough and yet not too hot. These two conditions are addressed by keeping the water stored at a suitably high temperature, above 60°C, and controlling the outlet temperature by using a tempering valve. Storing water at too low a temperature risks the growth of Legionella. Delivery of stored water at too high a temperature is also unsafe as there are risks of burns or scalding. This is especially the case when the water is heated by an uncontrolled heat source such as a wet-back or solar heated system. If Legionella enters the respiratory system, it can cause Legionnaires’ disease. Legionellae exist as free-living planctonic organisms. They can enter reticulated water systems from their natural water and soil environments. Water temperatures in the range 20°C to 45°C favour its growth. The risk is minimised when the water is heated above 60°C. Legionellae remain dormant at temperatures below 20°C and multiply when the temperature and nutrients reach a suitable level. Legionnaires’ disease can lead to a number of life-threatening complications, including respiratory failure. The most common causes of infection in New Zealand are from airborne bacteria that are found in air conditioning systems and working with compost. Some hot water supply piping systems risk the growth of legionella. Dead legs in hot pipework, and some flow and return systems, store heated water for a length of time and so can allow bacteria growth. Stored water should therefore be kept at a temperature high enough to kill most of the bacteria. This can be achieved by keeping the stored water temperature permanently above 60°C or by ensuring the temperature is periodically raised to at least 60°C once a day. This latter situation can be achieved, for example, by use of dual elements in conjunction with an electrical timer. Plumbing and Gasfitting Essential Skills 207 Book 1.indb 207 19/07/12 4:17 PM  chapter 7: Hot water However, unless the storage vessel is oversized for demand, water stored at only 60 °C may not contain enough heat for normal average demand, for example several showers plus a bath in reasonably quick succession. To store more heat in the storage vessel, it is common for water to be stored at temperatures between 65 and 70°C. This presents the problem of possible burning or scalding from exposure to that heated water. The outlet temperature of a storage water heater is typically controlled by use of a ‘tempering’ valve. This device meets the requirements of G12 AS1 and G12 AS2. A tempering valve is referred to as a ‘thermostatic mixing valve’ in AS/NZS 3500 Part 4. This valve mixes heated water, which is too hot to use safely, with cold water to a temperature which suits the user. The NZBC states a maximum temperature of 45°C for various institutions, including early childhood centres and old peoples homes. Special high temp valves for solar and wetback systems/check for temp rating etc. The Health Act 1956 states temperature requirements for the preparation of food. There must be sufficient hot water at 63°C at all sinks and other appliances and a temperature of 83°C for every other purpose. Wash-hand basins may have tempered water from 38 to 55°C. Provision for seismic movement Seismic movement is ground movement caused by an earthquake. Such ground movement causes buildings to move as well as the objects within. Note that NZBC clause B1 considers storage water heaters to be building elements. If heavy objects within a building are not restrained they will move with dangerous force in an earthquake. Such movement of heavy objects, and in particular heavy objects in ceiling spaces, will cause damage and possible danger to human life. NZBC Clause G12 AS1 gives acceptable solutions of structural support for water heaters up to 360 litres capacity. Storage hot water systems which are which are outside a building also require seismic restraint. 208 Plumbing and Gasfitting Essential Skills Book 1.indb 208 19/07/12 4:17 PM  chapter 7: Hot water Structural support for water heaters up to 360 litres capacity Fittings and pipework attached to water heaters must be free to move with seismic activity, and G12 AS1 specifies clearances to allow that movement. Open vented systems Open vented systems are systems where the outlet of the water heater is open to atmosphere. Such systems allow the expansion of heated water to be accommodated within the open vent. Wetback and push through heating systems are examples of open vented systems. Push through systems will be covered in a later section of this chapter. Open vented storage vessels or water heaters are generally rated as low pressure, that is, up to 12 m head. Open vented systems are generally fed from either a supply tank or a pressure reducing valve. However, some of these systems are fed by mains pressure water, for example push though systems. Because water expands when heated, the extra water must be accommodated. A vent pipe provides for the escape of air (and steam if produced) and for expansion of the water. Where a cold water supply tank is used the water will find its own level as it expands so some of the water returns to the supply tank. If a pressure-reducing valve is installed then the vent pipe has to take all the extra volume of water and may overflow. Plumbing and Gasfitting Essential Skills 209 Book 1.indb 209 19/07/12 4:17 PM  chapter 7: Hot water Tank-fed systems As hot water is drawn off the supply tank water level lowers, and is then maintained by means of a ballcock filling the supply tank. This water level determines the working pressure of the system. This pressure is measured in metres head (m head) The pipework needs to be sized and graded to avoid air locks. Draw-off points should be kept below the line of the hydraulic gradient to avoid restricted supply at those points. The water level in the supply tank should be set no higher than the pressure rating of the storage water heater rating. Because a hot water supply system is a low-pressure system, the magnitude of its hydraulic gradient is particularly important. As the starting point of the hydraulic gradient, the supply tank should be sited as high as practicable. When a supply tank is fitted in the space between the roof and a ceiling, note the following details: The tank should be supported in a safety tray on a proper platform resting on a strongly constructed stand on the ceiling joists over a partition wall and secured against seismic forces. The tray should preferably be of the same material as the tank, or an inert material such as plastic and should have an overflow pipe terminating in a conspicuous position outside the building. To stop draughts and prevent the entry of birds, the overflow should be fitted with a self-closing hinged flap. To deaden the noise of running water a silencer tube should be fitted which extends to near the base of the tank. The overflow should not discharge into the tray and the tank should be placed so that there is ample working space above it to allow for servicing. So that the ball valve cannot be submerged, the high-pressure cold water service pipe should connect with the tank above the level of the overflow outlet, and there should be a stop tap as close as practicable to the ball valve. To allow for any sludge deposit, the tank outlet should be above the level of the tank bottom. Pressure reducing valve fed Mains pressure water can be fed to a low pressure storage water heater through a pre