Building Services By John Bradley– licensed under the Creative Commons Attribution –...
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Building Services By John Bradley licensed under the Creative Commons Attribution Non-Commercial Share Alike License http://creativecommons.org/licenses/by-nc-sa/2.5/ Slide 2 John Bradley Slide 3 INTRODUCTION Slide 4 Hot water Slide 5 Slide 6 Slide 7 Slide 8 E =Expansion (m 3 ) C=Volume of water in system (m 3 ) 1 =Density of water before heating (kg/m 3 ) 2 = Density of water after heating (kg/m 3 ) Slide 9 Slide 10 DHW Systems Instantaneous Multi point Single point Stored VentedUnvented Slide 11 CONVENTIONAL VENTED SYSTEM Slide 12 Conventional vented system In a conventional vented system, the mains cold water from the rising main supplies a header tank or Cold Water Storage Cistern (CWSC), usually located in the roof space, to give the system a head of pressure. A cold water feed pipe from the CWSC feeds the base of a hot water cylinder. The water in the cylinder is heated by a heat exchanger coil in the cylinder, in which circulates water that has been heated by the boiler. In this indirect system the water in the boiler is kept separate from the water in the cylinder, allowing the water in the boiler and central heating radiators to be treated with chemicals and kept free of dissolved oxygen to prevent corrosion of radiators and boilers. When the water is heated it rises to the top of the cylinder where it can be drawn off through the hot water supply pipes and tap or shower outlets. The water is automatically replaced in the cylinder via the CWSC. Heating causes the water within the cylinder to expand. A vent pipe allows a safe route for excess pressure, air bubbles and steam should the system overheat. It runs from the top of the cylinder back up to the CWSC where its open vent is located just above the water level. The CWSC is sometimes called a feed and expansion tank, reflecting the functions it performs. Water pressure and flow within the system is driven by gravity, i.e. the weight of stored water in the CWSC is usually sufficient to push water down the pipe that feeds the hot water cylinder and back up to any tap or shower outlet, providing it is lower than the stored water level. Slide 13 Cold water supply from rising main Cold water storage cistern Boiler Cylinder Coil heat exchanger to hot water outlets Cold feed pipe Immersion heater Vent pipe (at least 19mm) Overflow pipe Primary circuit Secondary circuit Slide 14 Heating and hot water system Slide 15 Slide 16 Slide 17 Slide 18 CWSC Normal expansion space Additional expansion space Overflow pipe Vent pipe Rising main Cold feed pipe 80mm Support for CWSC Flat, level, rigid platform to extend > 150mm beyond cistern Slide 19 Conventional vented system: direct An alternative to the indirect system that is not now commonly used, but occurs in many older properties, is a direct system. In direct systems the water in the cylinder is heated directly, either by an electric immersion heater in the cylinder, or by the water being circulated around a boiler. This was a common arrangement with back-boilers behind open fireplaces and ranges such as Agas and Rayburns, but is not generally used with central heating boilers. The hot water from the boiler mixes directly with the water in the cylinder. Water that has circulated in the boiler and primary circuit is drawn off through the taps and can therefore be contaminated. If used in a soft water area the boiler must be rust-proofed. This system is not suited to hard waters. When heated the calcium in the water precipitates to line the boiler and primary pipework, eventually furring up the system making it ineffective and dangerous. Slide 20 Immersion heaters Immersion heater Slide 21 Legionella There is an increased risk of bacterial growth in water held at temperatures between 20C and 46C for prolonged periods. This can cause Legionnaires disease. The elderly are particularly vulnerable. Control of the bacteria is therefore vital in settings such as hospitals and care homes. The following measures are recommended for use with hot water systems: Stored hot water temperature 60 to 65 o C throughout the storage vessel Pipework dead-legs to be minimal All pipework to be insulated to reduce water temperature losses Distribution temperature to outlets >55 o C Slide 22 Slide 23 UNVENTED SYSTEM Slide 24 Unvented Systems Until the 1985 Building Regulations and the new Model Water Byelaws of 1986, a domestic hot water storage system in the UK was required to have an open vent pipe (a vented system). The majority of new houses now built in the UK are designed with sealed, unvented mains pressure hot water systems: norm in Europe and USA. Expansion of water dealt with by the use of an expansion vessel. This replaces the CWSC. The system is normally supplied direct from the mains and is sealed to the atmosphere (rather than being vented to the atmosphere). There is therefore no need for a vent pipe. Hence the term unvented system. A full description of the system would therefore be a mains pressure, unvented sealed domestic hot water system. The installation of an unvented system is notifiable building work. Installers registered with a competent person scheme can self-certify that the work complies with relevant Building Regs and the owner/occupier will be given a Building Regs certificate of compliance. Slide 25 Unvented hot water and space heating system No roof space required for CWSC or feed and expansion tank Expansion vessel replacing CWSC for hot water system Expansion vessel replacing feed and expansion tank for heating system Slide 26 Unvented Systems Water in the unvented cylinder comes directly from the cold water main and is at (nearly) mains pressure. To contain this pressure the cylinder has to be much stronger than in a gravity-fed system. Unvented cylinders are therefore made of thick copper or stainless steel. The outlet of the cylinder is to hot water taps which are normally closed. The inlet is from the cold water main which may incorporate non-return (check) valves or other devices preventing expansion back into the supply pipe. Therefore measures have to be taken to accommodate the expansion of the hot water which could otherwise give rise to enormous pressure in the cylinder. These take the form of some type of container of gas which can be compressed as the water expands. This may be arranged as a bubble of air in the cylinder or a separate expansion vessel. Slide 27 Unvented Systems: expansion vessel An expansion vessel contains a diaphragm and a volume of air or nitrogen to absorb the expansion. It should be able to accommodate >4% of the systems overall water content. The photograph below shows an unvented hot water cylinder and two expansion vessels: one for the primary circuit and one for the secondary circuit. Diaphragm expansion vessels Slide 28 Unvented Systems: air gap A purpose made hot water storage cylinder designed with provision for an air gap is an alternative to installing a separate expansion vessel. As the water expands on heating, the volume of trapped air is compressed to provide adequate delivery pressure and flow. Some manufacturers fit a floating baffle between the water and the air, to reduce the effect of turbulence. Hot water cylinder incorporating air gapBaffle Slide 29 Safety of unvented systems At atmospheric pressure water boils at 100 o C. At higher pressures boiling point increases so that pressurised water can be heated to over 100 o C and remain liquid. However if the pressure is released (when a tap is opened) it will turn to steam, expanding and causing a steam explosion. Therefore unvented systems must have safety systems to control the temperature and pressure of the water. The expansion vessel is fitted with an expansion relief valve in case the vessel should fail. Beyond this there are 3 levels of safety : 1.Thermostat, set to operate at 60 to 65 o C 2.Non self-resetting energy cut-out, set to operate at 85 to 90 o C, to disconnect the supply of heat to the cylinder in the event of the thermostat failing and the storage system overheating, by turning the boiler off 3.Temperature/pressure relief valve, to discharge water to a safe and visible place open to the atmosphere, through a tundish (a small funnel with a pipe discharging into it to provide an air break in the overflow) if the water temperature reaches 95 o C Slide 30 Unvented system with safety features Pressure reducing valve (to keep pressure less than the expansion valve opening pressure) Line strainer (to remove dirt) Cold water main Stop valve Check valve (to stop water returning to cold water main) To cold taps Expansion vessel Expansion relief valve To drains Tundishes To hot taps Boiler Thermostat Energy cut out Hot water cylinder Temperature/pressure relief valve Slide 31 Temperature and pressure relief valve Slide 32 Safe discharge from T&P relief valve Discharge from T&P relief valve The diagram shows the method prescribed in AD G for the discharge of water from safety devices The tundish should be: Vertical; Located in the same space as the cylinder; and Fitted as close to the valve with no more than 600mm of pipe between the valve outlet and the tundish. The discharge pipe from the tundish should: Have a vertical section of pipe at least 300mm long below the tundish before any elbows or bends in the pipework; and Be installed with a continuous fall thereafter of at least 1 in 200. Slide 33 AdvantagesDisadvantages Balanced hot and cold water supply to all outlets at high pressure No storage back up if mains water fails Allows greater system design flexibilityPoor mains water pressure will cause poor performance Cylinder can be sited almost anywhereRequires high level of maintenance High performance showers without need for pumpsNeed for discharge pipe (overflow) Simple plumbing system, less pipework No risk of water stagnation No risk of frost damage to pipework No noise of filling cistern