Building Services

1. 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/

2. Hot water systems John Bradley

3. Introduction

4. Hot water

8. ρ1 – ρ2 ρ2 E = C x E = Expansion (m3) C = Volume of water in system (m3) ρ1 = Density of water before heating (kg/m3) ρ2 =Density of water after heating (kg/m3)

11. Conventional vented system

12. Cold water storage cistern Overflow pipe Vent pipe (at least 19mm) Secondary circuit to hot water outlets Cold water supply from rising main Immersion heater Boiler Cylinder Cold feed pipe Primary circuit Coil heat exchanger

13. Heating and hot water system

17. CWSC Support for CWSC Vent pipe Overflow pipe 80mm Additional expansion space Normal expansion space Flat, level, rigid platform to extend > 150mm beyond cistern Cold feed pipe Rising main

18. 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.

19. Immersion heaters Immersion heater

20. Legionella • There is an increased risk of bacterial growth in water held at temperatures between 20°C and 46°C 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 65oC throughout the storage vessel • Pipework ‘dead-legs’ to be minimal • All pipework to be insulated to reduce water temperature losses • Distribution temperature to outlets >55oC

22. UNVENTED SYSTEM

23. 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 supplieddirectfrom 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 .

24. 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

25. 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.

26. 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 system’s 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

27. 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 gap Baffle

28. Safety of unvented systems • At atmospheric pressure water boils at 100oC. At higher pressures boiling point increases so that pressurised water can be heated to over 100oC 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 : • Thermostat, set to operate at 60 to 65oC • Non self-resettingenergy cut-out, set to operate at 85 to 90oC, 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 • 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 95oC

29. Unvented system with safety features Temperature/pressure relief valve To hot taps To cold taps Expansion vessel Pressure reducing valve (to keep pressure less than the expansion valve opening pressure) Thermostat Energy cut out Expansion relief valve Line strainer (to remove dirt) Stop valve Boiler Hot water cylinder Check valve (to stop water returning to cold water main) Tundishes Cold water main To drains

30. Temperature and pressure relief valve

31. 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.

33. MAINS PRESSURE VENTED SYSTEM

34. Thermal stores • Unvented systems are normally at mains pressure. Thermal store systems have been developed that are at mains pressure but are vented. • A container of water (the thermal store) is heated by the boiler via a heat exchanger coil at the bottom of the store. Mains pressure cold water passes through a second heat exchanger coil in the top of the store where it is heated by the stored hot water surrounding it and supplies the hot water outlets at mains pressure. • Expansion of hot water in the store is accommodated by a feed and expansion cistern located just above the store (ie it is a vented hot water system).

35. Thermal store • The thermal storage system is supplied with primary water from the boiler which heats the store via a primary coil. • Secondary water flows directly from the cold mains into a secondary coil where it is heated by the store before being delivered to the taps at mains pressure. Thermal store (configuration for a sealed central heating system) Feed and expansion cistern for DHW system Secondary coil Thermal store Primary coil heat exchanger

36. INSTANTANEOUS SYSTEMS

37. Multi-point instantaneous heaters: Combi boiler • The ‘combi’ gas boiler functions as an instantaneous water heater only heating water as required. Water supply is from the mains, providing a balanced pressure at both hot and cold water outlets. Ideal for showers. • System is sealed and has an expansion vessel which is normally included in the manufacturer's pre-plumbed, pre-wired package. • Saves space: no need for cisterns in roof space, no hot water storage cylinder and associated pipework

38. SOLAR HOT WATER SYSTEMS 0 – 50 7.37 – 10.07 23.50 – 25.15

39. Solar irradiation 811 kWh/m2/year Contours show solar irradiation in kWh/m2 pa 1115 kWh/m2/year

40. 30o – 40o

41. The proportion of the total DHW load that can be met by the solar system is the solar fraction.

42. System efficiencies Solar irradiation Useful energy to taps 100% 60% 45% 30% Losses Glazing Collector Reflection Conductivity Losses Pump Primary pipes Losses Storage Secondary pipes

44. Solar collectors Flat plate collector Evacuated tube collector

45. Flat plate collectors • Insulated metal box with either a glass or plastic covering and a dark absorber plate usually made out of copper or aluminium. This absorber plate transfers the heat to a tube where the heat transfer fluid flows, picks up the heat from the plate, and returns it to the storage tank. • The main distinction in the types of solar collector is between glazed and unglazed: • The most common type of unit for DHW purposes is the glazed type

46. Evacuated tube collectors • Evacuated tube collectors are more efficient than flat plate collectors and can provide higher output temperatures, but are more expensive. They are more efficient because the absorber is mounted in an evacuated and pressure-proof glass tube which reduces conductive and convective heat losses.

47. Evacuated tube collectors • There are two main types: direct flow and heat pipe: • In a direct flow collector, cold return fluid passes through the manifold and circulates round the absorber tubes in series and is heated in the process, returning to the flow stream of the manifold

48. Evacuated tube collectors: heat pipe • Heat pipe evacuated tube collectors contain a copper heat pipe, which is attached to an absorber plate, inside a vacuum sealed solar tube. The heat pipe is hollow and the space inside is also evacuated. • Heat pipe contains liquid. Vacuum enables liquid to boil at low temperature. • When sunlight falls on surface of absorber, liquid in heat tube turns to hot vapour and rises to top of pipe. Water or glycol, flows through a manifold and picks up the heat. The fluid in the heat pipe condenses and flows back down the tube.

49. Evacuated tube collectors: heat pipe Heat pipe collectors must be inclined at an angle of > 25o to allow internal fluid of the heat pipe to return to the hot absorber