1. Building Services
By John Bradley– licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License

2. Hot water systems
John Bradley

3. Introduction

4. Hot water

8. E = C x ρ2 ρ1 – ρ2 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)

10. DHW Systems
Instantaneous
Multi point
Single point
Stored
Vented
Unvented

11. Conventional vented system

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.

13. Primary circuit Secondary circuit Cold water storage cistern
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
Secondary circuit
Primary circuit

14. Heating and hot water system

18. CWSC Support for CWSC Vent pipe Overflow pipe
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

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.

20. Immersion heaters
Immersion heater

21. 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. Advantages Disadvantages
Easy to maintain
Poor shower performance because of the relatively low pressure at which water is delivered
Low maintenance costs especially with electric heating
Possible water contamination (from debris entering the feed and expansion tank)
Relatively easy to install
Noisy cistern filling as water is drawn off and replenished from the mains supply
Less to go wrong when compared with other systems
Fixed system design and limited positioning options because of the need to generate a head of pressure
Less risk of being without a hot water supply due to breakdown
Potential freezing hazard of the feed and expansion tank
Gain the traditional airing cupboard
Likelihood of unbalanced hot and cold water supplies

23. UNVENTED SYSTEM

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 .

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

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.

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 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

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 gap
Baffle

29. 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-resetting energy 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

30. 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

31. Temperature and pressure relief valve

32. Safe 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.
Discharge from T&P relief valve

33. Advantages
Disadvantages
Balanced hot and cold water supply to all outlets at high pressure
No storage back up if mains water fails
Allows greater system design flexibility
Poor mains water pressure will cause poor performance
Cylinder can be sited almost anywhere
Requires high level of maintenance
High performance showers without need for pumps
Need 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 cisterns
Less space consuming and saves installation costs as there is no CWSC
Potable (drinking) water at all taps

34. MAINS PRESSURE VENTED SYSTEM

35. 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).

36. Thermal store (configuration for a sealed central heating system)
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

37. INSTANTANEOUS SYSTEMS

38. 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

39. SOLAR HOT WATER SYSTEMS
0 – – – 25.15

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

41. 30o – 40o

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

43. 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. Component
Option
Solar collector
Flat plate
Evacuated tube
Heat transfer
Pumped
Thermo-siphon
Heat storage
Combined solar pre-heat and boiler heated water
Separate solar pre-heat and boiler heated water
Expansion
Unvented
Vented

45. Solar collectors
Flat plate collector
Evacuated tube collector

46. 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

47. 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.

48. 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

49. 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.

50. 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

52. Flat plate Evacuated tube
Benefits
Simple design and proven technology
Higher water temperatures (can be used for space heating)
Cheaper than evacuated tube
Higher efficiency
Limitations
Lower efficiency, requiring more roof space than evacuated tube
More expensive
Lower water temperatures
High stagnation temperature
Cannot be embedded in roof structure

53. Heat transfer
The system contains water and low toxicity polypropylene glycol (anti-freeze for frost protection) and corrosion inhibitors as a heat transport mechanism.
If the solar collectors can be mounted below the level of the solar store circulation will occur by natural circulation (the thermo-syphon effect). This system is often used in Mediterranean countries.
In the UK, the solar collectors are usually mounted on the roof so circulation in the solar primary circuit is pumped.
Pump controlled by a differential temperature controller which compares the temperature at the collector panel outlet with the temperature of the water in the solar store and switches pump on when sufficient thermal gain.

54. Heat storage
A solar DHW system requires the solar heat to be stored in a vessel to allow the heat to build-up during the day.
Because of the pattern of solar gain, a back-up heat source will also have to be integrated into the system. This can either be in separate storage cylinders or in a combined cylinder.
Separate solar cylinder Combined cylinder
Solar coil at bottom of cylinder pre heats water

55. Combined cylinder heat storage
The solar coil should be the below the boiler coil, allowing the solar pre-heated water to rise up the cylinder for any top-up heating from the boiler coil.
Boiler coil
Solar coil

56. Safety of solar systems
Solar energy can quickly accumulate enough heat in a high performance collector to convert the circulating liquid to steam under significant pressure.
Sometimes a high temperature situation arises during a circulation failure, eg a faulty pump, power cut and both the primary and secondary system may overheat. The state in which there is no net heat extraction from the collector is described as ‘stagnation’.
This can be dealt by either:
Vented system: hot water vented to the atmosphere into a CWSC
Unvented sealed system, either:
An expansion vessel and safety valve
Drain-back: switch-off pump and drain away from a collector into a vessel

57. Vented system
A solar DHW open vented primary system has features that distinguish it from conventional open vented heating systems:
If antifreeze is used, the feed and vent cistern is not normally connected to the cold water mains via a float valve, in case of dilution due to long-term evaporation. Hence, re-filling of the header cistern with antifreeze has to be undertaken manually.
Cistern needs to be fitted high up to gain the greatest static pressure (head) above the collector. It is not always practicable to achieve sufficient height.
Schematic pipework layout of open vented solar primary system

58. Unvented system
An unvented system contains a vessel capable of holding the primary fluid contents of the collector at all the permitted pressures in the system. This vessel can be of the membrane expansion type or of the drainback air-pocket type.
Schematic pipework layout of unvented expansion vessel type

59. Unvented system: drainback
The location of a solar collector normally above the store, allows the possibility the ability to drainback. The system is only partially filled, leaving an air pocket permanently present in the circulatory circuit. When the pump is off, the fluid does not fill the collector but instead rests wholly within the lower part of the circuit. When on, the pump pushes the air out of the collector replacing it with fluid, the air is then displaced down to inside a ‘drainback’ vessel
Schematic layout of drainback solar primary

60. Solar hot water with a combi boiler
Because a combi boiler heats the hot water directly from the mains, there is no hot water cylinder. It is therefore more difficult to install a solar system. It is possible to pre-heat the mains water inlet to a combi boiler but the boiler must be solar compatible ie have approved temperature blending equipment and be able to monitor the temperature of water entering the system and modulate the flame accordingly.
An alternative arrangement is shown below whereby a twin-coiled cylinder supplies the majority of outlets and the combi directly supplies a shower for example.

61. FITTINGS

62. Fittings: taps
About 20% of domestic water flows through sink and basin taps.
Long, un-insulated pipe runs (dead-legs) waste water whilst waiting for the tap to run hot.
Spray taps (aerators) can save about 80% of the water and energy used for hand washing
The amount of water used by a tap is related to frequency of use, flow rate and duration. Flow restrictors that reduce flow rate do not necessarily reduce water use, since some functions of taps are volume dependent (e.g. filling sinks, kettles) rather than duration dependent (rinsing, hand washing).

63. Fittings: showers and baths
In new houses, showers and baths account for around 45% of water used.
Showers can use a third of the water of a bath but people tend to take them more frequently and power showers and mains pressure systems have increased flow rate: long shower can use more water than a bath.
Showering is single largest use of hot water in modern homes. Water saver showers introduce air or atomise the water drops to improve wetting for a given flow rate. Feels like a ‘power shower’ but uses less water.
Or, flow rate can be limited. Minimum acceptable 4 litres/minute.
Typical standard bath capacity is 225 litres. Reduced capacity (shallow) baths are available with a capacity of 140 litres which retain a standard bath footprint.