TEHVA The Electric Heating & Ventilation Association
DOM 9:
GUIDE TO THE DESIGN
OF ELECTRIC WATER
HEATING SYSTEMS
FEBRUARY 2006
The Electric Heating and Ventilation Association
Westminster Tower, 3 Albert Embankment, London SE1 7SL
t: +44 (0) 20 7793 3008 f: +44 (0) 20 7793 3003
www.tehva.org.uk
DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
Introduction The cylinders must comply with relevant British
Standards, including BS 1566: 2002.
Electricity can be a clean, quiet, efficient and When pressurising the hot water cylinder with a
convenient way of heating water. Whether it is for cold water tank is not easily achievable, or where
small quantities of water from a sink heater or an higher pressures and flow rates are required to
instantaneous shower or whether it is for larger enjoy the benefit of mixer showers and continental
quantities for general household use, there is a wide style taps, an unvented hot water cylinder may be
variety of products and methods of heating the installed. In this case the cylinder is pressurised by
water economically. Where smaller, and infrequent the incoming mains cold water. Because the mains
quantities of water are required, convenience and pressure can be much higher than a vented cylinder
space are generally the overriding factors to be can withstand, pressure reducing valves and safety
considered, so accordingly storage is not required devices have to be installed. A simplified diagram of
and the tariff applying would be that applicable at an unvented hot water cylinder layout is shown in
the time of use. However, where larger quantities Fig 2a. A diagram of an unvented hot water cylinder
are required, hot water cylinders would be used to layout with internal expansion is shown in Fig 2b.
store the water, which would generally be heated Unvented cylinders must comply with Building
by utilising available off-peak tariffs. Regulation G3, compliance can be shown by
approval to an accredited test regime (for example
Typically, electricity suppliers offer a 7-hour night
as operated by the British Board of Agrement, the
tariff and some may also offer a 10-hour tariff which
Water Research Centre or KIWA) or BS 7206.
provides periods of off-peak supply during the day,
however, not all suppliers offer this tariff, so the Unvented cylinders typically have two factory fitted
availability should always be taken into immersion heaters. Each immersion heater must
consideration when changing energy supplier. be separately thermostatically controlled. The
The following systems are included: lower element has to be capable of heating at least
a) Hot water storage systems 85% of the cylinder contents and the upper
b) Air to water heat pumps element has to be capable of heating at least 60
c) Thermal stores litres of water. The lower element is connected to
d) Solar water heating the off-peak electricity supply and the upper
element to the on-peak for boost operation. The
This guide considers the principles of the design of positions of the immersion heaters are intended to
hot water heating rather than detailed design assist the efficiency and economics of off-peak
guidance, particularly with regard to heat pumps water heating. Open vented cylinders would
and solar water heating. normally also have two immersion heaters,
although these are frequently fitted at the time of
Hot Water Storage Systems system installation. The thermostat settings are
similar to those for unvented systems.
Vented/unvented hot water cylinders
In order to achieve maximum efficiency it is
The most common hot water system is the open
necessary to maintain good temperature
vented cylinder, where the pressure required to
stratification of the water within the cylinder. A
drive the hot water to the taps is provided by the
baffle or deflector on the cold inlet may be required
height of the cold water storage tank above the
in order for this to be achieved, as it will prevent
cylinder. The cold water tank is usually located in
the incoming cold water from causing turbulence
the loft. A simplified diagram of an open vented hot
and lowering the temperature within the cylinder.
water cylinder layout is shown in Fig 1.
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
Fig 1 – Typical cistern fed heated
vented hot water storage FLOAT
OPERATED
system VALVE
MAINS COLD INLET WARNING PIPE
VENT PIPE
COLD WATER
CISTERN
IMMERSION
HEATER 2
HOT WATER
DISTRIBUTION
PIPE IMMERSION
HEATER 1
HOT WATER STORAGE
VESSEL
Fig 2a – Typical directly heated TEMPERATURE RELIEF VALVE
OR TEMPERATURE & PRESSURE
unvented hot water storage system RELIEF VALVE WITH DISCHARGE
with external expansion volume OUTLET TO TUNDISH
HOT WATER
SUPPLY
IMMERSION
HEATER 2
EXPANSION
VESSEL
IMMERSION
HEATER 1
STOP VALVE PRESSURE SINGLE
REDUCING VALVE CHECK VALVE
HOT WATER STORAGE VESSEL
EXPANSION VALVE WITH
DISCHARGE OUTLET TO
TUNDISH
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
Fig 2b – Typical directly heated TEMPERATURE RELIEF VALVE
OR TEMPERATURE & PRESSURE
unvented hot water storage system RELIEF VALVE WITH DISCHARGE
with internal expansion volume OUTLET TO TUNDISH
HOT WATER
SUPPLY
INTERNAL EXPANSION
VOLUME
IMMERSION
HEATER 2
IMMERSION
HEATER 1
STOP VALVE PRESSURE SINGLE CHECK
REDUCING VALVE VALVE
HOT WATER STORAGE VESSEL
EXPANSION VALVE WITH
DISCHARGE OUTLET TO
TUNDISH
Combination Units
A combination unit is a hot water cylinder with an units and are selected as an economic like-for-like
integral cold water feed tank situated immediately installation. Units should be positioned such that
on top of it. Combination units are installed there is always sufficient head to supply a good flow
primarily in dwellings that may not have a cold of water to the hot taps. In practice the
water cistern available for pressurising a hot water combination will often need to be placed on a
cylinder and where the householders may not wish plinth so that the top of the unit is as high as
to have an unvented system. The majority of possible. A simplified diagram of a combination unit
combination units are replacements for existing layout is shown in Fig 3.
Fig 3 – Typical vented
combination hot water
storage system
FLOAT OPERATED VALVE
MAINS COLD INLET
WARNING PIPE
COLD WATER CISTERN
VENT PIPE
HOT WATER
DISTRIBUTION
PIPE
IMMERSION COLD WATER CISTERN AND
HEATER 2 HOT WATER STORAGE VESSEL
IN SAME UNIT
IMMERSION
HEATER 1
HOT WATER STORAGE VESSEL
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
Insulation 1.28 x (0.2 + 0.051V2/3)
It is essential that cylinders are well insulated to where V is the nominal water capacity of the
minimise heat losses. Current Building Regulations cylinder in litres
state that the 24-hour heat losses in kWh from
insulated cylinders must be no more than: This equates to the losses shown in Table 1.
Table 1 – Maximum heat loses from insulated cylinders
Cylinder size
British Standard rating BS7 BS8 BS9E
Cylinder capacity (litres) 120 144 210
24-hour max heat loss (kWh) 1.84 2.05 2.56
Immersion heaters temperature, firstly to obtain the maximum
Domestic immersion heaters are typically rated at storage, without risking dangerously high
3kW, although lower ratings are available, temperatures; and secondly, to allow for the very
particularly as replacements for existing heaters. In small drop in temperature of the water during the
hard water areas, calcium carbonate is deposited as day, due to standing heat loss and the introduction
the water is heated, resulting in the well-known of cold water to replace the hot water that has
scale formation. This can build up on an immersion been used.
heater giving some measure of protection from
corrosion. If the scale builds up too much however, The top immersion heater does not switch on
the heating element overheats and premature automatically as soon as the temperature at the
failure can occur. Scale builds more quickly at thermostat drops below 60ºC, as this would
temperatures in excess of 60ºC and thermostat restrict the amount of off-peak charge required to
settings should not generally exceed this unless heat the cold water in the cylinder, thereby
using immersion heaters with a metal sheath having increasing the on-peak use and therefore the
low surface adhesive properties, for example running costs. When householders are aware that
titanium. Titanium sheaths should also be used in the hot water storage has been depleted, they
areas with naturally occurring soft water, owing to manually operate a one-hour boost on the
the presence of free oxygen and carbon dioxide, controller, which will be sufficient to heat the top
which is acidic and aggressive to common metals. section of the cylinder without the risk of
Immersion heaters should comply with BS EN forgetting to switch it off.
60335-2-73: 1997 or be specifically designed for and
tested as part of the water heater to which they During installation it is recommended that the
are fitted. settings of any water heating time controls are
matched to the electricity supply tariff switching
Controls times and usage/occupation requirements, or the
Each immersion heater has its own thermostat. metering equipment itself directly switches the
The top heater, used for a boost if the off-peak water heating circuit. This is to avoid the risk of
heated water has been used up, is generally set at time slippage, which could result in on-peak energy
60ºC, whilst the lower heater, used for heating the being used – unnoticed by the householder –
whole cylinder at the off-peak rate, is set at 65ºC. increasing running costs. A schematic wiring
The lower heater thermostat is set at the higher diagram of a hot water controller is shown in Fig 4.
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
1 hour boost timer
Timer
Neon
Neon Boost
timer
On-off
switch
24 hour L N Single
supply N L Element
24 hour L L (Night element) Night only L
supply N L (Boost element) supply N
N
Off peak water heating controller Off peak boost controller
Fig 4 – Internal wiring diagrams for off peak water heating controllers
Cylinder sizes
If hot water is supplied using an off-peak tariff then 10-hour tariff usually provides 5 hours of off-peak
it is usual to install a 210-litre cylinder, this will electricity overnight and two periods of off-peak
minimise the amount of on-peak use, which may be during the day. This means that a smaller quantity
necessary from time to time. If smaller dwellings of water needs to be stored as it can be heated at
are specifically designed for low occupancy, such as more frequent intervals. For similar sized cylinders
sheltered accommodation, then smaller sizes of on the two different tariffs, the cylinder on the 10-
cylinders can be used. Also, if a 10-hour tariff is hour tariff will use significantly less electricity at the
available, a smaller cylinder size may be used. The on-peak rate, as shown in Table 2.
Table 2. Comparison of on-peak energy consumption
Proportion of on-peak energy (%)
Occupancy Cylinder size 7-hour 10-hour
120 litres 13% 7%
2 Persons 144 litres 10% 0
210 litres 3% 0
120 litres 17% 10%
3 Persons 144 litres 14% 1%
210 litres 4% 0
120 litres 22% 14%
4 Persons 144 litres 17% 2%
210 litres 6% 0
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
Thermal stores
A thermal store is a cylinder, or rectangular storage Safety devices will prevent water with too high a
tank, with a primary store of water, which is heated temperature from being delivered to the taps,
on available off-peak tariffs. The heated water particularly when a tap is first opened. As the
remains in the vessel; it is not intended for end use. temperature of the water inside the vessel drops,
Mains cold water is then circulated through a heat so the flow rate will reduce to enable the water at
exchanger within the vessel and delivered to the the taps to maintain the required temperature. For
taps. The thermal store is helpful where there is an more detailed information on thermal stores the
insufficient head of water to provide adequate hot Water Heater Manufacturers Association has
water supply to the taps and householders do not produced a guide entitled Performance Specification
wish to have an unvented hot water supply system. for Thermal Stores. A simplified diagram of a thermal
store is shown in Fig 5.
In order to obtain maximum storage capacity, the
primary water can be heated to temperatures in
excess of those for standard open vented cylinders.
Fig 5 – Thermal store system
for domestic hot water
FLOAT OPERATED
VALVE
WARNING PIPE
VENT PIPE
COLD WATER CISTERN
BLENDED HOT OUTLET
DOMESTIC WATER HEAT EXCHANGE COIL
MAINS COLD
INLET
BLENDING VALVE
IMMERSION HEATER
IMMERSION HEATER
PRIMARY THERMAL STORE
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
Air To Water Heat Pumps those from either gas or oil fired boilers. With this
performance heat pumps are of great benefit in
Domestic heat pumps are electrically driven complying with Building Regulations.
machines which utilise environmentally free heat
existing in the air, water or ground. A vapour SAP currently recognises air to water heat pumps
compression cycle boosts the temperature of the for space heating and specifies a CoP of 2.5 for
free heat to a higher, more useful level so that it is their average seasonal performance. However, as
capable of supplying sufficient heat to provide space an air to water heat pump dedicated to supplying
and/or water heating to a dwelling. The heat is hot water only would be operating throughout the
transferred to a water circuit, which allows it to be year rather than just in the cold weather, it should
circulated as required. The prime advantage of heat achieve a significantly higher CoP because for the
pumps is that, depending on the application, they summer months the external temperature will be
can operate with efficiencies ranging from 200 to much higher. The higher the heat source, the
500%. The efficiency of a heat pump is generally greater will be the CoP of the heat pump.
referred to as the coefficient of performance
(CoP), and rather than expressing it as a The environmental benefits of air to water heat
percentage, such as 350%, it is usually expressed as pumps are also recognised by the Chancellor of the
having a CoP of 3.5. This means that for an energy Exchequer by reducing the VAT on them to 5%.
input of 1 kWh, the heat output is 3.5 kWh. This This is applicable when they are used in residential
guide focuses on air source heat pumps supplying buildings or in buildings used solely for charitable
hot water only. If hot water is to be provided from purposes.
the same unit that supplies the space heating, then
ground source heat pumps are frequently used; for Controls
further information on ground source heat pumps, When providing the water to heat the cylinder, the
contact the Heat Pump Association (HPA). temperature of the primary water must be within
the range of 60-65ºC. If the heat pump is not
Storage cylinders capable of supplying water at this temperature, then
The hot water storage cylinders for use with air to supplementary heating would be installed; this
water heat pumps would generally be indirect would generally be in the form of an immersion
cylinders, enabling the primary water supplied by heater. An immersion would very often be installed
the heat pump to circulate through the coil. As the for emergency use in case of a break down, as is the
heat from the heat pump is available economically case with gas or oil fired boilers when providing hot
throughout the day, a 144-litre cylinder would be water.
sufficient. However, if full advantage of the off-peak
tariffs is to be achieved the larger 210-litre cylinder The controls for the hot water cylinder are simple.
could be used. Insulation requirements are the They include a thermostat to switch on the heat
same as for direct cylinders. pump as and when required. The thermostat is
normally positioned about one third up the cylinder
Environmental benefits to allow a reasonable quantity of water to be
Because of the very good coefficients of heated at once, rather than cause the heat pump to
performance that can be achieved with air to water cycle too frequently. A hot water timeswitch is
heat pumps, carbon dioxide (CO2) emissions are advantageous in making maximum use of available
greatly reduced to the extent that CO2 emissions off-peak tariffs.
from heat pumps can be significantly less than
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
Solar Water Heating heating would usually supply the supplementary
heating to the hot water. If this were electricity
For even greater environmental benefit, water then the supplementary heating would be an
heating can be provided by solar panels. Because of immersion heater. Care would have to be taken to
the complexities of a solar water heating system, it prevent the supplementary heating from taking
is essential that the manufacturers are fully involved priority for heating the water.
throughout the design and installation process as
there are a number of factors to be considered, Typically, the indirect coil from the solar collector is
including the positioning of the solar collectors in located at the bottom of the cylinder, so that the
relation to shading, collector performance complete volume can benefit from this ‘free’ heat,
requirements, primary circuit fluid considerations, or preheat incoming cold water during poorer
circulation pump requirements and heat exchanger solar conditions. The cylinder therefore has a
performance. dedicated solar heated volume. The supplementary
heating heats the upper section of the cylinder so
The solar collectors used for domestic water that if it is necessary to switch it on, it will not
heating would normally be either flat plate inhibit the use of solar heating to any great extent.
collectors or evacuated tubes. Ideally they would Good cylinder insulation is again essential to
face south for maximum benefit, although if they maintain water storage temperatures and to limit
face south-east or south-west the effectiveness will the introduction of supplementary heating. An
not drop noticeably. The optimum tilt angle is electronic controller constantly compares the
usually in the range of 20-50° and most systems temperature of the solar collectors with the
follow the roof pitch. Even if the collectors face temperature in the cylinder, and whenever the
due east or west, they should still produce sufficient collectors are hotter than the cylinder, the
hot water, although avoiding shading becomes more controller switches on the system’s circulating
important. pump.
The hot water cylinder would have a A simplified diagram for typical solar water heating
supplementary source of heating in the event of is shown in Fig 6. Further information on solar
insufficient heat being provided from the solar water heating can be obtained from the Solar Trade
panels. The same fuel that supplies the space Association.
Solar collector
To taps
Controller
Immersion heater
Cold water feed
Pump
Fig 6 – Schematic diagram of solar heating for hot water supply
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
Point-of-Use Water Heating pressure from the cold water cistern is sometimes
insufficient to provide a high enough water flow, and
Point-of-use water heaters are usually fitted into can be a more economic solution than the power
existing dwellings. They are particularly useful shower.
where extensions and alterations are being
undertaken, as they need only a cold water supply, Point-of use water heaters can also be appropriate
usually mains cold water. Probably the most for the growing markets of holiday lets and second
common point-of-use water heater is the homes, where usage can be very varied and
instantaneous shower, which is often used on the irregular.
first floor of two storey dwellings as the head
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DOM 9: GUIDE TO THE DESIGN OF
TEHVA
The Electric Heating & Ventilation Association
ELECTRIC WATER HEATING SYSTEMS
References
Heat Pump Association
2 Waltham Court, Milley Lane, Hare Hatch, Reading, Berkshire, RG10 9TH
Tel: 0118 940 3416
Solar Trade Association
The National Energy Centre, Davy Avenue, Knowle Hill, Milton Keynes, MK5 8NG
Tel: 01908 442290
5
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TEHVA The Electric Heating & Ventilation Association
The Electric Heating and Ventilation Association
Westminster Tower, 3 Albert Embankment, London SE1 7SL
t: +44 (0) 20 7793 3008 f: +44 (0) 20 7793 3003
www.tehva.org.uk
A Association
