Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
Unvented Hot Water systems
Index to section 1
Page
Short history of our hot water supply
2
Instantaneous water heat heater, inlet controlled
3
Outlet controlled, multipoint
4
Storage water heaters under 15 litres.
5
Under sink heater
6
Bulk storage water heater heaters, combi tanks
7
Bulk Storage Heaters, floor mounted
8
Traditional indirect cylinder, calorifier
9
Water jacketed tube heater, heat bank, thermal stores
10
Combination boilers
11
Diagrams of storage type and non storage combi boilers
12
Conventional vented water systems
13
Balanced and unbalanced cold water systems
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
1
DOMESTIC HOT WATER SUPPLY
1.1 Short History of Our Hot Water Supply.
Over the past century hot water systems have developed from the old coal burning
fireside boiler through various types of tank and cylinder systems heated by back
boilers or range boilers.
Modern way of life has significantly changed the way heating and hot water systems
are used. Our use of water for personal hygiene purposes has increased significantly
we now take more baths and showers than in the past. We have modern fitted kitchens
that include water-consuming devices like washing machines and dishwashers etc.
Also, during the last two decades an increase in air travel for the masses has meant
people have been to USA or Europe and experienced their unvented systems. They
will no longer accept a trickle of water for a shower or a system that will half fill a bath
and then require an hour or so until the system has recovered enough to draw another
one.
People insist on having hot water available in unlimited quantities, whenever they want
it.
Development of space heating design coupled with hot water supply has resulted in
many types of systems being available and an even wider range of ways that they can
be controlled. System efficiency is now a required part of system design.
Building Regulations Approved Documents. L1 & L2 lay out minimum standards, which
must be achieved.
The advent of combination boilers and more importantly their ability to deliver
reasonable quantities of hot water at mains pressure has had a significant impact on
system design.
Unvented domestic hot water systems are not a new innovation; they have been used
successfully and safely for over 50 years in the USA, Europe and Australia. The United
Kingdom has been one of the last of the industrialised countries in the world to adopt
the use of unvented hot water systems.
The use of unvented hot water systems in the UK has been on the increase over the
last few years, particularly in new build housing and conversions where it has some
significant advantages over the traditional types of systems.
Unvented hot water systems must be kept in perspective, they are another option of hot
water supply and not a one size fits all panacea suitable for every occasion. With
correct design and application there is very little that a traditional vented system cannot
achieve particularly when high pressure alone is not the overriding requirement.
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
Domestic Hot Water Supply – The Options.
1.2 There are many different types of domestic hot water systems available and each one
has its own merits, applications, requirements, advantages and disadvantages.
The following pages contain details of the main types of domestic hot water systems
that have been available to-date. Some of them have had fairly limited usage, some
have been limited by their design and others are now out dated and seldom considered
as a viable alternative.
INSTANTANEOUS WATER HEATER (GAS OR ELECTRIC)
There are two common types of heaters available
Inlet Controlled, (Fig 1)
These are most commonly over sink type with a single outlet at the point of use and are
usually mains fed. Gas fired types are the most common but electrically heated units
are available
.
When the inlet valve is opened water flows through the heater at a stipulated minimum
flow rate and activates the heat source triggered by the flow or occasionally the
pressure drop of the water. The water is heated as it passes through the heat
exchanger and discharges at its outlet. This will continue as long as water flows
through the heater. When the control valve is closed, the flow of water stops and the
heat source shuts down.
This type of water heater is suitable as a single point unit where it may be difficult to
supply hot water from a conventional hot water system due to long dead legs, or where
small quantities of hot water are required, such as hand wash basins or showers.
From the point of view of safety, should the heat source fail to shut off, any build up of
temperature or pressure is relieved through the hot water outlet, which is permanently
open to atmosphere.
Figure 1
Inlet Controlled
Gas or electric
Instantaneous
Water
Heater
Hot supply
Spout is permanently
open to atmosphere.
Inlet Valve
Cold supply
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
Outlet controlled multipoints (Fig 2)
This type of water heater, commonly called a multi-point water heater is capable of
supplying water to more than one outlet. Unlike the inlet controlled instantaneous water
heaters this type is under pressure normally from the water mains. It needs no vent
pipe, as there is no storage of hot water requiring to be vented or to accommodate
expansion. The water is heated on demand. Any expansion of the water caused by
latent heat in the boiler after the tap has been closed can be accommodated by back
pressure into the cold water supply. It is a requirement that no check valves or
stopcocks with loose jumpers are upstream of the water heater as this could obstruct
the expansion and this may lead to a dramatic increase of pressure which may cause
something on the system to rupture.
In order to prevent this possibility some heaters are supplied with a pressure relief
valve and a small expansion vessel to accommodate expansion and prevent any loss
caused by the pressure relief valve operating, and water discharging to waste.
With the advent of modulating gas valves these heaters have widened their usable
output potential and control of the temperature is more finely tuned than before, making
them a lot more flexible to use in a greater number of installations.
NB. When used to supply a shower, great care should be taken in selecting the
shower mixer. Experience has shown that in some circumstances, use of a nonthermostatic, manually operated type control valve may be preferable, as thermostatic
controls can have an undesirable effect on the operation of the heater and it's ability to
maintain an even temperature.
This is caused by the two controls acting against each other, one adjusting the flow of
water to the thermostatic shower and the other trying to modulate the gas flow to the
appliance burner. The end result is erratic operation
Figure 2
Outlet controlled
Instantaneous
Multi point
Water
Heater
Hot
Isolation
Valve
Cold
Other outlets may
be fed from these
pipes by cutting in
tee pieces.
Cold supply
(Inlet)
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Supply to hot & cold taps
or shower mixer.
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
STORAGE WATER HEATERS (ELECTRIC)
The most common types are the 'over' or 'under' sink water heaters which are referred
to as displacement heaters and are still commonly found in situations such as factory,
garage or workshop toilet areas, in bedroom extensions or loft conversions or as standalone units in kitchens etc. They are simple to fit and require only a water (usually
mains) and electrical supplies.(13 amp for the three kilowatt unit). Most are either One,
Two or Three-Kilowatt outputs.
These are non-pressurised water jackets or bodies and must be permanently vented to
atmosphere. There are some makes that are of a closed pressure type similar to the
multi point operation except heated water is stored for use and not heated
instantaneously. These units are restricted to a maximum of 15 litres storage capacity.
Over Sink Storage Heater (Fig 3)
Fig 3 shows the over sink version of an electric storage water heater, the outlet, always
open to the atmosphere to allow the expanding hot water to vent itself and prevent a
build up of pressure in the heater. The water is heated electrically to approx 60° and
has a thermostat for temperature control based on the same principle as an immersion
heater.
Operation:
As cold water enters the bottom of the heater the water level in the heater rises
displacing the hot water out through the outlet. When the inlet valve has been closed,
water will continue to flow from the outlet until the water level inside the heater settles
to the top of the outlet pipe.
Having drawn water, the unit re- heats, controlled by a thermostat.
As the water in the unit is heated, it expands, and can often be mistaken for a dripping
tap as it vents through the outlet. This is perfectly normal and should be clearly
explained to the customer. This should hopefully avoid service call-backs to a heater
that is working exactly as intended.
Fig 3
Over sink storage
heater
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Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
Under Sink Heater (Fig 4)
The same principle of water displacement applies to under sink water heaters but they
differ by the way the water is delivered. These are normally single point water heaters.
Always open to atmosphere.
Some manufacturers use the same basic unit to provide over or under sink installation
by simply changing the position of the heater element and adding a spout kit for over
sink or vented tap kit for under sink operation. This gives real flexibility to installation
choices.
The hot water outlet pipe is terminated at the point of use, and as stated must be
permanently vented. A special mixing tap or valve operating on the inlet controls water
flow. (See Fig 4)
This type of heater are usually installed close to the point of use and are suitable for
delivering small quantities of hot water (15 litres or less) and prevent long dead legs.
Figure 4
Under Sink
Storage Heater
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
BULK STORAGE WATER HEATERS (COMBI TANK)
Bulk storage water heaters are generally available as either wall mounted or floor
mounted models. This is basically a traditional hot and cold vented system but shrunk
down into one package. The heat source is usually an electric immersion heater. The
storage cistern has a vent pipe and cold feed being an integral part of the unit.
Often wall mounted (on angle iron brackets built into the wall fabric), usually made of
copper. Looks like a cylinder, with a copper cistern as an integral part. The cistern only
satisfies the hot water requirements, with the rest of the cold taps in the house being
mains fed. Frequently found in blocks of flats or converted bed-sit type properties.
Good, simple operation but limited volumes and pressure available which is entirely
dependent on the static head and as these units are normally just under the ceiling
height of the property, this may be less than two meters head. When fitted tight up to
the ceiling the ballvalve can be difficult to change. This system is not good for showers
due to low static head and therefore pressure inbalance between hot and cold supplies.
Often during updates these are replaced by combi boilers or unvented units.
Safety vent pipe
Cold water
storage
Figure 5
Bulk Storage
Wall Mounted
(Combination tank)
To Hot water Draw off’s
Cold
supply
Electric
element
Drain cock
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
Bulk Storage Heaters, Floor Mounted
Floor mounted bulk storage heaters are usually available in several types.
A self-contained unit similar in principle to the wall mounted unit in Figure 5
or the type of unit where the cold water cistern is fitted remote from the water heater
and all cold feed and hot water draw off pipework is external to the heater itself. This is
essentially a direct cylinder. Water pressure and flow is dependent on the head of
water generated by the siting of the cistern
One other floor type is the pre-plumbed unit with angle iron framework and Fig 6 shows
a typical unit of this type. These only require connections to be made to the house
pipework to complete the installation. Water pressure is limited by the static head
pressure available from siting the unit. They can frequently be found installed in 70’s
style three bed type houses or flats and often in industrial units feeding toilet areas.
The units are electrically heated and tend to be used in conjunction with the cheaper,
overnight Economy 7 tariff.
Figure 6
Bulk Storage Heater
Floor Mounted
Angle iron frame
Safety vent and
expansion pipe
Cold water
storage
cistern
Heater with
electric element
Cold feed
To hot taps
Cold mains
supply
Draw offs
D/O
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
Water Jacketed Tube Heaters
Traditional indirect cylinder (Calorifier)
In a traditional design of hot water cylinder, hot water from a central heating boiler
passes through a coil inside the cylinder, which is surrounded by the secondary water
for the domestic hot water system. The hot water in the primary flow passes through
the primary heating coil and indirectly heats the secondary water for the domestic draw
off. (See Figure 7)
Found in the majority of UK residences, these systems are the ultimate for flexibility
and add on options. Sometimes can be limited by static head available, however, if
powerful showers are required it is usually a fairly straightforward job to fit a shower
pump. With correct pipe sizing, volumes of water available can match or exceed any
that unvented systems offer.
Maintenance is minimal but may experience problems in cold loft areas if precautions
against freezing are not applied.
Safety vent
Pipe
Hot supply to taps
Secondary
hot water
Primary heating coil
Primary flow
Primary return
Cistern fed
Cold supply
Figure 7 Indirect Hot water cylinder
High performance water cylinders
High performance water cylinders have heat exchangers which incorporate a larger
surface area, the benefits of this include a reduction in recovery time which also helps
to increase efficiency.
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
Heat Bank or Thermal Stores
Thermal Stores works in exactly the opposite way to an indirect hot water cylinder. The
domestic hot water is passed through a tightly coiled finned copper tube (heat
exchanger). This is housed within a cylinder, through which the primary water passes.
This is known as a heat bank or Thermal store. So the surrounding water heats up the
water in the coil.
A variant to this, is the plate type heat exchanger, which is usually kept externally from
the cylinder, and the heat bank water is pumped through the plate heat exchanger at a
controlled rate, which heats the secondary water, to give the required temperature of
hot water at the taps.
In simple terms it is an instantaneous heater with a heat bank reserve within the
primary cylinder. As hot water is drawn off the motorised valve closes the heating
circuit and the entire boiler output is directed into the primary hot water store, heating it
up as it flows through the boilers heat exchanger. A smaller version of this principle is
used in combination boilers.
It is a Water-to-Water Heat Exchanger. These have good flow rates and volumes
available until the thermal store becomes exhausted, then the recovery time becomes
critical.
A smaller boiler may be utilised as hot water priority is used and so the load will be
either Heating or Water but not both simultaneously.
Primary flow
Cold supply
Heat
Bank
Hot supply to taps
Primary
return
Heat
Exchanger
Motorised valve
Figure 8 Thermal store
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
COMBINATION BOILERS
There are two main types of combination boilers currently available. Storage, which
uses a water-jacketed tube heater (Fig 9) and Non-storage, which uses a water-towater, type heat exchanger (Fig 9a).
Combination boilers consist of a sealed central heating system (including all
components) and a domestic hot water system combined within its casing,
When the boiler is in central heating mode, water is heated in the main heat exchanger
and pumped around the heating circuit to the radiators. In hot water mode, a sensor in
the cold water flow pipe in the boiler senses the demand for hot water and a diverter
valve is actuated. The full heat output of the boiler is directed into a small heat bank
where it heats the incoming cold water via a very efficient coil or plate heat exchanger.
The heated water is then carried to the hot water taps. While the hot water is running
the boiler remains in this mode. The heat transfer to the heat bank is rapid as the full
output of the boiler is used and is only heating a small water quantity. Input rates in
excess of 30Kw is heating a relatively small volume of primary water. However we are
all subject to the laws of physics and it requires far more energy to heat water
dynamically as opposed to statically. This can erode the benefits of economy offered by
the instantaneous heating method.
Several combination boilers incorporate a storage facility using a small reservoirs (Heat
Banks) of pre-heated water to top up flow rates. When hot water demand ceases the
diverter valve switches the boiler output back to the heating circuit.
Combination boilers are generally fitted with modulating gas burners to give improved
control over the heat output in both heating and hot water modes. When the boiler is in
heating mode the firing of the burner is modulated to match the heat output to the
system requirements, giving efficiency improvements. In hot water mode the
modulating burner gives improved control over the heat output and allows for small
quantities of water, as low as 1 litre per minute, to be heated.
Most combination boilers have electronic temperature monitoring and control to allow
the automatic adjustment of burner output to match water flow.
Due to the ease of fitting, many combination boilers were given a bad name because
installers were guilty of grossly exaggerating claims about their performance and
specified them wrongly. This then created problems by not being able to meet
consumer's expectations of them.
When used in the right situation combination boilers are an excellent option but system
designers must take into consideration the hot water flow rates that is available when
specifying the boiler and if it meets the customers needs, use it, if not select another
type of system.
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Section 1
Storage Type Combination Boiler Fig 9.
Reasonable quantity of primary water
in heat exchanger.
Sometimes as much as 80 litres
Combi boiler non-storage type.
Heating and hot water are heated
within the appliance case by a single
burner and there is no hot water
storage. A diverter valve selects the
heat source required and a water to
water heat exchanger may be used
to transfer heat to the domestic hot
water. The hot water takes priority
and is kept at mains pressure
because it is controlled by the outlet
taps.
Water to Water Heat Exchanger Fig 9a.
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
CONVENTIONAL HOT WATER SYSTEMS
There are two basic arrangements for these types of system, Figure 10 shows an
unbalanced vented system and Fig 11 shows a balanced unvented system. When we
talk about balanced we are referring to the hot and cold water supplies coming from the
same source and thereby equalising the water pressure available to the outlet fittings.
Remember that at we are only considering the provision of hot and cold water supplies
and not how the hot water is generated.
Unbalanced System (Vented)
In an unbalanced system all the cold water taps in the property are supplied from the
mains supply (direct cold) and all hot taps are supplied via the cold water storage
cistern.
The pressure available at the hot water taps is dependent on the head of water
available and is determined by the height of the cold water storage cistern above the
outlet. The main disadvantages of this type of system is that in many instances the
height of the cistern is governed by the design of the property and it’s siting is a
compromise between that which is desirable and what can actually be achieved.
To give an adequate flow of hot water at the highest draw off point, normally in the
bathroom, the storage cistern is usually positioned in the roof space. Even with the
storage cistern at its highest possible in the house, the pressure of the incoming cold
water main and the hot water supply will be unequal. This unequal or unbalanced
pressure can affect the performance of mixing taps and showers and can result in
fluctuating temperatures, which can be at least, be very frustrating to the user or may
even be a danger. Some manufacturers of showers advise the pressure differences
between cold and hot to be a maximum ratio of 5:1 to achieve a balanced shower.
Safety vent pipe
C.W.S.C.
To hot
Water taps
Cylinder
Cold mains
supply
To cold
Water taps
Drain off
Fig 10 Unbalanced (or direct cold) System (Vented)
Balanced (or indirect cold water) System (Vented)
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© NIC Certification 02/08
Certification Scheme
For Individual Unvented Hotwater Operatives
Section 1
In this system (Fig 11) all taps, with the exception of the kitchen sink tap, are fed via the
storage cistern. This gives equal or balanced pressures, desirable for mixing taps and
showers resulting in better temperature control. The pressure is still determined by the
head of water available.
This system with correctly sized pipes will deliver water simultaneously to multiple
outlets in large volumes, but generally at lower pressure than an unvented system.
N.B. "Every storage Cistern shall be so installed as to minimise the risk of
contamination of stored water". (Water Regs 2000) Schedule 2 Reg.16 (Formally
covered in Bylaw 30)
This means it should be suitable for domestic usage and of wholesome quality.
Reg. G 4.2. (2000) States: "All cold water fittings located within the building but outside
the thermal envelope, or those outside the building must be protected against damage
by freezing"
This should help the burst pipe problem associated with “Wet lofts".
Safety vent pipe
C.W.S.C.
To hot
Water taps
Kitchen
Tap
Cold Mains
supply
Cylinder
To all other
Cold water taps
Figure 11
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Balanced System (indirect cold water) Vented
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© NIC Certification 02/08