Unvented Systems

Building Services
By John Bradley– licensed under the Creative Commons Attribution – Non-Commercial – Share
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John Bradley
Hot water
Expansion (m3)
Volume of water in system (m3)
Density of water before heating (kg/m3)
Density of water after heating (kg/m3)
DHW Systems
Cold water storage
Vent pipe (at
least 19mm)
to hot water outlets
Cold feed pipe
Coil heat
Cold water supply
from rising main
Heating and hot water system
Vent pipe
Overflow pipe
Support for CWSC
expansion space
Normal expansion
Cold feed pipe
Rising main
Flat, level, rigid platform to extend
> 150mm beyond cistern
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.
Immersion heaters
Immersion heater
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
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
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
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 .
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
vessel replacing
feed and
expansion tank
for heating
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
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
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
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
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 :
1. Thermostat, set to operate at 60 to 65oC
2. 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
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 95oC
Unvented system with safety features
To hot taps
relief valve
To cold taps
Pressure reducing valve
(to keep pressure less
than the expansion
valve opening pressure)
Energy cut out
relief valve
Line strainer
(to remove
Stop valve
Hot water cylinder
Check valve (to
stop water
returning to cold
water main)
To drains
Cold water main
Temperature and pressure relief valve
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
– 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
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
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
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).
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
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
0 – 50 7.37 – 10.07
23.50 – 25.15
Solar irradiation
811 kWh/m2/year
Contours show solar
irradiation in kWh/m2 pa
1115 kWh/m2/year
30o – 40o
The proportion of the total DHW load that can be
met by the solar system is the solar fraction
System efficiencies
Primary pipes
to taps
Solar collector
Flat plate
Evacuated tube
Heat transfer
Heat storage
Combined solar pre-heat and
boiler heated water
Separate solar pre-heat and
boiler heated water
Solar collectors
Flat plate
Evacuated tube
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
The most common type of unit for DHW purposes is the glazed type
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.
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
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.
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
Flat plate
Evacuated tube
Simple design and proven technology
Higher water temperatures (can be
used for space heating)
Cheaper than evacuated tube
Higher efficiency
Lower efficiency, requiring more roof
space than evacuated tube
More expensive
Lower water temperatures
High stagnation temperature
Cannot be embedded in roof structure
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
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
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
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
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
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
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
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
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.
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).
Fittings: showers and baths
In new houses, showers and baths account for around 45% of water
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
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.