to identify your hot water system
you are considering installing, upgrading or revamping a shower room or
bathroom it is important you understand what type of water system you
have and what equipment you may use (or need to use) as a consequence.
the years as the technology behind heating and storing water has
developed so have the various components and ancillary controls required
to allow the system work correctly.
able to select the correct type of product for your installation it is
essential you understand the type of water system to which you intend to
the UK there are two basic methods of supplying water to outlets in the
average household scenario (these examples assume the property is
supplied from a water main rather than a borehole, well or water storage
hot water system
Probably the most common hot water
system in the UK.
What is a gravity hot
This type of system
hot water cylinder (often situated in an airing cupboard) being fed
with cold water from a header tank (often situated in the loft of a
The contents of a hot water cylinder
is often heated either by immersion heaters (electric elements) or by an external heat source such as a boiler.
But your cylinder may also be heated by other heat sources such as
Solar, AGA, a ground source heat pump or energy an recovery system.
No matter which method is
employed to heat the cylinder is is important to note that the cylinder remains open
vented. This is to say the system is under nothing more than the
pressure of gravity pushing water down from the header tank (which is
open to atmosphere).
system relies entirely on the height of the header tank above
the outlet point to create pressure. For those of you who are interested
in the physics, a 10 metre head of water will produce 1 bar of pressure
at the outlet.
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Advantages of a gravity
hot water system
A gravity system is about
the safest type of hot water system you can install in you house. It is
fully vented and therefore carriers none of the risks associated with
it's unvented counterpart. A gravity hot water system can have it's
pressure improved with the addition of pressure boosting pumps (such as
a shower booster pump). Gravity systems are the ONLY type of systems
that can have a booster pump fitted.
Disadvantages of a gravity
hot water system
Biggest drawback is
pressure. Gravity hot water systems are also know as 'low pressure'
systems. Given that a 10 metre head of water produces 1 bar of pressure
most household bathrooms would be lucky to achieve somewhere around 0.2
Pressure can of course be
improved with the addition of a booster pump, but this involve
additional expense, maintenance/replacement in time and then there is
the matter of noise. Booster pumps can be noisy.
If you try to blend a
mains pressure cold water feed with a gravity hot water feed (in a
shower mixer valve for example) you can experience problems. Adding a
booster pump to the hot supply can sometime help, but there are problems
water is sometimes delivered to outlets in a property from the
header tank in a loft, although there are various practical,
operational and health reasons why such a gravity feed is not always a good idea.
Conclusions about a
gravity hot water system
Cheapest of the options to
install, but generally suffer from poor pressure. Considered the most
versatile because booster pumps can be added, but this of course incurs
additional expenditure and bring with it the possibility of noise.
Note: NEVER connect a mains pressure
cold water supply to a cylinder or device specifically designed for connection
in a gravity
supply system (one with a header tank). The results can be catastrophic and may result in
severe injury and/or death.
hot water system
The most obvious way to
supply taps and outlets with cold water is a direct feed from the
incoming cold water main. From the cold water perspective this is simple
enough to do. But things become a little more complicated when you want
to create hot water from a direct mains cold water feed.
Requirements for a mains
pressure household water system
Matters begin with the
pre-requisite for good incoming mains pressure (1) and flow rate (2).
Without these you have a problem (the system will either work poorly or
won't work at all) and the higher cost of installation will have been a
complete waste of money. The added problem with a system fed from the
mains is you are not permitted under Water Bye Laws to add a booster
pump to enhance performance, so if you find you are less than happy with
your mains pressure system I'm afraid you have a problem.
Advantages of a mains
pressure hot water system
Providing you have the
requisite mains supply there are certain advantages to having a 'mains
pressure hot water system'.
Firstly, the delivery of
your hot water is at a pressure very similar to that of the cold water.
This not only means the hot water pressure is greatly enhanced when
compared to your typical gravity hot water system, but the balanced
hot/cold pressures negate many of the problems one finds in a mixed
pressure system (where you have mains pressure cold and gravity pressure
Secondly, mixer showers
and mixer taps (monobloc type - many of which now require higher
pressures to operate) work correctly. This is to say, with equal
incoming pressures on the hot and cold supplies mixer valves can operate
as intended. Problems can be experienced in a mixed pressure system
where higher pressure cold water dominates the second you open a mixer
valve. Lower pressure hot water effectively gets pushed back up the
delivery pipe - the result being little more than cold water at the
outlet. When the incoming pressures are equal the user of the appliance
decides what the blend of hot and cold water should be, not the system.
Thirdly, with a mains
pressure hot water system in your property the performance may be
likened to 'power shower' performance without the need for booster
pumps. Indeed, Water Bye Laws prohibits the use of such pumps on a mains
fed system. If you convert from a pumped gravity system to a mains
pressure system remember to remove all booster pumps that may already be
in use. Booster pumps may only be connected to a gravity system.
Fourthly, it might be
considered an advantage by many house holders not to have the need for
header tanks in lofts. This releases valuable space and development
potential. You might not even have a loft, so the advantage is clear.
Disadvantages of a mains
pressure hot water system
To every action there is
an equal and opposite reaction. In other words there are pro's and
con's. The pro's have been examined above, but then there are other
Yes. Mains pressure
systems can be expensive to install. This is to say, more expensive than
their gravity counterpart. The technology and materials that go into the
construction of mains pressure equipment is far more expensive and
complicated, so your investment will greater.
Mains pressure hot water
systems are at the mercy of fluctuations in mains water pressure. This
is a constant issue that varies the force at which water is pushing
through the system, both inside and outside your property. Local demand
will affect supply pressure, whereas a person opening a tap within the
house will further alter the pressure in your shower for example.
Although these pressure fluctuations will vary in magnitude according to
the cause you should be aware that appropriate shower equipment is
necessary to cope. Even changing the hot/cold mix ratio in your shower
will have an affect on the respective line pressures.
There are two types of
shower equipment designed to be used on mains pressure hot water
- where the valve balances/equalises the pressures of the hot
and cold supplies giving the operator a fighting chance to adjust
shower temperature accurately
where the shower valve makes internal compensation to hold it's
output temperature stable. These are by far the most popular types
of shower these days and will usually operate on all types of
systems (please check manufacturer's specifications for minimum
inlet operating pressure requirements)
The use of an ordinary
'manual' shower valve on a mains pressure system can result in sudden
and wild temperature fluctuations as a result of changes in system
pressure. This can be annoying or surprising at best or lethal to
persons with reduced sensation, the very young or elderly and infirm. It
is folly not to use thermostatic equipment where users are vulnerable.
You may even have a legal obligation to do so.
Advice is always
available. For more information please
Remember, pumped shower
equipment (manual or thermostatic) may not be used on a mains pressure
hot water system.
How do you heat mains
pressure hot water?
This is the single biggest
issue with a mains pressure hot water system. There are essentially 3
methods of heating a mains supply, each of which should be examined for
it's pro's and con's.
1. Combination boiler
A combination boiler (also
known as a 'combi') has a cold water feed directly from the main. As you
open an outlet that demands hot water the boiler senses the call and
fires up, heating the incoming cold water as required and delivering as
Advantages to combi
The biggest advantage with
a combi boiler is endless amounts of hot water on demand with no hot or
cold water storage tanks. This makes them ideal for apartments, flats
and smaller homes where space is at a premium and header tanks and
cylinders are either impossible or undesirable to site. But don't get
too carried away.
Disadvantages to combi
Remember, pro's and con's?
The down side to ALL combi boilers is the rate at which they can heat
and therefore actually deliver hot water.
It is fair to say that the
domestic hot water flow rate gets higher as the kW rating of the boiler
gets bigger, but extreme care is advised if you are leaning towards such
a device. Most (if not all) combi boilers deliver their domestic hot
water via a 15mm pipe. This speaks volumes about their delivery
capability. Even the biggest combi boiler has a modest delivery rate.
Furthermore, they are generally hopeless at supplying two demands
simultaneously. So if you wish to run a bath and fill the sink at the
same time you could be very disappointed.
Conclusions about combi
For houses with more than
one bathroom... forget it! If a plumber advises you otherwise be very
cautious. After all, you are the person who has to live with the
limitations of this type of system - the plumber may simply see it as an
easy install. Get another opinion!
Limited delivery rate on
the hot water but handy where space is limited or loft space
Combi boilers are regarded
as mains pressure devices.
2. Unvented hot water
A popular method of
heating mains pressure hot water is by utilising an 'unvented cylinder'.
This, in keeping with the item's description, is a cylinder that is
pressurised (hence the term unvented). The cylinder is fed with and
pressurised by cold water directly from the incoming main. Contents of
the cylinder are heated (by various methods) and as you open an outlet
in the house the force of the incoming water pushes the contents out of
the cylinder to the outlet point.
Advantages of an unvented
hot water cylinder
Firstly, it provides mains
pressure hot water. The advantages have been outlined above.
Secondly, because the
cylinder is fed from the incoming cold water main there is no
requirement for a header tank. This means no wasted space and makes them
ideal where good mains pressure is available but loft space isn't.
Disadvantages of an
unvented hot water cylinder
As mention, an unvented
cylinder is a pressure vessel. It stores a large volume of heated water
under pressure. Providing it works correctly and within operational
parameters it will do it's job nicely.
However, the dangers are
unimaginable if an unvented cylinder is installed incorrectly, has
untrained persons tamper with it's operation, has non-specific equipment
fitted, goes over design temperature or pressure. The risk of explosion
exists. For this reason the installation of unvented hot water cylinders
is governed under G3 Building Regulations.
Conclusions about unvented
hot water cylinders
Good performance on your
hot water system providing you have decent incoming pressure to the
property. Also check you flow rates.
Minimal space required as
no header tanks are necessary.
Unvented hot water devices
require an annual service to ensure that the unit and safety equipment
fitted are fully functional. This of course carries with it cost of
Mains pressure hot water from a Thermal Store
Growing in popularity in
the world of mains pressure hot water are a family of products known a
Thermal stores gather and
store thermal energy and can do so from many different sources - many of
which are simply incompatible with unvented cylinders.
How does a thermal store
Essentially, a thermal
store is a cylinder full of water which is heated by whatever means
(boiler, solar, wood burner, heat pump, etc) into which a large and very
efficient heat exchanger is immersed through which your cold mains
pressure water is passed. When it emerges it has drawn heat from the
store and become hot mains pressure water.
The advantages of a
The beauty of this type of
system is the store (the large volume of water contained in the
cylinder) is vented to atmosphere (unpressurised) and therefore is
inherently safe. The only pressure that exists within the store is
contained within the mains pressure heat exchanger. Because a thermal
store presents no risk of explosion (because there's no way for pressure
to build up within it) they are simpler to install, cheaper to install
(requiring no special qualifications) and require no annual maintenance
thereby reducing cost of ownership.
Thermal Stores have many
advantages which we won't go into here as the primary purpose of this
article is to look at shower compatibility. If you would like further
information on the capabilities of Thermal Stores please visit our
Disadvantages of a Thermal
Because a Thermal Store is
a mains pressure hot water device they aren't cheap to buy. Having said
that, they aren't any more expensive to buy that an unvented cylinder.
A thermal store requires a
small header tank. The purpose of this tank is to keep the store topped
up (as the vented contents would over time evaporate away if you
didn't). This is however the only function for this tank (also know as
an F&E or feed and expansion tank). The F&E tank can be either remote (a
separate little tank the size of a small toilet cistern) or joined to
the top of the cylinder - a combination type. The F&E tank can either be
filled manually on a periodic basis or can have a live water feed to it
via a float valve (ball cock) in which case an overflow pipe will be
Other than this thermal
stores share the same requirements for mains pressure devices including
things like requisite incoming flow and pressure and the types of shower
equipment that must be fitted.
Conclusions about Thermal
Thermal stores arguably give you all the benefits of mains pressure
hot water without all the nasty baggage that you inherit with unvented
cylinders. Simple to install. No risk of explosion. No ongoing costs of
ownership. Silent operation.
We hope this section has helped to explain
the various considerations and how your household plumbing system
operates. If things remain unclear or your system differs to those
mentioned please don't hesitate to
call for advice.