A guide to household hot water system types

The Shower Right guide to shower products and their compatibility with different water system types

Before making any decisions, or indeed a purchase, it is wise to familiarise yourself with the type of system to which you will be adding components. Whilst we are always at the end of a telephone to provide help showerright respect your desire to quietly research the subject. For this reason the following guide will provide you with a grounding in the various types of water system and the appropriate products for connection. Remember, this is a guide an not intended to be a definitive instruction manual.

Your biggest decision is whether to go for a mixer type shower (one that blends pre-heated water and cold water) or an electric shower (one that heats cold water as you require it).

Mixer Showers

Mixer showers require supplies of cold water and hot water (which is generated and/or stored by other equipment). For a more comprehensive explanation of the various system type please read down this page.

Electric Showers

An electric shower (with few exceptions*) connects directly to your incoming cold water main. An electric shower relies entirely on the incoming pressure to push water through the unit, heating it as it goes for as long as you keep it switched on. One advantage of an electric shower is the 'on demand' nature of operation and the fact that no storage of water is necessary.

While an electric shower requires no storage of hot water, heating only what you use, you should consider performance and the cost of installation when making your choice. The bigger the kW rating of an electric shower the more water it can heat in a given instant so bigger might be considered better (provided your supply is good enough). Conversely, it might be your goal to reduce or restrict the amount of water used in a shower - in which case a smaller kW shower might be better.

Because they are connected directly to the water main all electric showers will be subjected to pressure fluctuations that can manifest themselves as temperature variations unless the equipment is thermostatically controlled.

Electric showers also require a dedicated electrical supply from the consumer unit. This can be costly as very heavy cabling is required. You are advised to obtain costings before you proceed.

*There are devices known a 'pumped electric' showers. These are not to be confused with power showers and must be connected to a gravity cold water supply.

There are also 'all-in-one' power showers that require an electrical supply but only to boost a hot and cold feed passing through the unit. These look remarkably like electric showers but as they do not heat water they operate entirely differently.

How to identify your hot water system

When 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.

Over 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.

To be 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 connect.

In 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 tower).

Method 1:

A gravity hot water system

Probably the most common hot water system in the UK.

What is a gravity hot water system?

This type of system comprises a 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 house).

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).

The 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.


Visit our sister website www.gasapplianceguide.co.uk for a comprehensive range of discounted heating products, including vented and unvented hot water cylinders.


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 bar.

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 here too.

Cold 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.


Method 2:

A Mains pressure 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 hot).

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 considerations.

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 systems.

  1. Pressure balancing - where the valve balances/equalises the pressures of the hot and cold supplies giving the operator a fighting chance to adjust shower temperature accurately

  2. Thermostatic - 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 call.

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 systems

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 required.

Advantages to combi boilers

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 boilers

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 boilers

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 unavailable.

Combi boilers are regarded as mains pressure devices.


2. Unvented hot water cylinder systems

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 ownership.


3. 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.

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 work?

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 Thermal Store

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 sister website www.gasapplianceguide.co.uk.

Disadvantages of a Thermal Store

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 necessary.

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 Stores

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.


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