Hot Water Tanks
Indirect and Direct Cylinders
Combination Copper Cylinders
Economy 7 Copper Cylinders
Economy 7 Combi Water Cylinders
AGA Cylinders
Primatic Copper hot water cylinders
Gledhill Sunspeed Solar Cylinder
British Standard Water Cylinders
CondenCyl Water Cylinders
ShowerCyl gravity with mains
Horizontal Water Cylinders
Twin Water Cylinders
Cylinders Explained
Bespoke cylinders
Design your own

Hot water cylinder terminology

You may find the terminology used to describe a hot water cylinder a little confusing.

Direct....  Indirect.... Combination..... Solar....  What does this mean? Click on your question below -


Know enough?

If you feel comfortable with the terminology and wish to make a product selection click on the button below to go to the cylinder page....


Cylinder Page

1. Categories of Hot Water Cylinder

What is a DIRECT cylinder?


What is an INDIRECT cylinder?


What is a COMBINATION cylinder?


What is a SOLAR cylinder?


What is an ECONOMY 7 cylinder?


How are copper cylinders graded?


Configuring or specifying a cylinder to your own design


How are cylinders insulated?

....It all depends where the cylinder gets it's heat from.

Please note: Information contained on this page is intended to assist with your understanding of cylinders and terminology. It does not represent a definitive description of each and every product available. If in any doubt, please call for assistance - we will be pleased to help.

Categories of hot water cylinder
There are several categories of cylinder that can be split into types. To simplify matters we will firstly deal with Category.
  1. Vented
  2. Unvented
  3. Thermal store

The category depends on how you intend to supply cold water to the cylinder which in turn will determine what you can expect in the way of performance from the cylinder.

There are essentially two methods of supplying water to a cylinder...

  1. Directly from the incoming cold water main - known as a mains pressure supply
  2. From a holding tank (also known as a Cistern, Header Tank or Storage tank) - known as a gravity supply

It is most important that you never connect a cylinder designed for a gravity supply to a mains pressure supply. To do so is highly dangerous and the risk of explosion exists.

The only type of cylinders you may connect directly to a mains pressure supply are the Unvented Type and Thermal Stores. Both of these are designed to handle the higher pressures associated with a mains supply and will deliver mains pressure hot water - albeit is slightly different ways.

Unvented cylinders: These are essentially pressure vessels. You fill them with water at constant mains pressure, heat the contents (by whatever means) and when you open a tap the incoming pressure forces the contents (hot water) out - hence mains pressure hot water.

Thermal Stores: Deliver mains pressure hot water but achieve the same result in a different way. A thermal store could be likened to a large bucket. The water in the bucket is heated by whatever means. By immersing a heat exchanger in the bucket and passing mains pressure water through it the heat from the water in the bucket is transferred to the water passing through the heat exchanger, which emerges as it leaves the thermal store as mains pressure hot water.

The advantage of a Thermal Store over an Unvented Cylinder is the Thermal Store can remain open vented - that is to say there is no pressure in the stored water, only in the heat exchanger (unlike the Unvented Cylinder that has a huge amount of very hot water under constant pressure). This makes Thermal Stores inherently much safer. Whilst rare, Unvented Cylinders have been known to go 'over temperature/over pressure' which has lead to explosion. For reasons of safety the installation of Unvented Cylinders is governed by Section G3 of the Building Regulations.

Vented Cylinders (also known as Gravity Cylinders): Get their name from the source of the water that supplies them. When water is supplied from a header tank it relies on gravity to get where it needs to go. The height of the header tank above the final outlet point determines the pressure of the water at that point. A 10m head of water will produce 1 bar albeit very rare to be able to achieve such gravity pressure in most domestic houses. Gravity cylinders are designed to handle lower pressures than mains water is normally supplied at. Gravity cylinders are Graded for the head of water they can handle - Grades of Cylinder


Cylinder Configurations: (Please refer to the rest of this section for more detailed explanations of types)

Unvented: High performance pressure vessel. Can be Direct or Indirect. Heat sources must be controllable. Solar Compatible. More information and product guide here

Thermal Stores: Mains pressure hot water from open vented store. Requires F&E tank*. Can be configured to handle multiple heat sources (Direct & Indirect, Solar) simultaneously. Particularly suitable for uncontrollable heat sources such as AGA, Rayburn, Wood Burner etc. Available as 'cylinder' or combination' types. More info and product details

Vented: Must be fed from header tank. Low pressure output which can be pumped if necessary. Suitable for all types of heat input. Guide to available products


*F&E tank = Feed and expansion tank required the enable cylinder to breathe.


The meaning of 'Direct Cylinder' has change somewhat over the years. In the early days (when fires used to have copper back boilers) a direct cylinder might simply have had flow and return from the back boiler feeding directly to/from the cylinder (no heat exchange within cylinder). Your domestic hot water would have been heated in the back boiler then passed back to the cylinder - eventually providing a cylinder full of hot water heated 'directly' by the back boiler. Such systems were typically unpumped - relying on gravity circulation (also known as 'Thermosiphoning' where hot water rises, cooler water falls creating circulation within the system).

Although the traditional back boiler became rare through the inconvenience of having to light a fire every time hot water was required the concept has enjoyed somewhat of a revival more recently with appliances like wood burning stoves, although the more traditional appliances such as AGA and Rayburn have always persevered with the concept. However, the modern back boiler heat exchanger tends to be made from cast iron rather than copper. For this reason it is necessary to separate the primary (heat source) water from the domestic water (what you see coming from your taps) by a heat exchanger within the cylinder - thus preventing contamination.

Cylinders that work with the principle of thermosiphoning are slightly different to those designed for a fully pumped system. More information is contained in this section.

The term 'direct' continues within the industry, it's just the meaning that has changed.

A modern Direct Cylinder is typically heated by electrical immersion heater/s located within the cylinder. Because the Immersion Heater is heating the water 'directly' within the cylinder it too is known as a 'Direct Cylinder'. In a direct cylinder there are no other external heat sources.

The diagram shows a 'direct' cylinder with two side mounted immersion heaters.

Please note: Variants may include:

  • a side mounted single immersion heater boss

  • a top mounted single immersion heater boss

  • one side mounted and one top mounted heater boss

  • two side mounted heater bosses

  • more than two heater bosses

You should specify the number of heater bosses you require and their desired position. The diagram is only intended for the purpose of explanation. Immersion heater elements are not normally fitted or included in the price of a cylinder and should be specified if required.

Direct hot water cylinder with two side entry electrical immersion heaters

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A cylinder is deemed 'indirect' whenever a heat exchanger is fitted within. This is to say, whenever a heat source is external to the cylinder and utilises a heat exchanger to impart heat to the contents of the cylinder the cylinder is deemed to be 'indirect'.

A cylinder could be both 'direct' and 'indirect' in certain scenarios.

Although heat may be imparted to the cylinder by a heat exchanger located internally, the heat source is actually remote - such as a gas boiler, solar panel, heat pump, wood burning stove, Aga etc etc. It is therefore deemed that such a cylinder is 'Indirect'.

Indirect cylinders may be fitted with immersion heaters as a supplementary 'direct' heat source but the cylinder is still deemed to be 'Indirect'.

An indirect cylinder may have more than one heat exchanger (also referred to a a coil) depending on the number of remote heat sources feeding the cylinder. (i.e. 1 x boiler coil and 1 x solar coil = twin coil indirect cylinder, or, 1 x boiler, 1 x AGA and 1 x solar = triple coil indirect.)

The term 'indirect' may apply to a vented cylinder, an unvented cylinder or a thermal store. It simply refers to an external heat source heating the cylinder via a heat exchanger.

An Immersion heater boss may be fitted through the top of the cylinder. You should specify your requirement. Immersion heater elements are not normally fitted or included in the price of a cylinder and should be specified if required.

Indirect hot water cylinder heated by boiler via coil and a side mounted electrical immersion heater


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A gravity-fed household copper cylinder is typically supplied from a tank sited above the cylinder - often located in the loft. This height of this water store (also referred to as a 'cistern' or 'header tank') is what creates the pressure in a gravity water system. The higher the header tank the greater the 'head' of water and hence the higher the gravity pressure when measured at the outlet point. (Also refer to Section on Grades of Cylinder).

Pressure in a gravity system varies throughout as the height differential between the header tank and the outlet point varies. Example: A 10m head of water will create 1 bar of pressure at the outlet. Using this simple formula you can calculate the approximate pressure at any outlet by measuring the distance from the outlet point to the header tank above. You might begin to appreciate why gravity systems are often referred to a 'low pressure' systems as it is most unlikely that 10m is anywhere near achievable in most domestic properties.

Where the property is unable to accommodate a header tank (for example in an apartment or flat) the header can be accommodated literally on top of the cylinder and built into the unit. The cylinder therefore combines the 'header' and the 'storage' elements and becomes a COMBINATION cylinder.

Note: The output water pressure from a combination cylinder is often poor due to the small distance (head) between the header section and the outlet point (i.e. A tap). This can be exacerbated when supplying hot water to a shower outlet when the height of the cylinder header section and a wall mounted shower handset can be virtually the same - resulting in zero head and therefore no pressure.

Combination Cylinders may be Direct, Indirect or Economy 7. They simply combine the header function.

Thermal stores may also be configured to be 'combination type' but for a very different reason. Please refer to 'Thermal Store section' for further info.

Immersion heater elements are not normally fitted or included in the price of a cylinder and should be specified if required.

Combination indirect hot water cylinder (fortic tank) with built on cold water compartment. Heated by boiler coil and immersion heater.

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A 'solar cylinder' is a hot water cylinder that has been configured to accept 'wet' solar energy.

To explain, there are essentially two types of energy we try to harvest domestically from the sun - radiant energy (the warmth from the sun) and photovoltaic (using the light - also referred to a 'pv'). The latter is where sunlight is converted to electricity. The resulting electrical energy can be utilised in numerous ways and is therefore not exclusive to the production of hot water. The former (the radiant energy) is typically captured by 'wet' solar panels - so named because they contain and utilise a fluid (rather like antifreeze) to capture and transport the gathered energy.

Although pv energy can of course be use to heat a cylinder of water with electrical immersion heaters, no special modifications need to be made to a standard cylinder to accommodate an electrical immersion heater - a fairly standard feature. To store solar energy on a 'wet' system is however somewhat more complex and requires a cylinder to be equipped with a 'solar coil' in order that energy may be imparted to the cylinder contents.

A solar coil is usually located at the very bottom of a cylinder. Although 'free', solar energy is not as vigorous an input as say that of a gas boiler so is generally regarded as a low grade heat source. By placing the heat exchanger at the bottom, energy when available is percolated throughout the entire cylinder through a high efficiency coil with a large surface area that facilitates maximum heat transfer.

For the discerning, the presence of a solar coil in a cylinder facilitates the future addition of a wet solar system even if the necessary hardware is not immediately available. Known as 'future-proofing' this is becoming popular (and arguably logical) as it saves major disruption and further expense by having to change the water storage cylinder when solar collectors are eventually introduced to the system.

Solar coils can be fitted in most types of cylinder (vented, unvented and thermal store) at the time of manufacture. It is not however a feature that can be retrofitted or bought as an accessory.

Also see:


Indirect Solar hot water cylinder diagram shower solar coil located underneath the main boiler coil

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ECONOMY 7 hot water Cylinders
An Economy 7 cylinder is designed to exploit 'off-peak' electricity - typically available throughout the night.

Because you are heating your cylinder overnight it is important to not to loose the stored heat. Economy 7 cylinders are therefore fitted with an extra thick jacket of insulation to minimise standing heat loss.

Economy 7 (or indeed Economy 10 where available) cylinders usually have two heating elements. The lower element energises as the economy tariff becomes available (via an off-peak switching device). This heater heats the whole cylinder over several hours - taking longer because it heats the entire volume. The upper heater is normally used as a 'boost' element providing a quicker response because it only heats water at the top of the cylinder and is manually energised when required.

Economy 7 cylinders are available as direct and indirect models of 'cylinder' type or 'combination' type.


Economy 7 Direct hot water cylinder showing upper and lower immersion heater positions

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Configuring or Specifying a hot water cylinder to your own design
Never buy a hot water cylinder believing it to be a 'standard' configuration. Different manufacturers and indeed different areas of the UK have their own interpretation of what 'standard' means.

For example, cylinders made in Yorkshire by one manufacturer might fit 1" bosses as connections with boiler connections to the right and cold feed to the left, whereas another made in Hampshire might use compression fittings and have connections the opposite way round! Try replacing one for the other and you may have your work cut out!

It doesn't pay to be complacent. This is especially important if you are replacing an existing cylinder. The one you take out might have been made to special order thereby facilitating pipe connections in an awkward installation. Or worse still, the last plumber didn't think the original job through well enough which meant a difficult install and even more difficult replacement.

Part of the service we offer is to supply a cylinder with connections (tappings) configured to your requirement. This is a fairly simple process and make life so much easier when it comes to retrofit.

Simply use our online form to sketch your requirement and post, fax or email it to us.


Help is always at hand. Simply call us for assistance if the going gets confusing or you wish to discuss matters in detail.

Design your own hot water cylinder using our online form

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Grades of gravity hot water cylinder
Gravity hot water cylinders are available in three grades. The grades refer to the pressure of water they can withstand from the header tank (the pressure created by the height of the header tank above the cylinder).

In simple terms, this translates to the gauge (thickness) of copper used in construction. Do not confuse a gravity cylinder with mains pressure or unvented cylinders.

Connection of mains pressure water to any cylinder designed for a gravity feed can be extremely dangerous.

Grade 1 - Can withstand a 25 metre head of water

Grade 2 - Can withstand a 15 metre head of water
Grade 3 - Can withstand a 10 metre head of water

It is possible to supply a gravity cylinder that will withstand a 31 metre head. This is achieved by doming the bottom of the cylinder. These cylinders sit in a ring mount. Please specify the working head of the system into which the cylinder will be installed.

A normal domestic cylinder  is usually a Grade 3.

(All prices quoted on this website refer to Grade 3 cylinders unless otherwise stated)

Insulating hot water cylinders

Insulation is absolutely essential on a 'hot water' cylinder. It is simply ridiculous to consider heating a large volume of water only to let it go cold again. Many older properties will indeed have an un-lagged cylinder, but their owners are simply burning money each and every day as they warm the water then allow the heat to escape. It is almost a blessing when an old cylinder fails, as the replacement will invariably arrive lagged and go on to demonstrate the difference in efficiency (saving money) over the following years.

But the term 'insulation' can lead to disagreement.

There are many arguments between manufacturers as to what type of insulation is best. Some swear by mineral insulation which is physically wrapped around the cylinder then held in place by a tight polythene jacket. Others will argue that polyurethane foam is more efficient.

Which ever type you choose you can rest assured that all new cylinders manufactured in the UK must conform with British Standards and Part L of the Building Regulations.

Beware of cheap imported cylinders that fall a long way short of the quality demanded by UK Institutions.

While standards determine the minimum required performance of a cylinder's heat loss, you don't have to settle for it. We can supply cylinders with enhanced insulation thereby reducing heat loss to a level lower than even the 'good book' demands. For details give us a call or refer to the relevant cylinder section for details.

In short, if you heat it then insulate it. The more you insulate the longer hot water stays hot and the lower your operational costs.

Remember: When considering the size of cylinder you require you must take into account the thickness of insulation. Cylinder sizes stated on the website do not include the thickness of insulation which must be added in order to determine the finished diameter, length and height.

A 450mm diameter cylinder with 50mm of insulation actually measures 550mm (because you have the thickness of insulation on either side to add). So be mindful of this when placing your order.

If you measure an existing cylinder ensure you take account of whether your measurements include insulation (especially if old insulation has fallen off) and also measure the size of entry to the location - remembering that your new cylinder needs to pass through doorways, cupboard doors etc.

Another pitfall: It is not unknown for plumbers to install a cylinder and then for a chippy to build a cupboard around it. This often leads to immense difficulty when replacement is necessary as the access does not represent the size of the cylinder. Look out for this problem and consider your best course of action before deciding on your purchase. Do you buy smaller to fit through the entry or might you need to destroy and rebuild the cupboard afterwards?

Hot water cylinder insulation


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