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Hot Water Storage Explained - tanks and cylinders

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Domestic Copper Hot Water Tanks, Cylinders and Storage Vessels - Tanks and Cylinders


Water Cylinder Terminology

to the Hot water cylinders - sizes and prices

We often hear people refer to....

  • The 'Hot Water Tank'

  • 'Immersion Tank'

  • 'The Immersion Heater'

  • The 'Hot Water Boiler'

.... when in fact they are actually talking about a Hot Water Cylinder.


Water storage vessels are of course available in many shapes and sizes, but it is common these days to find them cylindrical in shape and often made of copper. Stainless steel is fast becoming a favoured metal in the cylinder industry, but not necessarily for some of the reasons given.


Stainless steel and Copper water cylinder construction

Traditionally, copper has been the favoured metal for hot water cylinder construction. More recently stainless steel has featured in cylinder construction. With the advent of unvented water storage stainless steel is used for it's tensile strength. In other words, stainless steel is a much stronger material and by comparison with copper will withstand the forces and pressures contained within an unvented cylinder with a much thinner wall thickness. (Copper can do the same but because it is softer the wall thickness must be much greater.)

To simply state stainless steel is however a little simplistic. Stainless steel is alloy and its chemical composition determines different properties, albeit we just call it all stainless. For example, one type of stainless might be more resilient to salt water than another so is better for use in maritime applications such as ships railings for example. Whereas, another type is better in clean or clinical environments (e.g. Food Grade Stainless). In certain environments stainless steel might be more resilient than copper, but don't be fooled into believing that it will never corrode. Stainless just takes longer.

The simple fact is, Stainless steel is a cheaper material to make a cylinder out of than copper - albeit much more demanding to work with. So it is fast becoming popular with manufacturers who can realise higher profit margins. Oddly enough, these savings don't filter through to the retail price of stainless cylinders!

For water storage copper offers many advantages over stainless steel. Copper, as a metal, has an inherent antiseptic property. That is to say it makes the environment within a cylinder a difficult place for bacteria to breed. Copper in concentration can be poisonous - just think what happens when you bang a copper nail into a tree stump, it dies! So in the small quantities that dissolve into water within a system it actually purifies.

Within the industry if you spoke honestly with experienced tradesmen who make cylinders they would concede that copper is still a better material for water storage.

Interestingly, now that stainless steel has been used in cylinder production for some time certain issues are becoming evident giving cylinder manufacturers cause for concern.


Immersion heaters

Because many cylinders have at least one electrical 'immersion heater' fitted they are mistakenly referred to as 'immersion tanks' or 'boilers'. Just for the record, an immersion tank is a tank in which you might immerse something or someone (for example - hydrotherapy utilise an immersion pool or tank), and although a boiler is an appliance that heats water, such a device is typically used to heat water in a wet central heating system with radiators.

A hot water cylinder is neither an immersion tank nor a boiler as water in a domestic hot water cylinder should never reach anywhere near boiling point. These terms have been wrongly assumed because many hot water cylinders contain an immersion heater (an electrical heater that is screwed in the side or top thereby being immersed in the water contained within) and may also be heated by a remote boiler. Plumbing terminology can be most confusing at times!

Electrical immersion heaters are simply a way to heat the water in a cylinder electrically. Some cylinders use an immersion heater as a backup or emergency heat source whilst utilising something like a gas boiler as the primary method. Such a cylinder is generally referred to as an 'indirect' cylinder. If there is no external heat source and the cylinder is heated exclusively by electrical immersion heater or heaters the cylinder is typically referred to as a 'direct' cylinder. (Historically the definition of a 'direct' cylinder has changed, but the modern understanding is as mentioned).


Economy 7 cylinders

These are specified as such because they are designed to make the most of 'off-peak' electricity - saving the user money with cheaper running costs. Economy 7 cylinders can be 'direct' or 'indirect' (exclusively heated by electricity, or with the addition of an external heat source), but they invariably utilise a two heater setup. The lower heater typically being connected to the off-peak supply, the upper heater being used as a manual 'boost' element and intended only for use when a depleted cylinder needs to be partially reheated in a hurry.

Whichever type of hot water cylinder you have, referring to it as an 'immersion tank', 'boiler', 'water boiler' etc is inaccurate and can be misleading. Try and refrain from such descriptions as you might end up ordering and receiving the wrong thing.


Hot water cylinder insulation

To avoid wasting energy and to comply with Building Regulations, domestic hot water storage vessels must be insulated.

Insulation helps the contents of the vessel stay hot for longer and reduces the need to re-heat. By reducing loss they work more efficiently and cost less to operate. Insulation comes in several forms. Common types of insulation include:

  • Sprayed-on foam

  • Insulating Jacket

The better your insulating jacket the longer your water will remain hot. This saves energy (reducing carbon emissions from generating heat and/or electricity) and in turn reduces the cost of maintaining a tank full of hot water.

In years gone by it was possible to purchase a bare cylinder (one without insulation). This now contravenes regulations. All new domestic hot water cylinders will be supplied insulated. You should however understand although insulation is included when you purchase a cylinder it is not included when cylinder sizes are stated or discussed. For example, a 1200 x 450 cylinder will actually be bigger according to the thickness of the insulation. Maybe you can appreciate how confusing or misleading a cylinder volume might be if insulation was included in dimensions by one manufacturer but omitted by another. The more efficient the insulation is the lower the standing heat loss.


Specifying the size of a replacement Hot Water Tank or Cylinder

Remember when you are specifying the size of a replacement not to include the insulation in your measurements. The industry see insulation as an addition and therefore only refer to the size of the bare cylinder or tank in specifications. Remember to check the thickness of the insulation when ordering as this must be added to the over all dimension of the cylinder or tank to establish the finished size. For example:


  • If the insulating jacket on a cylinder is 30mm thick , you must add 30mm to the height of the cylinder (no insulation on the bottom) and 60mm to the diameter. Remember, there is insulation on either side of the cylinder.

Failure to observe this advice may result in you ordering a cylinder that simply won't fit through a hatch or an airing cupboard doorway.

So remember

If you order a cylinder with dimensions of 1050 x 450 you will receive an insulated cylinder measuring:


Height = 1050 + the thickness of insulation, by

Diameter = 450 + (2 x the thickness of insulation)





Other terminology commonly used with domestic hot water cylinders:

Immersion heater:

An electric heater that typically screws in the top or the side of the cylinder, is immersed in the water contained within and when energised heats the contents. Immersion heaters are typically fitted with a thermostat which can be adjusted on the cap by turning the indicator dial. When power is connected, an immersion heater will heat water until the set temperature is reached then automatically cut out - turning on again automatically if the water temperature should drop - all the time power remains connected.

You don't really want or need to have your immersion heater on all the time. To do so would be costly. So, immersion heaters are normally controlled by a timer (off peak controller in the case on Economy 7) which energises the heater at times you can program. Heaters can be controlled by use of a simple on/off switch, but beware, if you forget to switch off you can expect a thumping great electric bill next time round!

Immersion heaters are prone to corrosion caused by the water they heat. Periodically, immersion heaters will need to be replaced. As a rule, cheap heaters are more prone to the corrosive effects of water than the dearer 'Incoloy' types. Incoloy heaters have an alloy shroud over the element that protects from corrosion. Although they cost more they represent a good investment as they last a lot longer.

Boss, or cylinder boss:

This refers to a threaded fitting on the cylinder into which an immersion heater, fitting or device will screw.


A threaded hole that derives its name from the tool (a 'tap') used to cut a thread. Also refers to where one might 'tap into something' creating an outlet or inlet. In this instance, a tapping is where something connects into the body of the cylinder. A 'tapping' is essentially a direct connection to the cylinder and is made typically via a threaded fitting that can be male or female. Compression fittings are also popular these days.


As the term suggests, anything that offers a means of connecting to the cylinder.


This is a term that typically describes a heat exchanger - because the heat exchanger is coiled within the cylinder. Coils can be used to impart or extract heat depending on design function. A cylinder (or process) is normally referred to as 'indirect' if a heat exchanger is used.

Heat exchanger:

A device that facilitates the transfer of heat from one medium to another whilst keeping the two separated. It terms of a cylinder this is typically a heat exchange between the heat generated by a boiler (source) and the water contained within the cylinder. Heat is exchanged or transferred.


In cylinder terms this typically refers to a cylinder heated by a remote heat source. (see coil)


In cylinder terms the modern interpretation of 'Direct' means the cylinder is heated electrically with no external heat source. In the past this term meant water from the cylinder was taken out, passed through something like a back boiler (that heated it ) then put back into the cylinder. The latter process was often unpumped and known as 'gravity circulation' or 'thermo siphoning'.

Gravity cylinder:

On a modern system this often refers to the fact that water to the cylinder is supplied under gravity pressure from a header tank sited above the cylinder (often sited in the loft).

Gravity flow (heat source):

This has a slightly different meaning to the above. Gravity flow (also known as Thermo siphoning) is a process where water circulates through the pipework by the principle of hot water rising and cooler water falling. This process does not involve or require pumps. Quite common with appliances such a wood burning stoves, AGA's, Rayburns, Back Boilers, some oil boilers etc.

Gravity coil:

A larger bore heat exchanger to facilitate good flow on a gravity flow system - as outline above. Standard cylinders these days are fitted with a 22mm boiler coil suitable for a pumped system. A gravity coil is typically 28mm and must be requested when purchasing.

Do not overlook the bore size of a cylinder's heat exchanger when purchasing. A cylinder designed to be used on a pumped system will not perform well on an unpumped system. Do not be persuaded otherwise!

Pockets and sensor probes:

When referred to as a 'sensor pocket' this is usually a dry pocket located on the cylinder into which a sensor probe can be inserted. Some cylinders require multiple dry sensor pockets to monitor water temperature gradients.

If 'Wet' sensor probes are used these will typically require a tapping into which they will screw. The tip of the sensor probe will therefore be immersed in the water contained within the cylinder.

Secondary return:

This is a connection on a cylinder where return water from a pumped 'secondary return circuit' connects. SR circuits are typically used to shorten the 'dead leg' or draw off time of hot water at locations distant from the cylinder. Not normally required in most domestic properties.

Solar input:

This normally refers to an additional coil placed low down in the cylinder through which liquid from a wet solar collector system is passed - imparting solar heat to the cylinder.

PV solar energy (electric) can be supplied to the cylinder via an immersion heater.

Vented cylinder:

An open vented cylinder (or one designed as such) is (or should be) vented to atmosphere. i.e. must not be connected directly to mains pressure water or configured in such a way that pressure can build up within. Vented cylinders are filled by and normally breathe back into a header tank (or F&E, a reservoir of cold water) sited above the cylinder, often in the loft. Or in the case of a combination cylinder is a compartment on top of the cylinder - physically attached.

Never connect a 'Gravity Cylinder' directly to a mains pressure water supply or prevent the cylinder from breathing to atmosphere.

Unvented cylinder:

An unvented cylinder derives it's cold water supply directly from the cold water main. An unvented cylinder is a pressure vessel. There are special requirements with regard to installing and maintaining this type of cylinder. Malfunction and/or incorrect installation can lead to explosion, severe injury and death.

Advantage of unvented over vented cylinders is better hot water pressure/performance but comes with cost and strict rules .


If you have any questions or would like further explanation please don't hesitate to call

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