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Hot Water Cylinders

Replacing your Hot Water Cylinder? Which is the best option for you?

Our guide will help you choose which hot water cylinder (commonly called hot water tank) best suits your needs and plumbing system. Find out the difference between vented and unvented cylinder types. How much will replacing your current hot water cylinder cost? What cylinder size is needed to comfortably meet you hot water needs. The difference between copper and stainless steel. Immersions and other considerations and best pratices.

Brief overview of common hot water cylinder types

Indirect Cylinders

Used with central heating systems, these cylinders utilise a coil to seperate the domestic hot water and central heating water

Direct Cylinders

These cylinders do not posses a coil for water seperation and are only suitable for back boiler or electrical immersion heating

Vented Cylinders

If you have a gravity feed plumbing system your hot water cylinder will be open vented to prevent an pressure increase when heated

Sealed Cylinders

Sealed or Unvented cylinders are designed for use on pressurised plumbing systems and incorporate an expansion vessel

Hot water cylinder replacement benefits

Replacing your hot water cylinder can mean a saving of up to 25% on you annual hot water fuel expenses. New hot water cylinders now come highly insulated meaning your stored hot water will stay hotter for longer, meaning a reduction in your fuel bills.

Available in both copper and stainless steel with the option of a rapid recovery coil, this will an optional extra will reduce the heat up time for you stored hot water meaning those days of waiting around while the hot water heats will be a thing of the past.

 Replace your old cylinder with a energy efficient stainless steel cylinder!

96 Litre (30"x18")

10 Year Warranty

120 Litre (36"x18")

10 Year Warranty

140 Litre (42"x18")

10 Year Warranty

160 Litre (48"x18")

10 Year Warranty

Benefits and Features of Stainless Steel

☑ Exceptionally low heat loss, meaning you water STAYS HOT!

☑ Faster heat up time due to the rapid recovery coil

☑ 10 Year warranty on all stainless steel cylinders installed

☑ Manufactured from Stainless Steel eliminating the risk of corrosion

☑ Fitted with an high quality Incoloy Immersion

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A finned copper coil above. The fins create a larger surface area, increasing heat transfer.

What size cylinder do you need?

The Hot water cylinder need to be correctly sized to meet both the demand and the heat source. For example a gas or oil boiler will heat the hot water cylinder quite quickly so the volume of stored hot water can be quite small (80 – 120 Litres).

Where a solar panel system will heat a quite a large volume of water for free in a short space of time dependent of course on the outside weather, therefore a solar cylinder will be quite large in comparison, often with a storage capacity of 300 Litres.

If you are unsure about your size requirement contact us and we will be happy to assist you in your decision making.

Key Sizing Considerations

  • The amount of occupants
  • The number of bathrooms
  • The number of showers
  • Heat Source (Gas,Oil, Solar, Electricity)

Cylinder Sizing Guide


No. Of Bedrooms


No. Of Bathrooms



No. Of Showers



Cylinder Size

2 Bedrooms 1 Bathroom 1 Shower 96 Litre
3 Bedrooms 1 Bathroom 1 Shower 120 Litre
4 Bedrooms 1 Bathroom 2 Showers 140 Litre
4 Bedrooms 2 Bathrooms 2 Showers 160 - 300 Litre
Larger Properties 2 Bathrooms 3 Showers 300 Litre

Hot Water Cylinder Guide

A hot water cylinder(also known as a hot water tank) is a basically a tank in which your hot water is heated and stored for use. They are pre-insulated and are usually made from either copper or stainless steel. They are the most popular method of delivering ample amounts of hot water for showers, baths and general hot water usage.

The size and type of cylinder you require will depend on current your current heating arrangement and the amount of hot water you are likely to require on a daily basis. There are two main types of hot water cylinders, unvented cylinders and open vented cylinders with open vented being the most common.

Open Vented Hot Water Cylinders

Open vented cylinders are usually manufactured from copper but are now commonly available in stainless steel. They are available in almost any size, grade and arrangement and usually open vent hot water cylinder come pre insulated unless otherwise requested.

The Higher grade the cylinder the longer the expected life span and durability, additionally you can expect a more costly installation.  An open vented hot water cylinder is used in property’s that have a gravity fed plumbing system.

In a gravity fed system a cold water storage tank is placed at the highest accessible point, usually in the attic space. The water stored in the cold water storage tank supplies the hot water cylinder. The supply to the cylinder is reliant on gravity and care must be taken to ensure sufficient head of water. The cold water contained in the cylinder can then be heated and stored for use.

An open vent cylinder will have an open ended pipe that is routed form the hot water take off on the cylinder to above the cold water storage tank, this pipe acts as a vent pipe but more importantly maintains the low water pressure within the cylinder.

The pipe size should be no less than 3/4 inch and be constructed from a material capable of withstanding very higher temperatures (such as copper). It should not be constructed of Pex or plastic piping as in the event of a thermostat failure the water temperature can exceed the safe design parameters of Pex piping.


  • More affordable to install
  • Minimal maintanance required
  • Cost effective to replace


  • Relies on gravity

Unvented Hot Water Cylinders

Unvented hot water cylinders are used on pressurised plumbing systems, where the system pressure far exceeds that of a gravity system. They are manufactured from high grade stainless steel and are capable of withstanding very high pressures. A pressurised system is basically a water system that has a pressure pump installed. These systems typically have an unvented hot water cylinderoperational pressure of between 2 – 3 Bar and are capable of delivering a very high flow rate. This high flow rate makes for a very satisfying user experience particularly where showering is concerned. Unvented hot water cylinders are available in a range of sizes and can be installed in both horizontal and vertical positioning with vertical being the most popular. Being a closed hot water storage tank safety is of the upmost importance, for this reason temperature and pressure relief valves are preinstalled on the cylinder. Additionally a potable water expansion vessel is also required, this allows for the increased pressure that will naturally occur as the water stored inside the hot water cylinder is heated. An annual safety check should be carried out, this ensures the operational performance of temperature relief valves, pressure relief valves and ensures the correct pre-charge pressure of the expansion vessel. You can expect a minimum 25 Year warranty on a good quality unvented hot water cylinder when maintained and inspected annually.


Direct hot water cylinder

Direct hot water Cylinder

Direct Hot water cylinder is usually found in property’s that do not have an gas or oil boiler heating system such as an apartment that uses electric storage heater for the property’s primary space heating source. A direct hot water cylinder does not possess a heating coil that would be found an indirect cylinder and is instead heated by an immersion heater which can be either side or top entry. The immersion heater heats the water contained inside the cylinder directly and it is for this reason it is known to be a direct cylinder. Once the water has been heated to the required temperature, it can then be used at the property’s hot water outlet taps, showers etc. The amount of hot water required will affect the size of the hot water cylinder and for larger hot water demands additional immersion heaters will be required, in some installations this could be up to 3 immersion heaters. Direct hot water cylinders are available in both open vented and unvented models.

Indirect copper cylinder

Indirect hot water cylinder

Available in both open vented and unvented models indirect hot water cylinders are fitted with a heat exchanger or coil as otherwise known. The coil allows for the connection of external heat sources such as gas and oil boilers, stoves and solar panels. These external heat sources can then be used to heat the water contained within the cylinder. Indirect cylinders can be manufactured to contain multiple coils, allowing for the connection of multiple heat sources. Indirect hot water cylinders may also have an immersion fitted, which can be useful in the event of an external heat source failure, with the immersion be utilised as a backup heat source. An Indirect cylinder will also have a superior heat up time than a direct model, and the external heat source will usually offer a cost saving factor too when compared to an electrical immersion heater.

Unvented solar hot water cylinder

Solar Hot Water Cylinders

Solar cylinders are for both open vented and unvented plumbing arrangements. A solar hot water cylinder should be sized according to the heat output of the solar hot water system. The rule of thumb for solar panel systems in a domestic dwelling is usually 50lt per person. So if a family consists of 2 adults and 3 children the solar cylinder will have a water capacity of 250lts, provided the solar panels are correctly sized for the same. Because of the amount of stored water required for maximum efficiency in a solar system, consideration must be given to the placement of the cylinder due to the excessive weight associated with it. 1 litre of water equals 1 kilogram in weight so a 250 litre cylinder will have a weight of 250kg plus the weight of the cylinder itself which for such a size can be up to 50 kg. Solar cylinders are available in both direct and indirect models, immersion heaters are also available as well as additional coils. For maximum solar panel efficiency a solar cylinder will be installed with a rapid recovery coil. This ensure maximum heat transfer from the solar system to the stored hot water.

Thermal Store Cylinder

Thermal Store Cylinder

A thermal store works in the opposite way to a standard hot water cylinder in that the water contained within never leaves the cylinder, instead the water contained inside is heated to a set temperature and this heat is then transferred to the domestic hot water system through a heat exchanger rather like a combi boiler. A thermal store is ideally suited to heat pumps and solar system as the need for a minimum heat requirement of 60°C to eliminate the risk of legionella is not required, this ensures optimum heat pump efficiency. A thermal store can be used with heat sources and offers a very high flow rate for domestic hot water purposes. Because a thermal store cylinder is not pressurised there is no need for pressure and temperature relief valves.