Domestic hot water storage tank

Domestic hot water storage tank or vessel

hot water storage tanks or cylinders for domestic hot water accumulation from Flexiheat UK


With regards to a domestic hot water storage tank or vessel , first we have to define what domestic hot water is in relation to a storage hot water tank or hot water cylinder.

They are a hot water storage tank / hot water storage vessel / hot water accumulator tank or thermal store dhw tank to provide hot water for commercial buildings or process applications that have a requirement for large peak demands of sanitary grade warm water for multiple bathrooms in hotels etc or a process application.

Domestic hot water

Domestic hot water is the heated potable water that feeds taps, showers, baths, or the kitchen hot taps. It is also referred to as sanitary hot water. The hot water is heated up from the cold water mains temperature to the recommended temperature for storing hot water of at least 60 degrees C.

The cold water enters or that feeds the hot water tanks can either be mains water supply at mains water pressure or via a cold water storage tank/cold water tank – (often called a header tank) that is used on a vented hot water cylinder.

It is then heated up by an external heat source such as an energy efficient water heater , which can use various heat sources such as gas, oil,heat pump or an electric heating system to heat water directly. This hot water is then stored in a hot water storage tank, which is designed to retain heat as a thermal storage tank for use when there is a demand for the hot water, which is often a large load, in a short period of time.

They essentially serve as water reservoirs, and they’ll be connected to either an indirect or direct fired hot water system to increase hot water storage volumes to accommodate high demand applications with thermal storage in hot water cylinders.

A hot water storage tank or hot water storage vessel is also known as a hot water buffer tank because it “buffers” hot water loads during peak hot water demand.

Hot water storage tanks

A hot water storage tank or vessel, just like its counter parts, such as indirect cylinders or heating products, must also be energy efficient water cylinders and must carry an ErP energy efficiency rating for standby heat loss.

Any hot water storage systems must comply with the Eco-design ErP Directive.

On September 26, 2017, restrictions on hot water storage tanks’ standing heat losses (heat lost to the surroundings equals wasted energy) were implemented to ensure maximum efficiency.

Storage tanks, or cylinders with a capacity of 2,000 litres or less must meet a minimum energy performance standard in order to be sold legally in the UK and the EU. The same applies for a thermal store.

The rate of heat loss determines the energy rating of a hot water storage tank , which is measured in watts. The insulation of the hot water tank is crucial to achieving a high efficiency band since each efficiency band has a minimum and maximum heat loss proportional to capacity. The best hot water tanks have a thick layer of high-performance polyurethane foam to cover the gap between the tank and the cover.

As part of the energy labelling directive, which says that all energy-related products, like home appliances, must show their efficiency rating in a clear way, hot water storage tanks with a capacity of 500 litres or less must also have energy labels.

Our entire hot water storage tank line complies with both ErP and the energy labelling directive and can be fitted with additional electric immersion heaters or a single immersion heater as additional heat sources to top up or as a backup heating system to the primary water heating system.

hot water cylinder

Is it a hot water cylinder, a thermal store, a hot water tank,hot water holding tank or a buffer tank?

Ultimately, they are just a storage tank for hot water—you can call them what you want.

hot water storage tanks or cylinders for domestic hot water accumulation from Flexiheat UK

In simple terms, they are hot water storage tanks heated by other heat sources designed to store hot water for peak hot water demands or loads, normally for domestic use hot water or a process water heating system such as warm washing down systems demand etc.

They are not an indirect hot water cylinder heating system that uses a boiler (normally condensing boilers) or solar panels to heat the surrounding water in the hot water cylinder via a heat exchanger coil within the storage tank or hot water cylinder. Thus the term an indirect cylinder.

Hot water tank range

We do various types of cylinder or tank, these being – a glass lined hot water tank ( vitreous enamelled ) or stainless steel in construction-

Vitreous enamelled DHW / glass lined hot water tanks

Vitreous enamelling is an impermeable, food-safe coating that looks like porcelain and protects the metal surface of a storage tank when it comes in contact with water.

Vitreous enamel, commonly known as porcelain enamel, is a material created by burning powdered glass onto a substrate at temperatures ranging from 750 to 850 °C. The powder melts, flows, and hardens to form a smooth, long-lasting vitreous covering, thus these units are also known as glass lined hot water storage tanks.

In addition to the test temperature required by current standards (22 degrees C), our vitreous enamel has a food grade certification at 120 degrees C. This makes sure that it is of the highest quality, even at high operating temperatures.

The maximum operating temperature of an enamelled storage tank

Because of their capacity for molecular interaction with the steel surface, these hot water storage tanks can withstand a maximum DHW storage heated water temperature of 95 degrees C without deterioration or detachment.

Pressure rating and warranty period

These warm water storage tanks, are 8 Bar pressure rated and come with a 5-year warranty


hot water storage tank 200 to 1500 litre dimensions – these cylinders are fed by a sperate water heater often tankless water heaters or instantaneous water heaters which heat the water and then it is stored in one of these tanks or cylinders

200 to 1500 Litre Warm Water Storage Tanks – CV-R range


Dimensional detail of our hot water storage tanks – The “R” range have a inspection / clean out port which is 146mm in diameter as standard – item “J” on the drawing – we can also fit an immersion heater in this port as well.

Domestic hot water storage vessel range 200 to 1500 litre technical data from Flexiheat UK

Rigid, mould-injected PU thermal insulation that has demountable sections that allow you to get the 800 and 1000 litre hot water cylinders through a standard 800mm doorway.

800 and 1000 litre hot water cylinders that can fit through a standard 800mm doorway

Demountable thermal insulation on the 800 and 1000 Litre

The full range can be seen on the following brochure –  Please see page 10 for units from 200 to 1,500 Litres , and page 36 for 1,500 to 6,000 Litres of storage cylinder capacity.

Download the Vitreous Enamelled DHW storage tanks or vessels brochure

Commercial hot water storage tanks

We do a full range of commercial hot water storage tanks or industrial hot water tanks all the way up to 6,000 litres

These large-capacity commercial and industrial hot water storage tanks have both side and top DN 400mm manholes to access the interior of these large hot water storage tanks for inspection, cleaning, and maintenance tasks.

Thermally insulated with rigid, mould-injected, 80 mm-thick, polyurethane foam, with an insulating piece in the same material on the DN400mm side manhole.

As standard our storage hot water cylinders are fitted with a cathodic protection system by sacrificial anodes with a charge meter , the anodes must be checked periodically for wear and replaced if necessary.

The anode charge metre is a straightforward and convenient method for determining the state of the anode. Check whether the dial indicator is in the green zone (anode with sufficient charge) or the red zone (anode with insufficient charge, requiring replacement of the anode).

As an option on our industrial hot water storage tanks, Correx permanent anode (cathodic) protection units can be included, which are special titanium anodes that emit the necessary current for the metal surface to be protected via an automatic potentiostat connected to the main power supply.

Maintenance free: since the cathodic protection system is permanent, unlike sacrificial anodes, there is no degradation, and no replacement of the anodes is necessary.

commercial hot water storage tanks 1500 to 6000 litres by Flexiheat UK

1,500 to 6000 litres commercial or industrial hot water storage vessels, tanks or cylinders


Dimensional detail of our commercial hot water storage tank range


commercial and industrial hot water tanks technical data 1500 to 6000 litres Flexiheat UK

stainless steel hot water storage tank

Stainless steel hot water tanks have become an increasingly popular type of domestic hot water storage tank as they have many benefits.

  • Stainless steel hot water storage tanks do not require a sacrificial anode for normal potable water conditions  , unlike an enamelled or glass-lined hot water tank or hot water cylinders. However there is an exception:

    Aggressive agents such as >150 mg/L chlorides in the water with storage temperatures of >75 ºC – in these circumstances cathodic protection is required.

  • This means that the inspection and maintenance regime of a stainless steel hot water tank is less than that of enamelled hot water cylinders.

  • Stainless steel storage tanks or hot water cylinders used to have a working pressure advantage over enamelled storage tank versions, but these days the hot water tanks are equal in the pressure rating.

stainless steel domestic hot water storage tanks flexiheat UK

Dimensional detail of our stainless steel domestic hot water storage tanks

For example our 500 litre hot water storage tank in stainless steel – is 770mm in diameter and has a height of 1690mm


Stainless steel domestic hot water storage tanks 200 to 1000 litre technical data from Flexiheat UK


Our range of stainless steel domestic hot water storage tanks are detailed more in the following brochure

Stainless steel hot water storage tank brochure

Insulation of the DHW storage tanks

The insulation keeps the temperature of the DHW storage at a constant level for extended periods of time without requiring extra energy input. It is an over-sized, rigid, mould-injected PU thermal insulation. This results in fewer external energy source start-ups (such as tankless water heaters), which reduces energy usage, which directly translates into actual energy cost savings when using an insulated hot water storage tank.

Hot water system

A vented cylinder must feature a vent pipe that allows the water to expand as it warms up in the storage tank and safely condenses any residual warm water into the cold tank. To maintain water pressure to the vented cylinders, there must be “head” or distance above the tank or vessel, which is why the stored cold water tank is often located in the attic space in UK homes for vented cylinders.

On the other hand, unvented cylinders, or buffer tanks, are a sealed domestic hot water heating system. The expansion of the domestic hot water as it heats up is taken up by an expansion vessel in an unvented cylinder or unvented cylinders.

Should you require any more support, selection advice or costs/ get a quote on our domestic hot water storage tank range , with fast delivery times then please use the contact us form page on the website, or feel free to give us a call on 01202 822221.