Domestic hot water storage tank or vessel

 

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

They are a hot water storage tank / hot water storage vessel / hot water accumulator tank, domestic hot water cylinder, hot water storage cylinders 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 commonly known as DHW (Domestic Hot Water) for multiple bathrooms in hotels, etc., or for a hot water process application.

They are often used in conjunction with tankless water heaters, instantaneous water heaters, or even heat pumps to store additional domestic hot water for peak water heating demands.

With a hot water storage cylinder, the water inside is heated by an external heat source, such as a gas water heater or DHW heat pump, prior to entering the storage cylinder, normally via the mains water (cold water) supply in an unvented cylinder configuration.

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 from the cold water mains temperature to the recommended storage temperature of at least 60 degrees Celsius in a hot water storage cylinder.

The cold water that enters or feeds the hot water tanks can either come from the 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 with a vent pipe to accommodate any hot water expansion.

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, a heat pump or an electric heating system to heat water directly so that they are not indirect cylinders or direct cylinders. They are just hot water cylinders that store heated water.

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 from the hot water system, in a short period of time.

Domestic hot water storage tanks 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/hot water accumulators.

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 counterparts, such as indirect cylinders or direct cylinders, must also be an energy-efficient water cylinder and must carry an ErP energy efficiency rating for standby heat loss.

Any hot water storage systems must comply with the ecodesign 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 a hot water storage cylinder with a capacity of 2,000 litres or less, must meet a minimum energy performance standard to be legally sold 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 for achieving a high efficiency band, as each band has a minimum and maximum heat loss that is proportional to capacity.

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

Storage hot water cylinder

Is it a hot water storage cylinder, a thermal store, a hot water tank, a 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.

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 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 is an indirect hot water cylinder.

A hot water holding tank is commonly used in conjunction with tankless water heaters or instantaneous water heaters to store additional domestic hot water for peak water heating loads.

Hot water tank range

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

Please note: Stainless steel domestic hot water storage tanks should be used in Scotland, the south-west and north-west of England and the west of Wales, where water is particularly soft.

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 inside.

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.

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 Celsius without deterioration or detachment.

Pressure rating and warranty period

These warm water storage tanks are either 10, 8 or 6 bar pressure rated depending on the model and come with a 3-year warranty.

“VS” model – with no inspection or cleanout flange.

“VS1” model – with one inspection or clean-out flange – has a 220 mm diameter orifice (our most popular unit)

“VS3” model – with three inspection or clean-out flanges – again has a 220 mm diameter orifice.

“VS DN” model – one large 400 mm actual orifice inspection or cleanout flange – is available from 800 to 5,000 litres only.

 

 

 

Technical details of our hot water storage tanks – note that ERP energy class regulations are only applicable to volumes up to 2,000 litres.

The “Smalver” range has a resin synthetic lining on the inside of the tank; this option has a maximum water temperature of 70°C.

The “Smaltech” range has a resin-ceramic nanotechnology treatment on the inside of the tank; this option has a maximum water temperature of 80°C.

Additional water heating options

Each tank comes with an immersion heater port as standard for additional heating power to the cylinder for top-up or back-up water heating requirements.

Alternatively, we can supply the units with a 40 mm tapping on the inspection/clean-out flange, as shown here, for additional immersion heaters.

 

 

The full hot water storage cylinder range can be seen on the following brochure – 200 to 5,000 Litres  commercial hot water storage tank units / industrial hot water storage tank capacity models.

Download the Glass lined or resin lined 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 5,000 litres

These large-capacity commercial and industrial hot water storage tanks have a 400 mm manhole for access to the interior for inspection, cleaning, and maintenance tasks.

Thermally insulated with 100 mm-thick polyurethane foam for higher energy efficiency for these DHW commercial buffer vessels.

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

As an option on our industrial hot water storage tanks, we can supply Correx permanent anode (cathodic) protection units, which are electrically powered.

Maintenance-free: since the cathodic protection system is permanent, unlike sacrificial anodes, there is no degradation, and no replacement of the anodes is necessary for our large hot water storage tanks with CORREX permanent anodes.

Technical detail of our industrial and commercial hot water storage tank range – low height option

Again, we have the option to fit an additional heat exchanger to the “VSR DN290” model or an additional immersion heater or heaters.

The full brochure on these low-height water storage vessels / tanks / cylinders is here

Flexitank low height domestic hot water storage and indirect water heater brochure – 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 and must be used in soft water areas.

• 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 even for a stainless hot water storage tank.

• Better corrosion resistance – this means that the inspection and maintenance requirements for stainless steel storage tanks are less than those for enamelled hot water cylinders.

• Our “XVS” range of stainless steel storage tanks or hot water cylinders’ working pressure as standard is 6 bar; however, we can produce the storage tanks with higher pressure ratings if required.

 

Dimensional detail of our stainless steel domestic hot water storage tanks

For example, our 500 litre hot water storage tank in stainless steel is 790 mm in diameter and has a height of 1690 mm.

 

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

Stainless steel hot water storage tank brochure 200 to 5000 Litres from Flexiheat UK

Insulation of the DHW storage tanks

The insulation keeps the temperature of the DHW storage tank at a constant level for extended periods of time without requiring extra energy input. It is an oversized, rigid, mould-injected PU thermal insulation. This results in fewer external energy source start-ups (such as our gas 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, an unvented hot water storage vessel, or an unvented hot water buffer vessel, is a sealed domestic hot water heating system. As the domestic hot water heats up, an expansion vessel connected to the storage vessel or buffer vessel absorbs the expansion.

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 our website, or feel free to give us a call on 01202 822221.