The Guide to Hot Water Cylinders Re October 19, 2021

The Guide to Hot Water Cylinders

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Doing research about which hot water cylinder is best for your home is essential. Installing the correct hot water cylinder ensures that you are provided with an efficient source of hot water without running out. 

Hot water cylinders are used to store water. They are used in main pressure (system) heating and gravity-fed (conventional or regular) heating systems. 

There are a few types of hot water cylinders, including direct and indirect hot water cylinders, as well as vented and unvented hot water cylinders. It is important to note that direct and indirect hot water cylinders are only installed within conventional (regular) boilers.

Direct Hot Water Cylinder

With a direct hot water cylinder, the water is heated using immersion heaters installed within the cylinder itself. Some hot water cylinders have more than one immersion heater to improve performance, this is referred to as a booster immersion heater. 

If you are unsure what an immersion heater is and how it works, we have written a little about them below, or you can read our blog.

Indirect Hot Water Cylinder

Indirect hot water cylinders are fitted with a coil inside the tank, however, they rely on external appliances such as a boiler to generate hot water. Some indirect hot water cylinders are installed with an immersion heater for additional heat.

The Difference Between a Direct and an Indirect Hot Water Cylinder

An indirect hot water cylinder has an immersion heater either at the top or the bottom of the tank. If the tank is an Economy 7 (a cylinder that features two immersion heaters that are located at different heights in the cylinder and programmed to work at different times of the day) or has a photovoltaic solar system installed, then it may have two, one at both the top and the bottom. However, the number of immersion heaters fitted doesn’t matter when it comes to an indirect hot water cylinder. 

With an indirect hot water cylinder, the main heat source comes from the heating water from your boiler. The hot water from your boiler travels up, usually through a two or three-port valve, which then directs hot water through a coil inside your hot water cylinder. This water will come through your taps, depending on the time you set it for, at the temperature you’ve set it at. 

The water travels through the coil ensuring that both the water in the coil and the water in the tank do not mix, the water that comes directly from your boiler is not clean, especially if it has been sitting for a while, so you wouldn’t want to use it to wash in. 

The water in the coil is introduced to the water in the tank at around 75/80 degrees celsius. The heated water from the coil then heats the water in the tank. Usually, you will find a thermostat on the side of the tank, which is typically set to around 55 degrees celsius. When the water reaches this temperature, it sends a signal to the control valve to shut off the flow of hot water to the coil and also tells the boiler and pump to turn off. 

Direct hot water cylinders have an immersion heater on the top and bottom of the tank. This is more commonly found in Economy 5 or 10 systems. Economy 5, 7 and 10 are the names for the different ‘time of use’ energy tariffs, which gives you a set amount of hours of ‘off-peak, cheaper electricity. 

The top immersion heater essentially acts as a booster heater, meaning that when you’re at a high rate of electricity usage, but you still want to use the bath, the top immersion heater will get into action and work to heat the water at the top of the tank only. At night, when you have a lower rate of electricity, you will have what’s called a ‘nightset-back’ immersion. This is when the bottom immersion heater will work to heat the entire tank of water. 

If you have a photovoltaic solar system fitted in your indirect hot water cylinder tank, during the summer, when the weather is hot, a lot of people turn off their electricity and allow the solar panels to heat the water. This can help you save money on your energy bills and become more environmentally friendly.

What is an Immersion Heater?

In simple terms, immersion heaters are devices that provide hot water for your home, powered by electricity. However, an immersion heater could be your knight in shining armour if your central heating fails, as it can still provide your home with hot water. Hot water immersion heaters can also be used as your main source of hot water or as a backup for your traditional gas boiler, as it is separate from your central heating boiler and radiators.  

Those who still have older gas boilers or oil boilers in their property often choose to install an electric immersion heater as a safety blanket, so that they always have a supply of hot water. You will also find that immersion heater tanks are the primary source of hot water in newly built homes that are not supplied by the main gas network or homes that are off the grid as a result of their remote locations.  

Immersion heaters are located within a large hot water cylinder, also known as an electric resistance heater that heats the surrounding water. They are connected to the main power supply and can be switched on and off on-demand, meaning you do not have to have it switched on and heating the water constantly.  

In addition to this, there are also vented and unvented cylinders.

Unvented Hot Water Cylinders

Unvented hot water cylinders are fed directly from the mains water pressure and deliver hot water to showers and taps at mains pressure. 

The cold water is directly pulled from the main water supply and is then heated by the selected heat source relevant to the cylinder type. For example, a direct unvented cylinder utilises electricity to heat the water using immersion heaters, whilst an indirect unvented cylinder utilises gas from a boiler to heat the water. Unvented cylinders are ideal for properties with limited space. This is because they do not need cold water storage. 

Unvented hot water cylinders require a series of safety mechanisms such as:

  • Relief valves that protect against excessive temperature and pressure
  • Thermostats that control and limit the cylinder temperature and usage
  • Expansion vessel which allows water to expand in the system

Modern unvented hot water cylinders are made from stainless steel. This material is efficient in that it is thin and can withstand extreme pressure and high temperatures. Heavy copper is also used as an alternative to stainless steel and is better for water storage, however, it is more expensive to purchase and install.

What are the benefits of an unvented hot water cylinder?

  • They are compatible with existing pipework
  • Easy to install
  • There is no requirement for gravity, which means that they can be installed anywhere in your home
  • There is no need for pumps due to a connection directly for the mains pressure

What are the downsides to having an unvented hot water cylinder installed?

  • They are more expensive to install and maintain
  • They can not be installed in conjunction with power showers (we have a separate blog that covers boiler and shower compatibility)
  • Older pipework may struggle due to the increase in water pressure
  • They require specialist installation

Vented Hot Water Cylinder

Vented cylinders are only installed alongside a regular, conventional boiler. Vented cylinders store and heat water that is fed directly from a tank in the attic.

As the tank in the loft is situated in a location higher than the vented cylinder, this provides a natural gravitational force of water to the system, enabling it to provide a flow of hot water to the tap outlets throughout the property. Thanks to the water tank in the loft, which provides a constant source of water, it can provide hot water to numerous bathrooms with consistent pressure. 

However, as a result of needing an additional water tank in the attic, this can restrict where the cylinder can be installed. 

What are the benefits of a vented hot water cylinder?

  • They are easy to maintain
  • Cheaper than other systems
  • Easy to install

What are the downsides of a vented hot water cylinder? 

  • They need an additional pump if the water pressure is low
  • They are becoming outdated
  • They take up more space in your home due to needing a storage water tank

How is Cylinder Capacity Measured?

The capacity of your hot water cylinder will depend on whether it’s being heated directly or indirectly. Larger houses will have larger cylinders. When selecting the size of the hot water cylinder, a simple rule of thumb is that for a typical domestic household, you should allow between 35 and 45 litres for every occupant. With that being said, a mains pressure system can use about 18 litres of water per minute at 40 degrees celsius, if a decent quality shower head is used. Certain brands of showers can use up to 25 litres of water per minute.

However, it must be noted that personal habits also play a big part in total hot water use. Two households of the same size can use completely different amounts of hot water, with one of them using twice as much as the other.

So, what affects hot water usage?

  • How many baths are in the property
  • How many people are using hot water at the same time
  • Temperature settings
  • The time the boiler takes to heat more hot water
  • The size of the property
  • The number of sinks, basins and taps

In determining how much hot water you require you should always consult with the occupants, and consider the following:

  • A bath uses 100 litres of hot water at 40 degrees celsius (equating to 60 litres at 60 degrees celsius)

  • Showers can use about 18 litres of hot water per minute at 40 degrees celsius (equating to 11 litres at 60 degrees celsius). So for a normal 5-minute shower, you will use around 90 litres of hot water.

    The following average consumption values can be used as a general rule (hot water requirements per person per day):

    • Low Consumption = 20 – 30 litres
    • Average Consumption = 30 – 50 litres
    • High Consumption = 50 – 70 litres

On this basis, a typical four-person household would often use around 200 litres of hot water a day and this is the figure that will generally be used under the new EU energy labelling scheme. 

This does not necessarily mean that a 200-litre cylinder is required as depending on the heating system the cylinder may be partially reheated during the day. It is up to the installer to match the correct size of the cylinder relative to boiler (or electrical) input to avoid running out of hot water.

What Size Cylinder Will I Need For My Home?

Choosing the correct size cylinder for your home is essential. If you choose a cylinder capacity that is too large for your home, then you could run the risk of storing more water than you need for your home, which will waste more energy during the heating process. If you choose a cylinder capacity too small for your home, this may leave you with a lack of hot water for your home. 

Here is a table to help you identify what size cylinder you will need for your home: 

Number of rooms in your homeCylinder capacity in litres
1 Bed & 1 Bath / Shower120 – 150
2 Bed & 1 Bath / Shower150 – 180
2 Bed & 2 Bath / Shower180 – 210
3 Bed & 1 or 2 Bath / Shower180 – 210
4 Bed & 2 Bath / Shower210 – 300

What are the best hot water cylinders?

When you’ve decided on the right type and size of cylinder, your next choice will be which brand to opt for. While the Megaflo Eco by Heatrae Sadia has been very popular for many years, several other brands are now rivalling them with cylinders of equal quality at competitive prices.

Here are the leading hot water cylinder brands in the UK today: 

Cylinder ModelCylinder MakeSizes Available
(Volume in Litres) 
ErP RatingHeating MethodWarranty Length 
Megaflo EcoHaetrae Sadia70 / 125 / 145 / 170 / 210 / 250 / 300BIndirect or DirectLifetime
GreenstoreWorcester Bosch90 / 120 / 150 / 180 / 210 / 250 / 300BIndirect Only25 years
Vitocell 200-VViessmann90 / 120 / 150 / 180 / 210 / 250 / 300CIndirect (includes a backup immersion heater)25 years (against corrosion)
StainlessLiteGledhill90 / 120 / 150 / 180 / 250 / 300 / 400CIndirect or Direct25 years
TempestTelford90 / 125 / 150 / 170 / 200 / 250 / 300CIndirect or Direct25 years

We hope that this blog has helped you to identify the difference between indirect and direct hot water cylinders, as well as how to differentiate vented and unvented cylinders.

As mentioned at the beginning of the blog, installing the correct capacity size and the correct type of hot water cylinder is essential. If you are unsure, discuss this with your Gas Safe Engineer.

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