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What size hot water cylinder do I need?

Some heating systems require a hot water cylinder in order to store and heat water ready for use, however, if you are planning to install a new heating system, you may be wondering what size cylinder would be appropriate for the property and household usage.

We will be exploring the different size cylinders available, their water capacity and how to decide which size would be best for you.

What is a Hot Water Cylinder?

Firstly, before we discuss the range of sizes that cylinders are available within, let’s cover what a hot water cylinder is.

A hot water cylinder is an insulated storage tank used within certain types of heating systems to store hot water for later use. There is a range of benefits of having a hot water cylinder within the heating system, including the ability to supply a range of water outlets without losing pressure or to have a reserve tank of hot water should the boiler itself fail. Due to the fact that the tank is insulated, the water should stay hot for a period of time should there be a power cut for example.

However, there are also other considerations with water cylinders such as space for the tank and also that the hot water within the tank needs to be heated in advance of the need to use it. In addition, there can be efficiency issues due to the whole tank needing to be heated even if the household does not need hot water that day.

Vented vs. Unvented Cylinders 

There are two different types of hot water cylinders as follows:

Vented cylinder

Hot water vented cylinders are fed from another tank that stores cold water and utilises a vent pipe to manage the water levels during the heating and subsequent expansion process. The cold-water tank is often located within the property’s loft due to two factors; for storage purposes due to the size of the tank and also as gravity plays a part in water distribution to the rest of the property.

Pressurised unvented cylinder

This type of hot water cylinder receives water directly off the mains which aid water pressure and does not require a separate cold-water tank. Unvented cylinders became common in properties from the 1980s due to their space saving and improved water pressure benefits.

In addition to choosing between a vented or unvented hot water cylinder, homeowners or landlords also should decide whether a directly or indirectly heated cylinder would be most appropriate. The differences are as follows:

  • Direct – A directly heated hot water cylinder has its own source of heating from an in-built immersion heater, rather than being connected to the boiler.
  • Indirect – An indirect hot water cylinder has an in-build coil to heat the water held within the tank, however, it is powered via an external source such as a boiler.

Should you need any specific advice regarding the choices of differing hot water cylinders, a qualified heating engineer would be best placed to provide property-specific advice.

Water Cylinder Tank Sizes

As we have briefly mentioned, water cylinders come in a range of sizes and therefore hold various water capacities. As a rough guide, the number of bedrooms within the property would determine the size of the water cylinder needed as follows:

  • 1 bedroom property – 120- 150 litre tank
  • 2-bedroom property – 150- 180 litre tank
  • 3-bedroom property – 180- 210 litre tank
  • 4-bedroom property – 210- 300 litre tank
  • 5 or more bedrooms within a property – 300+ litre tank

The number of bathrooms within the property may also be a factor as more than one bath or shower would increase the demand of water, therefore a larger tank would be needed.

Should you have any queries regarding the most suitable tank size for your property and hot water usage, please seek the advice of a professional heating engineer.

Estimating a Household’s Daily usage of Hot Water

In addition to the suggested generalised tank sizes above, the homeowner or landlord should also factor the household’s actual water usage before finalising the water cylinder tank size needed.

Estimating a household’s water usage is not an exact science, and for a landlord, this may be even more tricky if the incoming tenants are unknown, however in order to assist there are some industry averages per adult of water usage as follows:

Hot water usage level (per adult) Industry average of the amount of water used per day:

  • Low level 25 litres
  • Medium level 40 litres
  • High level 60 litres

In addition to these averages, there are a number of factors that should be considered when estimating a household’s water usage such as:

  • The number of baths and showers within the property
  • The number of sinks and taps within the property
  • The number of habitants within the property and their typical routine, including how many baths or showers would be used at the same time
  •  The household typical water usage at peak times, perhaps before work or school
  • The household circumstances and routine, for example, those that work from home more often
  • The temperature that the hot water is typically set at
  •  The time that the boiler takes to heat the water cylinder currently

Why does Getting the Correct Size Water Cylinder Tank Matter?

It is important to install a water cylinder tank that is the most suitable size for the property and household habitants needs otherwise there can be problems.

For example, a tank that is too small will not provide sufficient hot water for the household, whereas an installed hot water cylinder tank that is too large will result in larger heating bills by heating a higher quantity of hot water than the household requires.

In addition, the storage of the tank is an important factor, therefore selecting a tank that is too big will waste storage space also.

What Other Considerations are there to Installing or Replacing a Hot Water Cylinder?

  • Replacing like for like – Often the advice if replacing a current hot water cylinder is to purchase the same type of tank for the new system to save from changing any pipework or the location of the tank. However, should the household requirements have changed, for example, it is highly recommended that the advice of a professional heating engineer is sought to help plan any significant system changes.
  • Leaks – Leaks can spring from the hot water tank itself and maybe a sign that there is an internal issue taking place that would require immediate investigation from a professional heating engineer. As water tanks are often stored within a loft and out of sight, a visual check of the tank should be undertaken as part of the regular routine of household maintenance.
  • Water discolouration – Another sign of an issue with a hot water cylinder is when the water discolours. Discoloured water can be the result of the anode rode from inside a hot water cylinder requiring replacing or the tank itself may have started to deteriorate or corrode and therefore a professional heating engineer should be called to investigate.
  • Cold Water – Should a water tank not be heating up to the desired temperature, the habitants would not be getting enough hot water for their needs. A direct hot water tank could be suffering from a fault with the internal heating element and therefore a professional heating engineer should be called out to check the tank and its internal parts.

What size hot water cylinder do I need summary

Hot water cylinder tanks are common within UK properties and come in a range of sizes depending on the requirements of the household. Should you be installing a new hot water cylinder or, replacing a current tank, it is highly recommended that specialist advice is sought in order to find the most suitable tank for your needs.

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