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A guide to hot water cylinders

Researching which cylinder you need for your home is essential to cater for your hot water needs. Installing the correct cylinder will provide you with an efficient source of hot water without ever running out. In our hot water cylinder guide, you’ll find everything there is to know about domestic cylinders, e.g. the different types, pros and cons of each, the size you’ll need and some energy-saving tips.

How does a hot water cylinder work?

Hot water cylinders are used to store water, as well as being used in mains-pressure and gravity-based heating systems. You’ll come across two types of cylinder, these being either indirect hot water cylinders or direct hot water cylinders. The water in a direct hot water cylinder would be heated using immersion heaters within the cylinder itself, some having more than one to improve performance. An indirect hot water cylinder is reliant on external appliances such as a boiler to generate hot water. You will find some indirect cylinders also have an immersion heater installed for additional heat.

What is an unvented cylinder?

An unvented hot water cylinder is fed directly using the cold mains water pressure, delivering hot water to showers and taps at mains pressure. As a pressurised hot water system, it’s great for improving how your water outlets perform. The water within the system can either be heated indirectly through your central heating system as well as via solar technology. An unvented cylinder is also ideal if you are pushed for space since a cold-water storage tank isn’t required. The Megaflo cylinders are a prime example of unvented cylinders and the current market leader in the UK.

Being a pressurised system, the unvented hot water cylinder requires a series of safety mechanisms, such as relief valves (which protect against over temperature and pressure), twin thermostats (which control and limit cylinder temperature and use), and an expansion vessel to allow water to expand in the system as it’s heated. You will also find that most modern unvented hot water cylinders are made from stainless steel, which is not only thin but also withstands extreme pressure and increasingly high temperatures. Heavier copper is an alternative, however it’s much more expensive despite being better for water storage.

Advantages of unvented hot water cylinders:

  • Work easily with existing pipework
  • No requirement for gravity meaning it can be located anywhere in your home
  • No need for pumps due to direct mains pressure
  • Installed easily due to existing pipework

Whilst some of the downsides are:

  • Higher cost to install and maintain
  • Can’t be installed in conjunction with power showers
  • Require specialist installation
  • Pipework in older homes may struggle to cope with increased pressure

What is a vented cylinder?

A vented hot water cylinder uses a gravity-led system instead of water pressure, relying on a cold-water tank to feed the cylinder water then a heat source to heat the water. The water will be heated either directly using an immersion heater within the cylinder or indirectly via a central heating boiler. A cold-water tank is required in order to enable air to escape the system as well as acting as further storage for water during the heating process. This eliminates the need for an expansion vessel to cope with trapped air.

Vented hot water cylinders are relatively simple in terms of build and are slightly challenged by their lower water pressure. The vertical distance of this system determines the pressure of the water, which can be a problem if you’re already tight for space.

Advantages of vented hot water cylinders include:

  • Easy to maintain
  • Cheaper than most alternate systems
  • Relatively easy to install

Whilst some of the downsides are:

  • Require additional pump if water pressure is low
  • Consume a lot more space due to cold water storage tank
  • Becoming outdated

What size of hot water cylinder will you need?

Choosing the right size cylinder is essential to cater to the hot water demands of your home. Too big a cylinder will store more water than you need and also waste energy in the heating process. Too small a cylinder will leave you lacking available hot water. The capacity of a cylinder will change depending on whether it’s being heated by an indirect heat source (e.g. a boiler) or directly. The cylinder capacity is always measured in litres, which may not be entirely useful when thinking in terms of daily showers, baths or washing up. However, the easiest way to work out what size and capacity you’ll need is by looking at the number of bedrooms, bathrooms and showers you have.

It’s crucial to consider the combination of your cylinder’s capacity and re-heat times, especially during the periods of highest demand and also future proofing your cylinder purchase.

Insulating your hot water cylinder

It’s important to consider how a lack of cylinder insulation will affect the hot water stored after being heated by your boiler. For energy efficiency purposes, insulating your boiler will reduce the energy required to re-heat the water, especially in those cold winter months. Most modern hot water cylinders will already have some polyurethane foam insulation installed which is normally sufficient, but some older tanks will lack in insulation and could benefit from a jacket.

A typical cylinder jacket is made of polythene, and acts as an additional form of insulation for cylinders despite their existing insulation. Some jackets can range from £10 to £25, which is only a small investment considering how much you could potentially save a year. It may also be a good idea to check if the pipes running between your cylinder and boiler are also insulated. If not, you could be losing even more heat, which could be resolved using simple DIY pipe insulation.

Below we have provided a general guideline for domestic hot water cylinder sizes:

# of Rooms & BathroomsIndirect Hot Water CylindersDirect Hot Water Cylinders
1 Bedroom & 1 Bath/Shower75/120120/150
2 Bedroom & 1 Bath/Shower150180
2 Bedroom & 2 Baths/Showers180210
3 Bedroom & 1 Bath/Shower180210
3 Bedroom & 2 Baths/Showers180210
4 Bedroom & 1 Bath/Shower180210
4 Bedroom & 2 Baths/Showers210/250250/300

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