How Does a Hot Water Boiler Cylinder or Tank Works?

Hot water cylinders or tanks are used for storing hot water and keeping it warm when you need it. It is also used in mains-pressure and gravity-based heating systems. In UK homes, you will come across two types of cylinders—

  • Direct
  • Indirect

In a direct hot water cylinder, the water is heated using an immersion heater within the cylinder itself. Whereas, an indirect hot water cylinder is reliant on an external device such as a boiler to generate the hot water.

After settling on whether you need direct or indirect hot water cylinder, further there are two other types of cylinder you could choose—

  • Vented Cylinder
  • Unvented Cylinder

The key difference between them is that one needs a cold-water tank and one doesn’t.

Benefits of a Hot Water Boiler Cylinder

Having a hot water boiler has many benefits. Boiler cylinder can be placed in a cupboard and turn the loft space into an airing cupboard. Moreover, an airing cupboard is a brilliant place to store your clothes, towels and bedding.

A system with a hot water cylinder can tackle a high demand for hot water. For families of four or more, this type of system is more practical than other available options.

Hot water cylinders are extremely versatile and useful. They provide different ways to heat the water (direct or indirect), but the water can also be heated more sustainably through biomass fuels or solar power.

Furthermore, a backup heat source can be incorporated in an indirect cylinder in case the regular boiler fails, so the probabilities of running out of hot water are reduced significantly.

Boiler Cylinder— Difference Between Direct and Indirect Systems

Direct Systems

In a direct hot water cylinder system, the hot water is heated directly by an internal element, for instance- an immersion heater. In direct systems, the hot water is likely to be more expensive. For homes without any access to gas, such as a mid-level flat, have no other option than to choose a direct system to get the hot water.

Usually, this type of cylinder is installed with two separate immersion heaters, one meant for the peak electricity and the other one for the off-peak electricity. It is vital to make sure that the immersion heaters are fitted on the timers appropriately to make sure you are paying the lowest possible for the hot water. Heating water through your peak immersion heater in the middle of the night has no point.

You can get both vented as well as unvented direct systems.

Indirect Systems

Majority of the hot water cylinders are heated using an external heat source, like a solar thermal or gas boiler. In the indirect system, the hot water is heated and then passes through a copper coil in the hot water cylinder. The heat is further spread from the exterior heat source to the water in the hot water cylinder or tank.

Indirect cylinders are almost certainly fitted along with a direct backup (for instance- an immersion heater). In case the boiler is damaged, you can still heat the water whenever you need it.

You can get both vented as well as unvented indirect systems.

Now, let’s discuss about the vented and unvented systems.

Boiler Cylinder— Difference Between Vented and Unvented Cylinders

Vented Cylinder

A vented hot water cylinder relies on a cold-water tank to supply water into the cylinder for heating it either by the central heating boiler or an immersion heater within the cylinder. It uses a gravity-led system rather than water pressure.

A cold-water tank is needed in the loft as it is a system that works on gravity rather than pressure. It also lets air to escape the system and acting as additional storage for water during the heating process. Further, there’s no need for an expansion vessel to tackle trapped air.

Vented hot water tanks are comparatively simple in terms of build and are somewhat challenged by their lower water pressure. It means that your water pressure is better downstairs than upstairs. It could mean that boilers require an electric pump to keep good shower pressure.

Advantages of vented hot water cylinders include—

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

Some drawbacks include—

  • Becoming old-fashioned
  • Takes a lot more space due to cold water storage tank
  • Need additional pump if water pressure is low

Unvented Cylinder

There is no need of a separate cold-water cylinder in unvented cylinder system because water gets into it directly from the mains supply. It delivers heating and hot water to showers and taps at mains pressure. Mains pressure is better than the pressure generated in gravity-fed systems, so the shower performance is tended to be improved. Also, you’ll save enough space in the loft with the abolition of the water tank.

The water stored in the system can either be heated indirectly using your central heating system as well as via solar technology. An unvented cylinder is also perfect if you are struggling for space as a cold-water storage tank isn’t needed.

Unvented cylinders are quite more expensive to install since it demands extra safety mechanisms. To lower the risk of the water becoming overheated, you will need relief valves, and to control the temperature of the cylinder you need twin thermostats. Also, you need an expansion tank because the water will expand to some extent as it’s heated.

Advantages of unvented hot water cylinders include—

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

Some drawbacks include—

  • Can’t be installed in combination with power showers
  • Higher cost to install and maintain
  • Pipework in older homes may strive to handle higher pressure
  • Call for professional installation

Combi boilers are the only system that runs without a cylinder or hot water tank and heats the water internally. Unlike system boilers, the issue with combi boilers is that as soon as there is a pressure drop, then the hot water pressure gets split between the two hot water outlets because they work at mains water pressure. Turning the tap on, the boiler is activated into action, and it heats the water in a heat exchanger, for on-demand hot water instantly. However, they are ideal for small homes since there’s no need for storing hot water which is really not very energy efficient.

What Size of Hot Water Boiler Cylinder is Ideal for Your Home?

Choosing the ideal size cylinder is crucial to fulfilling the hot water demands of your home. Installing a too big cylinder will eventually waste energy in the heating process by storing more water than you actually need. Whereas, getting a too small cylinder will leave you running out of hot water during your shower or time of hot water need.

The size of a cylinder varies depending on whether it’s being heated by a direct source or an indirect heat source such as a boiler. Consider the number of bedrooms, showers and bathrooms you have in your home when choosing the size and capacity of the cylinder.

It’s very important to consider the blend of your cylinder’s size and re-heat times, mainly during the times of highest demand and moreover, future-proofing your cylinder purchase.

Also, it is vital to consider the importance of cylinder insulation as its absence will affect the hot water stored after being heated by the boiler. For high efficiency purposes, getting your boiler insulated will lower the energy needed to re-heat the water, particularly in those cold winter months.

In a nutshell, it is essential to research which cylinder you need for your home to fulfil for your hot water requirements. Installing the right cylinder will offer you an efficient source of hot water without ever running out!