What tank do you need to keep up with your home's hot water demands? Let's find out...
Are you looking to replace your old hot water storage cylinder and are wondering what type you need?
Or are you looking to have a brand new one installed alongside a system or regular boiler?
Whatever your circumstances may be, the technical jargon can be confusing, so in this guide, we are going to explain all the essentials you need to be aware of in simple terms.
What is a Hot Water Storage Cylinder?
As the name implies, a hot water storage cylinder is a water storage tank that holds larger quantities of hot water for specific types of boilers and central heating systems.
The hot water is stored in preparation to be used to provide hot water to water outlets like your shower and taps.
Typically, the hot water tank is located in an airing cupboard but is also commonly housed in garages and attic spaces.
Not every type of domestic heating and hot water system uses a cylinder either, for example, a combi boiler doesn’t, as the hot water is heated on-demand and when needed.
Storage tanks are installed together with other types of common boilers including system and regular boilers.
Different Types of Hot Water Cylinders
Just like the types of boilers available, there are also different types of hot water storage cylinders available and the one that is right for you will depend on a variety of different factors.
This mainly includes the following two factors:
Your current and future hot water demands
The type of boiler installed on your property
Direct vs Indirect Hot Water Storage Cylinders
What is the difference between a direct and indirect hot water storage cylinder?
Direct Hot Water Cylinders
A direct hot water storage cylinder has the ability to heat the water itself and is often referred to as simply an immersion heater.
These work in a similar way to a hot water kettle you’d find in almost any kitchen.
Inside they have a large heating element consisting of a metallic coil, which when heated using electrical voltage transmits heat to the surrounding water sitting in the tank and thus heats up the water contained inside.
The water is then delivered to the water outlets e.g. taps when they are turned on.
Typically an immersion heater must be switched on many hours before hot water is required, which is why they often work in conjunction with a timer.
Immersion heaters are rarely recommended as a primary hot water source since they are inconvenient and the cost of electricity is high in comparison to gas.
Indirect Hot Water Storage Cylinders
Another and perhaps the most common type of hot water storage cylinder is the indirect variety.
Indirect cylinders work by using an external source to heat the water prior to it being stored within the cylinder.
Typically external heat sources include a domestic boiler, which will heat the water via a heat exchanger, but it can also include renewables like solar panels.
The cold water is transported into the cylinder from a water source. This can be a separate water tank or can be fed from the mains water supply directly.
The storage tank contains a heat exchanger, often a metallic coil, which is where water from the boiler passes through and in turn heats up the water inside of the indirect hot water cylinder.
Once the water has passed through the coils, it then returns to the boiler and recirculates to repeat the process over again.
As the water cycles through the heat exchanger it heats up the water within the cylinder and since the heat rises, the hot water remains at the top of the tank, closest to the outlet to the home's water pipe network.
The cold water is distributed towards the bottom of the tank where the heat exchanger is. meaning that the water is kept warm.
Vented vs Unvented Hot Water Storage Tanks
What is the difference between an unvented and vented hot water cylinder?
Unvented Cylinders
An unvented cylinder is hooked up directly to the mains water supply, meaning it is a high-pressure system, with a strong water flow.
For this reason, they do not require a separate cold-water tank and they can also be connected to existing heating systems pipework, allowing them to be installed almost anywhere.
Typically, unvented cylinders are made from stainless steel, which tends to retain heat for longer than other metals such as copper. They also tend to have a longer lifespan than copper.
Unvented cylinders are typically used in combination with a system boiler and one of their major advantages is that they do not require a header tank in the attic, meaning that they are incredibly space-saving.
Vented Cylinders
This is a cylinder that is supplied with cold water from a cold-water tank stored in the loft, also often called a feeder tank.
This means water is fed into the cylinder with the help of gravity via a vented pipe which is then heated in the same way and fed to the water outlets via the pipework.
The vent pipe allows the water to expand as it enters the tank and enables any remaining water to be condensed safely into the cold tank.
Since this system relies on gravity to supply water to the tank, the cylinder has to be located directly beneath the cold-water tank.
It also means that water pressure can be low, which can sometimes make taking showers difficult, in which case additional water pumps can be installed to improve water pressure.
Normally, vented cylinders are made from grade 3 coppers and are usually used in conjunction with a conventional system.
Which Type of Cylinder is Right for Your Property?
Are you wondering what type of cylinder is appropriate for your home?
It largely depends on the type of heating system your home uses, for example, if you have a system boiler, you will likely need an unvented storage cylinder, while a system boiler will work together with a vented cylinder.
Here are some other factors that are important to consider:
Vented cylinders are more suitable for properties with older heating systems that are unable to deal with the high pressure of mains water. Installation costs also tend to be lower, as do the upfront costs of the cylinder itself.
On the other hand, unvented cylinders do not require a cold-water storage tank and so do not require as much space to install and where they can be installed is more flexible.
Since they are connected to the mains, they also tend to provide a much more powerful water flow.
With that said, it is worth noting that a major advantage of vented cylinders is that they can continue to be used, even if you lose access to the mains water supply.
It is also important to consider your current heating system, for example, its age. If it is an old system, it may not be able to handle the high pressure from the mains, in which case a vented cylinder would be preferred.
Similarly, if your mains water pressure is not sufficient enough, it does not make sense to install an unvented cylinder that relies on high pressure. It is still possible, but you may need to enhance the pressure by installing a pump.
What Size Hot Water Storage Cylinder Do You Need?
Like boilers, cylinders also vary in size, as in the capacity of water they can store and it’s essential you choose a tank that matches your home’s hot water demands.
If you live in a large 5-bedroom house and/or one with multiple bathrooms, you are going to need a cylinder with a capacity of at least 300 litres. Next, you add 40 to 45 litres for each person living in the home.
The amount of hot water two households use can vary tremendously, so it's worth considering your own and potentially considering how this may change in the future. For instance, if you are thinking about growing your family, it's like the hot water demands will also grow too.
For illustration, here is some average hot water consumption by appliance:
A bath uses 100 litres of hot water at 40 degrees Celsius (equal 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).
Under European Regulations, it is assumed that an average 4 person household will use around 200 litres of hot water per day.
However, if you want a more accurate determination of the size of cylinder your house requires, it’s always advised to seek the assistance of a registered Gas Safe Engineer.
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