How do hot water tanks work in your home? WarmZilla Blogs.

Currently, England is home to approximately nine million hot water cylinders. According to the Hot Water Association, since 2001, there’s been a shift from installing hot water tanks towards combination boilers, which produce hot water immediately.

For smaller homes, a combination boiler is a great option, but if you want hot water from multiple outlets around the house, only a hot water tank can fill this need.

The consensus is that cylinder water heaters are not as environmentally friendly, but nothing could be further from the truth. With improvements in thermal insulation, they are becoming more environmentally friendly as well.

Also, with smart home technology on the rise, homeowners can control when the cylinder is heated, its temperature, and more.

It’s also worth noting that you can retrofit older hot water tanks with smart technology, allowing the homeowner to keep their current units while ensuring they perform better environmentally.

And with the advancements in water pump technology, you can install one to ensure you get the most out of your hot water tank’s storage facility. There are countless avenues to explore when upgrading your home’s hot water cylinder.

But for those new homeowners who have never owned one, what exactly is a hot water tank?

What Is A Hot Water Tank?

A hot water tank, or hot water cylinder, is a system that stores heated water to use at a later stage.The council’s water supply is pumped into your home and is stored in a tank or cylinder. This water can either be heated before storing in the tank or inside the storage cylinder.

The storage cylinder is usually well-insulated to ensure the heated water remains at a pre-set temperature. As we mentioned, there are several methods of heating water for your cylinder.

So, what types of hot water tanks are available to the UK public?

Types of Hot Water Tanks

Like your options for boilers, there are also several choices available for hot water tanks.

When installing a new water cylinder system, you will want to consider your future hot water requirements. As an example: is your family growing, or are your children leaving home soon?

Or, if you just want to identify what type of hot water tank is installed in your home, knowing about the different kinds of hot water cylinders will help.

The first differentiator to determine is between direct and indirect.

Direct Vs. Indirect Hot Water Tanks

The type of water tank you can get will depend on the current heating system set-up of your home.This is because the terms "direct" and "indirect" refer to how the water source is heated.


Direct heats water in the tank, while indirect tanks heat their water source externally.

Direct hot water tanks use immersion heating elements. This immersion element will switch on when it detects that the water temperature drops.

At the moment, immersion heaters are not recommended for homeowners as their primary hot water source because they are inconvenient to repair and can be incredibly expensive to run compared to a boiler.


An indirect hot water tank uses an external heating source before the water is stored away. These are more popular because most homes in the UK have a boiler installed ready.

While most indirect systems retain warmed water, some can be fed directly from the council mains - but these are storage tanks. There are several boiler options you can fit into an indirect hot water tank system.

The next category of hot water tanks involves the difference between vented and unvented hot water cylinders.

Unvented Vs. Vented Hot Water Tanks

The core difference between an unvented and vented system is whether it is fed directly by the water mains.


Unvented cylinders are directly plugged into the main water supply, which means they operate under pressure. Because the water source is pressurised, it does not require a separate water tank.

Usually, unvented hot water tanks are made from stainless steel, which has the advantage of retaining heat for extended periods when compared to other materials. Another benefit of stainless steel is that it lasts longer because it doesn’t rust.


On the other hand, Vented hot water tanks are fed by a cold-water storage tank, which is usually installed in the home's loft.

These tanks gravity-feed water into the hot water cylinder. Once the cold water is circulated through the cylinder, heated, and stored, it can be piped to the outlets throughout the home.

The system is fitted with a vent pipe, which ensures the heated water vapour condenses and drips into the cold water storage tank rather than building pressure. Some homeowners will be advised to install an expansion tank.

This is because water vapour can expand up to 4% in volume. If your system converts too much water into steam, your plumber might suggest installing an expansion tank - for safety reasons.

Optimally, pressure in vented systems is low. And so, homeowners will often install additional water pumps to ensure the pressure remains at a safe level (we increase the volume of the pipes; thus reducing pressure).

Also, vented hot water tanks are created from copper and use conventional heating techniques, such as using an immersion element.

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How Does a Hot Water Tank Work?

Hot water tanks work in a specific sequence. Depending on the system in your home, this can vary widely.

But generally, hot water tanks work as follows:

Water Source

Water sources for hot water tanks are either pressurised (unvented) or gravity-fed (vented). A pressurised water source is fed directly from the mains and is piped into the mechanism.

With a gravity-fed system, the water is fed from a separate cold-water tank, usually directly into the cylinder, and is heated by an immersion heating element.

Heating the Water

As mentioned, direct cylinders heat their water inside the tank, while indirect will depend on an external source.

With indirect systems depending on boilers, their heating methods can be fed via gas, electricity, or supplemented by solar.

In contrast, direct systems heat the water with an immersion heating element.

Hot water tanks will have a thermostat, which regulates the water temperature. If the temperature dips, the heating mechanism will activate. Also, these thermostats will have built-in safety features to ensure the tank does not overheat or start if the cylinder is empty.


The insulation of a hot water tank is essential. When a hot water tank has been optimised for retaining heat, it automatically means a reduction in the energy required to maintain the set temperature.

Optimising your hot water tank means selecting a product with the best heat-retaining materials. As mentioned, stainless steel retains heat incredibly well. To improve your cylinder’s ability to retain heat, you can also wrap the tank in a specifically designed blanket.

Another element of your hot water tank’s system that can be insulated to retain heat is the piping that feeds your outlets.

Now that we’ve gone over the types of hot water cylinders and how they work in sequence, what size hot water tank does your home require?

What Size Hot Water Tank Do I Need?

Simply put, the bigger your home is, the larger the hot water tank needs to be.

Consdier the number of people in your home

As a basic guideline, each household member should be allocated between 35 to 45 litres of hot water daily. But with specific eco-friendly outlet attachments, you can reduce this.

As an example, some shower heads can use up to 25 litres a minute, while more environmentally friendly ones can reduce that to 18 litres per minute.

Personal habbits

This being said, these general guidelines might not match your home’s requirements simply because of personal habits. A typical bath can require 100 litres of hot water. If anyone at home is a regular bather, you could double your heated water consumption rate.

Thus, to determine your hot water tank size, you must talk with each household member about their habits.

Those who are low consumers of heated water may only use around 20-30 litres a day. In comparison, a high consumer can use between 50-70 litres.

Peak time usage

Another factor to consider is peak usage times in the home. Usually, these are in the morning before work and school and after work hours. Your hot water cylinder needs to store enough heated water for everyone.

Your location and climate

Your location is another factor to consider if your hot water cylinder is used to heat the home.

If your home is situated further north, you will want a bigger hot water tank to handle the extra strain on the system due to the lower temperatures.