Do you need a water tank with a combi boiler?

condensing boiler explained

A combi boiler is a type of boiler that provides both hot water and central heating. It does this by heating water directly from the mains, rather than storing it in a small storage tank built into the cylinder. This makes combi boilers more efficient than traditional boilers, as they don’t waste energy by heating an external hot water cylinder that isn’t being used.

Do you need a water tank with a combi boiler?

The short answer is no, you don’t need a water tank with a combi boiler. Combi boilers heat water directly from the mains, so they don’t need a separate storage tank. However, there are some situations where a water tank for storage combi boilers may be beneficial, such as if you have a large household or if you use a lot of hot water.

Can you store the water your combi boiler heats?

While you don’t need a water tank with a combi boiler, you can still store the water it heats. This is done by installing a cylinder for storage combi together. The cylinder is a large tank that stores hot water, and it can be installed in the back loft space, or under the stairs.

What is a hot water cylinder?

A water cylinder is a large tank that stores hot water. It is typically made of steel or copper, and it is installed small storage tank in the loft or under the tank in the loft or stairs.

How do system boilers store hot water?

They are a type of system boiler, that also provides both hot water and central heating. However, unlike combi boilers, system boilers do require cylinders. This is because they heat water in batches, and they need a hot water tank to store the hot water until it is needed.

Upgrade to a combi boiler with MML

If you are thinking about upgrading to a combi boiler, MML Plumbing can help. We are a leading new and storage combi boiler installation company in North London, and we have over 20 years of experience in the industry. We would be happy to assess your needs and recommend the best combi boiler for your home.

What are the reasons for installing a combi boiler? Benefits of the combi boilers!

There are many reasons why you need a water tank with a combi boiler you might want to install a new combi boiler here. Some of the most common reasons for new combi boiler include:

Efficiency: Combi boilers are very efficient, which can save you money on your energy bills.

Convenience: Combi boilers provide both a hot water supply and central heating, so you don’t need to have two separate appliances.

Space saving: Combi boilers are typically smaller than traditional boilers, so they can save space in your home.

Quiet operation: Combi boilers are very quiet, so they won’t disturb you or your neighbours.

If you are looking for a new boiler and a more efficient and convenient way to heat your home, then a combi boiler may be the right choice for you. MML Plumbing can help you assess your needs and recommend the best combi boiler for your home.

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What is an open-vented cylinder?

An open-vented cylinder, also known as a vented hot water cylinder, is a type of domestic hot water system commonly used in many homes. This system operates based on the principle of gravity, using the natural downward force to provide hot water around the house.


Here’s how it works: 

The open-vented cylinder system includes a hot water storage tank and a cold water storage cistern, usually located in the top loft space of a home. The cold water cistern is placed higher than the hot water cylinder, allowing the cold water to descend into the cylinder by the force of gravity. Once in the cylinder, the water is heated by either a boiler or an electric immersion heater.

The hot water cylinder and cold water cistern are connected by a vent pipe, which equalizes pressure in the system for heating and hot water and allows steam to escape in case of overheating. This is why it’s called an “open-vented” system. The vent pipe also serves as a safety feature, as it prevents the system from becoming pressurized.

The heated water in the cylinder is then supplied to the tank in the loft via taps and outlets when needed. Because the system relies on gravity, the water pressure might be lower compared to other systems, such as unvented cylinders or combination boilers, especially in a property with more than one story. This can be remedied with the installation of a pump.

Despite some limitations, open-vented cylinders are often favoured due to their reliability, simplicity, and lower cost. They are less prone to problems related to high pressure and do not require the same level of safety checks as unvented systems. They are also easier to install and maintain, making them a preferred choice for many households in North London.

What is an unvented cylinder?

An unvented cylinder is another type of domestic hot water system that operates without a cold water storage tank and provides a means to supply hot water at the main pressure throughout a property.


In an unvented cylinder system, cold water is supplied directly from the mains water supply, providing hot water demands a significantly higher flow rate than open-vented systems, which are gravity-fed. The cold water enters the unvented cylinder and is then heated by a boiler or an electric immersion heater. This is very useful in larger properties or buildings with multiple bathrooms where there’s a demand for hot water from multiple points simultaneously.

The hot water is stored under high pressure in the unvented cylinder. Because of the high pressure and the potential dangers associated with it, unvented cylinders come with multiple safety features. These include a pressure relief valve and a temperature and pressure relief valve, which prevent the system from becoming dangerously pressurized or overheated. If the cylinder becomes too hot or pressurized, these safety valves will discharge stored hot water out to a safe place, typically a drain.

One key advantage of unvented cylinders is that they deliver hot water at mains pressure, providing better flow rates for showers, baths, and hot water taps, regardless of their location in the building. However, they are generally more complex, more expensive to install, and require regular maintenance checks by a qualified professional to ensure they are functioning safely and efficiently.

In summary, unvented cylinders provide a solution for homes and properties with higher hot water demands, offering superior water pressure and eliminating the need for a conventional boiler or cold water storage cistern. However, their increased complexity and cost mean they may not be the best solution for every situation.

Reliable Engineers

Get Your New Boiler Installed Today – Trusted Services by MML Plumbing Ltd

Looking to upgrade your heating system? Choose MML Plumbing Ltd. for professional boiler installation services. Our experienced engineers will ensure a hassle-free and efficient installation, providing you with a constant supply of hot water and cosy heating. Enjoy the convenience and space-saving benefits of a new boiler. Contact us today for a free quote!

Frequently Asked Questions

2 thoughts on “Do you need a water tank with a combi boiler?”

  1. Bonnie Costello

    I had a combi boiler (Burnham Alta 136) installed last February to serve second and third floors of my house. But it does not seem to sustain enough hot water to provide a bathtub. Is it ok to install a hot water tank for indirect?

  2. The combi boiler is designed to provide hot water to one bathroom at a time, allowing for a single person to use the shower. If you attempt to open another faucet in a different bathroom or kitchen, it may not be able to supply an adequate amount of water.

    A properly functioning combi boiler should have the capacity to fill a bathtub with hot water.

    If you have multiple bathrooms and wish to use the shower simultaneously, you will require a separate hot water cylinder to meet the demand.

    Gas and Boiler Engineer

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