What is a combi boiler

The combination boiler, or combi boiler, has become the number one choice for domestic heating in homes across Britain. Combi boilers provide both heating and hot water directly from the mains.

It's easy to take your existing boiler for granted. They generally last for a good amount of time, and the only time you need to think about them is when they have a problem. That means many homes across the UK have an older boiler that only has one function. When it comes to replacing one of these older boilers a combi boiler may well be your best option, and they've quickly become one of the most popular types of boiler across the country.

So what makes them so popular, what sets them apart, and why should you consider upgrading to a combi boiler next time you replace your boiler? At Plumbnation we're boiler experts and we're here to run you through everything you need to know about combination boilers, whether you're a first time buyer or simply replacing your old combi boiler.

What is a combi boiler and how do they work?

A combi boiler is a unit that serves two functions, hence 'combination'. It is the combination of a water heater boiler and a central heating boiler all in one box. They are designed to supply you with hot water when you need it, rather than having to use a tank and heat that separately. This makes them both energy-efficient and space efficient, as there's no need for a separate hot water tank.

With this in mind, it's worth understanding how a combi boiler works. First of all, as there is no tank, a combi boiler takes water directly from the mains water supply. This means that when you turn the hot water tap on, your combi boiler kicks in and heats up a stainless steel heat exchanger. This quickly brings your water up to temperature and supplies you with fresh water at whatever temperature you need. Some combination boilers do have a reserve tank, but these are never very big. For your heating, the water is heated based on the instructions you give, whether it's your own custom settings or a heating system that uses a room thermostat to activate.

What's the difference between a combi boiler and a normal boiler?

There are actually three types of boiler; regular, system and combination. There are two main differences between a combi and either regular or system boilers. The primary difference is the water source. Where a combi boiler simply takes its water directly from the mains, a traditional boiler has a large feed and cold water expansion tank that's generally in the attic. The bonus of taking your water directly from the mains supply is that it generally gives you much stronger water pressure.

The other difference is similar, and that's the need for a hot water storage tank. While your combi boiler will heat up water on demand, normal boilers require a hot water cylinder or tank. A hot water storage cylinder is required to both heat up and store your hot water so it's there when you need it. As a combi boiler doesn't require a separate hot water cylinder you don't have to worry about running out of hot water.

Best Combi Boilers

There's an extensive selection of combination boilers available, from a wide array of manufacturers. While the best combi boiler for you will depend on your home, finances and other individual factors, the best combi boilers on the market are:

Rank Manufacturer Combi Boiler Model Available Sizes Efficiency
1 Main Main Eco Compact 30kW 25kW, 30kW AA
2 Worcester Worcester Greenstar 30i 25kW, 30kW AA
3 Main Main Eco Compact 25kW 25kW, 30kW AA
4 Vaillant Vaillant ecoTEC Plus 832 25kW, 32kW, 35kW, 38kW AA
5 Worcester Worcester Greenstar 32CDi 28kW, 32kW, 36kW AA

Types of Combi Boilers

While gas combi boilers are some of the most common, there are a number of different types of combi boilers available. Other types of combi boilers include electric, oil, and LPG. Not sure which is right for you? We're here to help you decide.

Gas Combi Boilers

A gas combi boiler is by far the most common in the UK. It's installed more often than any other type of combi. It connects directly to the gas supply and is designed to keep your bills and maintenance costs as low as possible.

The combination of using both water mains and gas mains makes this an excellent choice and it's understandable why it's so popular around the country. A gas combi boiler burns gas to activate the heating elements of the boiler.

Electric Combi Boilers

While the other types of combi boilers burn fuel to heat water, an electric combi boiler doesn't. It uses electricity straight from the grid and, just like in a hot water cylinder, it heats an electric element. Water then passes over this element to heat.

An electric combi boiler is advantageous in situations where gas or oil are either impractical sources or too expensive to use. It also makes it a green alternative as you can run it through a green energy supplier without burning any fossil fuels. The main disadvantage is that electricity can be more expensive than gas or oil. They're also more limited on the amount of water that can be heated at once.

Oil Combi Boilers

Like a gas boiler, these boilers burn fuel to heat water. They're not as widely used as other types of combi boiler, due to a number of more restrictive problems. While you are burning a fuel, the access to that fuel is more limited. You aren't simply tapping into a mains supply.

To use an oil combi boiler you'll need to have your own oil tank. This will need to be stored outside your home, for safety reasons, and you'll have to keep it filled yourself, by ordering from an oil supplier.

LPG Combi Boilers

The last of the fuel burning combi boilers. An LPG combination boiler uses liquified petroleum gas (LPG). This in itself is a combination of gaseous hydrocarbons and it's created from natural gas & oil extraction plus oil refining.

You'll need a tank of LPG on your premises, in the same way as an oil combi, and you'll have to keep it topped up.

How to check if your boiler is a combi?

Spotting a combi boiler is pretty simple. There are a few tell-tale signs that your boiler is a combi.

Step one: check your central heating system and whether you have a water tank

This is one of the easiest ways to check. Do you have a water tank in your loft, or a large hot water cylinder? If you do, then it's highly unlikely that you have a combination boiler. Combi boilers heat their own water, so there's no need for a water tank.

Step two: check your water pressure

As a combi boiler gets its water directly from the mains supply, and there's no tank involved, they usually offer much better water pressure. Simply turn on your taps and check. Low water pressure could mean two things; first of all that you are using a tank or secondly that your water pressure is too low for a combi boiler to be installed.

Step three: check your boiler pipes

Last but not least, combination boilers can be identified at the boiler itself. Check the copper pipes at the bottom of your boiler. Count them! A combi boiler should have 5 pipes, in comparison to other boilers that often only have 3.

Advantages of combi boilers

Combi boilers are a popular choice and there are a number of advantages to installing a combi boiler vs regular boilers and system boilers. These include:

  • Get hot water instantly - Providing the property with instantaneous heating to radiators and hot water supply, combi boilers are not only convenient but highly efficient as they only produce heat when required.
  • Only produce heat when needed and save money - This also helps you save money on your heating bills. Only producing hot water when it's needed means you aren't using excess energy that's just going to waste on heating water and then keeping it hot.
  • Keep installation costs low - Combination boilers can also save money through the installation process as it is not necessary to install a tank in the roof space, meaning less piping and a speedier fitting. It also means you haven't got to keep those tanks maintained.
  • Compact size saves valuable room - The boiler itself comes in a compact size, making them ideal for homes where space is at a premium. The unit will conveniently fit within a standard sized kitchen cupboard, which is a highly appealing feature, especially with the compact ranges on offer. Plus, there's no need for a large water tank or hot water cylinder, so you won't lose valuable storage space in your loft.
  • A choice of fuel types - There's a whole range of different fuel options, so even if you depend on a stored fuel source away from the mains gas supply you can opt for an oil or LPG combi boiler.
  • Optimised efficiency rating - No matter what sized combi boiler you choose, you can be assured that the efficiency rating is an optimal rating. New combination boilers are manufactured to rate highly for both ErP and SEDBUK. Designed to improve the efficiency of heating and hot water products, ErP and SEDBUK measure performance and then rate them. The ratings then allow for easy comparison of boilers between brands such as Worcester, Vaillant and Ideal.

combi boiler 1

Disadvantages of combi boilers

While we're big advocates of combi boilers, they may not always be the right choice for your home. A few disadvantages of combi boilers that you should keep in mind are:

  • Work best with good water pressure - If your home has poor water pressure, it's unlikely a combi boiler will be able to function properly. This means a number of older properties may struggle.
  • Work best where demand is moderate - Combi boilers work particularly well in households with a moderate demand for hot water. Combination boilers are not suitable for homes which exceed one bathroom as the higher demand will negatively affect performance.
  • Only suitable for houses where few outputs are expected - A combi boiler is also unsuitable for homes with multi-jet showers; this is due to an excessive demand for hot water which combi boilers cannot provide. If you have a number of bathrooms, or a pump assisted power shower a system boiler would be more suitable. Essentially, you won't be able to run two taps at once and expect hot water from both at a decent pressure.
  • Not suitable for power showers - You'll not be able to get a power shower if you use a combi boiler. They need an in-built pump that connects to a hot water cylinder, so they're incompatible with a combi.
  • Unsuitable for large households - Each of these issues combine to make combi boilers a poor choice for larger houses and households. If there are too many places that need hot water or too many people that need it at once, it's hard to make a combi boiler work. They are best in smaller flats.

What to consider when choosing a combi boiler

If you're interested in grabbing a combi boiler, then there are a few things you should consider. These include:

  • The size of your house
  • The space you have to install a boiler
  • The water pressure in your house and what it can mean for your boiler pressure
  • Your available fuel sources

There is a great range of combi boilers to suit any household size, available in a variety of heat outputs and flow rates. The correct boiler will entirely depend on the number of bedrooms and bathrooms in the property. This is determined by your gas safety registered engineer before installation.

If you're considering a new system it's a good idea to get to know the different types of boiler available before you decide on whether you should swap from a conventional boiler to a combi. Before you jump straight to replacing your boiler it could be worth checking for common boiler problems and how you can fix them. You may not even need to replace your existing boiler.