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What Is A Combi Boiler? All You Need To Know

Have you found yourself in the unfortunate (but sadly inevitable) situation of needing to replace your boiler? If so, why not consider a combi boiler?

Perhaps your boiler is well past its prime and starting to act up, or maybe you find yourself splashing out buckets of cash every month on energy bills due to a decline in efficiency.

Either way, a combi boiler is a great option for many homes - hence it is the most popular boiler type around. Believe it or not, combi boilers actually account for over half of all boiler installations across the UK.

But, what is a combi boiler? They’re simply compact, cost-effective, energy-efficient, and generally super convenient.

To help you understand what you could be missing out on by having a combi boiler in your home, we’ll tell you everything you need to know before you make any drastic boiler decisions.


How does a combi boiler work?

Combi boilers are actually pretty nifty, considering how compact and unimpressive they appear from the outside! They get their ‘combi’ name from the fact that they combine a highly-efficient water heater and a central heating boiler - all in one.

These boilers function by staying on standby and heating water from the mains, as and when you need it. When a hot tap or shower is turned on, a combi boiler will boot up and heat your water (almost) instantly - before sending it off to the relevant tap or radiator.

Combi boilers essentially do two things at once - heating your water and your home. So, you can have as many hot baths as your heart desires, whilst keeping your home snug too.

So, what is the best combi boiler for your home?


The typical boiler to be found in most homes across England is the gas combi boiler, which is the one to go for if you’re connected to the national grid - most UK homes are. Not only are gas combi boilers completely hassle-free when it comes to installation and storage, but they are also definitely the cheapest option.

Unluckily for some though, not everyone has access to the grid. If you live in a more rural area or frankly in the middle of nowhere, you can still have a combi boiler installed, but it will be a different type. Depending on your budget and the amount of space you have around your home, you could go for either an oil or Liquified Petroleum Gas (LPG) combi boiler, both which require a large storage tank to work alongside and a regular supply of fuel.

Don’t worry though, if you live in a small home and can’t afford to maintain the supply of oil or LPG and don’t have access to the grid, there is still hope. You can opt for an electric combi boiler instead. These work by passing cold water across an electric current to keep it warm enough for your taps and radiators.

What size combi boiler do I need?

The size of combi boiler needed will, of course, be determined by a number of components. Things to consider are the age of your property, the levels of insulation and the number of bedrooms and bathrooms.

For example, if you live in a well insulated new-build home with only one bedroom and bathroom, you would be best suited to an 18kW combi boiler. In the opposite extreme, if you live in an old house with little insulation and over five bedrooms and three bathrooms, a larger 40kW would be necessary.

As you can see, your combi boiler size is completely relative and there is a wide range to choose from. It’s easy to figure out which size/type of boiler your home would require using our simple Find a Boiler service - it just takes 2 minutes to be matched with your best boiler type.

How much is a combi boiler?

Now that you know exactly what a combi boiler is, you’ll probably want to know prices. How much is a combi boiler? As a guide, an A-rated new combi boiler replacement along with the initial installation can set you back around £2000 to £2500 minimum. At BOXT, we offer completely competitive prices. We can offer you the exact same boiler from £1,795 including installation.

You can benefit from BOXT’s highly competitive boiler and installation prices, as well as our quick and high-quality services. If your boiler has recently let you down and you need a replacement asap, our boiler experts can help with their convenient next day installation!

You’ll be pleased to know that there is actually little equipment or effort required, compared to larger boilers that require hefty storage tanks. You will only need a small storage space like a kitchen or airing cupboard, where pipes can connect the system to the required rooms.

What’s also great about combi boilers is that, by law, they must be at least 92% efficient, if not more. This means that you can minimise your carbon footprint and have peace of mind that your bill money isn’t going straight down the drain. The initial price is completely evened out by the money you can save in the long run.

Wondering how to use a combi boiler efficiently? 

What temperature should my combi boiler be set at for central heating?

For optimum efficiency, setting room temperatures to range from around 18 to 21 degrees Celsius is your best bet. For your radiators specifically, there’s no need to waste energy by having them all on full blast. Think logically, which rooms do you use the most? These should be turned up the highest. Focus most of your heat energy on the main living areas to avoid any going to waste.

We recommend using your thermostat timer to your utmost advantage too and setting specific times of the day for your heating to turn on and off. You can save a significant amount of money each month by getting your timing just right.

With the evolution of smart home technologies, you can now control your combi boiler from your mobile. If you’re out and about, you can turn the heating on via an app - so your home is nice and warm for when you get back.

How long does a combi boiler last?

The general expected lifespan of a well-made combi boiler is around fifteen years, but this life expectancy will vary depending on the level of care dedicated to it.

Firstly, you should make sure to invest initially in a high-quality boiler from a reputable brand, to avoid any problems presenting themselves earlier than they should. It is tempting to go for the cheapest combi boiler out there, but it will come back and haunt you when it starts to let you down.

On top of this, your boiler’s health will be determined by how well you look after it. There are certain things to keep in mind for ensuring your boiler has a long life, like keeping an eye on your pipes, pressure levels, and paying out for regular boiler servicing. Whilst it’s great to know how to look after a boiler yourself, having an engineer check it over once a year means any hidden faults can be identified and fixed before bigger boiler problems arise.

So, when considering how to use a combi boiler efficiently, taking good care of it is definitely the place to start.

How to care for your combi boiler

Check your pressure

If you invest in a quality combi boiler, it shouldn’t be too high maintenance. There are certain things to take into account with all boiler types though, for avoiding any long term issues.

One of these issues is boiler pressure, which can fluctuate for various reasons and prevent your boiler from behaving as it should. Luckily, boiler pressure is easy enough to maintain, so learning how to repressurise a combi boiler isn’t as scary as it sounds!

Has your combi boiler pressure dropped too low? Take a look at your pressure gauge, if it displays less than one bar and you haven’t noticed any leaks, it is worth re-pressurising. This is easily done using a filling loop, or keyless filling loop, to allow water to fill your boiler and boost your pressure back to normal. If you’re unsure which filling loop your boiler uses, check your boiler manual.

Take a look at our extensive guide on boiler pressure to make sure you keep your boiler in check!

What is the difference between a combi boiler and a condensing boiler?

Condensing boilers are simply those that have internal heat exchangers to minimise the loss of heat energy into the air. Less energy lost = less energy used! The less energy you use to heat your home, the less harmful CO2 emissions you are producing and the more you can reduce your carbon footprint (and save money).

You might have straight away assumed that a condensing boiler is different from a combi boiler, as many people do. Although, a combi boiler can be condensing too, and should be by law.

All it takes to turn your combi boiler into a combi condensing boiler, if it isn’t already, is the fitting of a Flue Gas Heat Recovery System, which prevents waste gases from entering the environment and instead reuses them to heat your home.

Pros and cons of installing a combi boiler 


  • Great for small homes

Although it may be hard to believe with how much they manage to do, combi boilers are pretty small things. They are an ideal option for smaller family homes or flats that don’t have the space to store water or oil tanks.

Many new build homes these days have minimal space and storage - so your combi boiler can slot right into a kitchen cupboard, no problem. This frees up plenty of living space for you/your family without having to compromise on the warmth of your home.

  • Uses your mains water pressure

Due to the fact that the standard combi boiler takes water straight from the mains to heat it, you won’t have to compromise on water pressure. That is, providing your home has a generally powerful water pressure to begin with.

If not, the fact that combi boilers use your mains water pressure could actually work against you. You can purchase water pumps to improve the speed of your water, but keep in mind that these do affect your energy bills.

  • Cost-effective

Combi boilers can save you money right now, as well as in the future. We’ve already mentioned how quick and easy the installation process is, hence you don’t have to worry about going bankrupt when your old boiler packs in - phew.

On top of the initial money, you can save having a combi boiler installed, your energy bills will no doubt improve too, as your boiler only kicks in when it’s needed.

You might sometimes see your combi boiler running when your heating and taps are turned off, but this is to keep a small amount of water warm in preparation for use and is nothing to worry about.

  • Simple  installation

With other boilers, an engineer will need access to a loft or garage to run pipework from your boiler to a storage tank. With your average combi boiler, you won’t need such a huge web of pipes spreading here there and everywhere, so there is a lot less effort involved - saving you money for an engineer’s time and materials.

  • Unlimited hot water

As a combi boiler heats water to meet your needs, you don’t have the risk of running out when you most need it. If you have a big family or family members that tend to hog a lot of hot water, a combi boiler can prevent you from being left with a cold shower to contend with. Your hot water simply cannot run out as it comes directly from the mains itself, rather than being stored.


  • May not suit larger homes

As handy as it may be to have an unlimited supply of hot water with a combi boiler, you are left heavily reliant on your mains water pressure. So, if you live in a large home (more than two bathrooms), it might not be your best option. If you’re asking the question of ‘what size combi boiler do I need’, and your home is too large, even the most powerful combi boiler wouldn’t be sufficient.

Let’s say you use three taps or showers at once in your home. Your boiler will split the water pressure across the three. As you can imagine, this doesn’t bode well and the pressure won’t be strong enough for use.

  • Can’t use hot water and heating together

Something else to keep in mind when considering ‘what is a combi boiler’ and ‘should I have one installed?’, is that your hot water and heating can’t run at the same time. Hot water cannot be supplied to your radiators and hot taps simultaneously, that’s why your central heating will pause as a tap is switched on.

This shouldn’t be a huge issue, except in the winter months - if someone decides to run a large bath, the rest of the family might be chilly for a while!

  • Heavily reliant on your boiler

The final thing to keep in mind when considering the downsides of combi boilers is that, due to them heating water on the spot, you are fully dependent on them for your hot water and warmth.

There is a small amount of hot water stored within your boiler, which is kept at a certain temperature throughout the day, but this wouldn’t go far, should your boiler fail. This might be worth considering if you live in an area that may be more prone to faults with your mains or grid; there is nothing worse than being stuck with no hot water!

What is a combi boiler? Is it the best option?

If you’ve come to the point where your boiler is causing more trouble than it’s worth, why not upgrade to a modern combi boiler?

At BOXT, we understand that the world of boilers can be overwhelming, so we decided to create a boiler matching service that takes all the confusion away. Spend less than 2 minutes answering some simple questions about your home using our ‘Find A Boiler’ tool and we’ll advise you on which boilers will suit you perfectly.

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