What is a combi boiler? Everything you need to know

The combination boiler (or combi boiler) is the most popular type of boiler in the UK thanks to its compact size, energy efficiency and ease of use. 

Combi boilers get their name because they provide homes with both hot water and heating - combining the two primary functions of a boiler into one small appliance.

Here, we explain the advantages of a combi boiler, including tips on how you can prolong its life expectancy, increase its efficiency, and decide whether it’s the right type of boiler for your home. 

How does a combi boiler work? 

Combi boilers function by staying on standby and heating water from the main water supply whenever you need it. When you turn a hot tap on, the boiler springs into action - heating the water and sending it to your hot taps. Essentially, it gives you hot water on demand.

­More technically speaking, combi boilers heat water by burning gas in a combustion chamber. Next, these hot gases move through the heat exchanger, which heats the cold water in the boiler - providing you with the hot water you need. 

Unlike a system or standard boilers, which store the domestic water in a cylinder until it is needed, a combi boiler only heats water when you need it as you turn on a hot tap and or shower. 

The benefits of having a combi boiler

Combi boilers are found in the majority of households in the UK for several reasons. Gas Safe registered engineers and plumbers will normally recommend having a combi boiler installed for these three reasons:  

1. They’re compact: Unlike other types of boilers, combi boilers do not require much space because they don’t require external hot and cold water tanks.
2. They’re efficient: New combi boilers are normally around 95% efficient - helping you to reduce energy waste.
3. They’re cost-effective: A combi boiler doesn’t need a water storage tank like a system boiler does, so there’s less to maintain and replace. Installing a gas combi boiler can be easier and much cheaper as a result.

What type of house is a combi boiler suitable for?

A combi boiler does not store hot water, so its performance may be affected if there is a high demand for hot water. For example, you may notice the water pressure and flow rate drops if multiple taps are on at any one time.  

With this in mind, combi boilers are typically most suited to homes with 2-3 bedrooms and one bathroom - to ensure the hot water demand can be met. 

This being said, if you need a larger size combi boiler, some appliances are available that will be more suitable for bigger homes, but the same still applies about the hot water performance.

How much does a combi boiler cost to buy and install?

A combi boiler and its installation will cost upwards of £1,500, with prices increasing based on the boiler’s power, brand and guarantee length.

What's also great about combi boilers is that, by law, they must be at least 90% efficient. This means that you can minimise your carbon footprint and be confident you’re not wasting any money. Although the upfront costs may seem expensive, you’ll save a lot more money in the long run.

How to use a combi boiler efficiently

Modern combi boilers are very energy efficient, which helps keep energy bills low and maximise the energy you are paying for. To keep your combi boiler in good working order and save money on your heating bills, there are a few things you can do to ensure longevity. 

1. Don’t have the central heating on all the time

Overuse of your central heating system will put excessive demand on your boiler and increase your heating bills, ultimately decreasing its lifespan.

2. Using a smart thermostat

A smart thermostat gives you greater control of your central heating. Not only can you set specific times for your heating to turn on and off, but you can control your heating even when you’re not in the house using your smartphone. You can decide when to turn on the heating without even having to use the boiler itself.  

3. Adjusting the boiler controls

As well as controlling when your heating should turn on, you can also manage your boiler’s controls. This allows you to set your boiler at the right temperature of the water in your central heating system - the lower the temperature, the less energy the boiler will use. 

4. Use thermostatic radiator valves

Thermostatic radiator valves (TRVs) are fitted to your radiators and allow you to control how hot each radiator gets, meaning you can manage the heating on a per-room basis. So, if you’re spending the evening in the lounge, you can turn down the radiators in the kitchen and be just as warm while saving on your energy bills. 

5. Regular servicing and system water treatment

Booking a regular boiler service by a Gas Safe registered engineer will ensure the boiler is working at its best throughout its lifetime, as well as ensuring your manufacturer-backed warranty is kept valid. Also treating the system water with inhibitor chemicals will reduce sludge build up, which can prolong the boiler's life as well as improving its performance.

What is the best temperature for a combi boiler?

Ideally, you should set your combi boiler at 75°C for the radiators and at a water temperature of around 55°C. You may choose to set your combi boiler at a higher temperature if you want to heat your home quicker, but this is likely to increase your energy bills.

How long does a combi boiler last?

Combi boilers will typically last around 15 years, although this will vary depending on the make, model and how well it’s maintained over the course of its life.

Extending the lifespan of your combi boiler ultimately boils down to making sure it’s kept in good working order year-round. This involves getting it serviced annually by a Gas Safe registered engineer and fixing any issues promptly before they have a chance to escalate into more serious problems.

Even though your combi boiler’s annual maintenance might seem like an inconvenience or unnecessary expense, it’s actually mission-critical to protecting the health and energy efficiency of your home’s central heating. 

What’s more, you may void your boiler’s warranty by failing to get it serviced as per the manufacturer’s terms and conditions. This might mean any repairs end up leaving you out of pocket, as the cost of replacement parts and labour won’t be covered.

To extend your boiler’s lifespan and protect your pennies at the same time, our combi boiler experts recommend taking out a boiler cover plan. Your monthly payments will help spread the cost of your boiler’s annual service. 

Should you need a boiler repair visit outside of your annual service, you can book a home visit from just £89.

The pros and cons of combi boilers

Want the no-nonsense pros and cons of combi boilers? Don’t worry, we’ve got you covered.

Pros	Cons
Ideal for small and medium-sized homes with 1-2 bathrooms	Unsuitable for large homes with multiple bathrooms
No need for a separate water cylinder or expansion tank	May struggle to cope with high hot water demands or multiple hot water outlets
A cost-effective home heating solution	Don’t work with power showers
Installation is usually relatively straightforward	Water pressure depends on your area’s mains water pressure
You get instantaneous hot water

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

In fact, a combi boiler is a type of condensing boiler because it uses condensing technology. There are also condensing standard boilers and condensing system boilers.

The term ‘condensing boiler’ refers to the system a boiler uses to recover the heat from the flue gases, which would otherwise be wasted. Under legislation passed in 2005, all boilers are required to extract much more energy from the flue gas to maximise efficiency and minimise wastage - essentially, every boiler made since 2005 has been a condensing boiler. 

Now, combination boilers are the go-to system for homes in the UK. This is due to their efficiency, cost-effectiveness and their compact size. If you live in a larger home, however, a standard or system boiler may be more appropriate as they are better equipped to handle a higher demand for hot water and heating.

Are you looking for a new combi boiler?

In the market for a new boiler? Now you have everything you need to know about combi boilers, take the first step to buying one by starting a quote for a new boiler. 

When it comes to searching for a new boiler, you want to be sure you’ve bought the right appliance - one that will produce heat and hot water without breaking the bank. To help you make an informed decision, BOXT has a library of guides with expert tips and insights to shed light on all things boilers. Check out our dedicated boiler guides here.