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Combi Boilers

Our complete guide to combi boilers and what we at Lacey Plumbing & Heating install will help determine what is best for you and your home.

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What is a Combi Boiler?

A Combi boiler, or Combination boiler, is a high efficiency water heater and central heating boiler together in one single compact unit.

Combi boilers heat directly from the mains when you turn on a tap, so you won’t need a hot water storage cylinder or a cold water tank in the roof space.


What size Combi Boiler do you need?

Choosing a suitable boiler size is one of the most important things to consider when getting a combi boiler.

If a boiler is too small, it won’t meet your home’s heating requirements and you may fall short of heating your radiators, hot water, taps and showers.

Combi boilers are typically available in 3 sizes of 24 to 27kw, 28 to 34kw and 35 to 42kw.

  • Most flats or smaller houses with about 10 radiators and 1 or 2 bathrooms will require a 24 – 30kw combi boiler.
  • Medium to large houses, with about 3-4 bedrooms, on average about 15 radiators and 1 or 2 bathrooms will require a 30 – 35kw combi boiler.
  • Any house with more than 20 radiators and 2 or above bathrooms requires a 35 – 45 kw combi boiler.

Where there is a higher demand of hot water, the higher the kw of the boiler is needed.

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How long should a Combi Boiler last?

A modern combi boiler should last for at least 15 years. However, there are some factors that can affect your combi boiler’s life expectancy.

These factors are:

The make and model:  If you can spend a bit more for a high quality boiler from a trusted and reputable brand, it will typically last longer.

Annual usage: The more you use your boiler, the shorter its lifespan is likely to be.

Regular services: Having regular boiler services regularly by a professional can help to extend your boiler lifespan – annual boiler services are so important. Engineers will check everything is working correctly, and repair or replace any components.

Our expert heating engineers can help you to prolong the life of your combi boiler through regular boiler servicing.

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Are Combi Boilers being phased out?

No, there are currently no plans to phase out combi boilers in existing homes.

However it is likely that from 2025, new build properties will not have combi boilers installed, and instead they will have renewable heating technology to reduce the amount of gas being used.

The government may encourage an increase for a switch to low carbon heating by pushing grants or incentives, such as the Renewable Heat Incentive, which gives a quarterly pay-out to homeowners who buy a renewable heating system.

There is also likely to be a move to hydrogen in the future. Some boiler manufacturers produce boilers that can take a percentage of hydrogen already.

If you would like to change your older boiler for a low carbon alternative combi boiler, we can assist you in finding the best boiler for your home and lifestyle.

Our expert team can also remove your old boiler, install your new one for you hassle free and provide you with a monthly new boiler payment plan to suit your boiler needs.


What size Combi Boiler do I need for a 3 bedroom house?

A house with 3 bedrooms and 1 or 2 bathrooms will typically require a 28-34kw combi boiler to be installed.

How does a Combi Boiler work?

Combi boilers work by signalling a sensor once you’ve requested hot water. This tells the boiler to burn fuel, whether it be gas, electric or oil. The heat exchanger then gets hot enough to heat the water when it flows through it.

The control valves within combi boilers work in different directions to one another, so the system will either transport hot water to your radiators or to your hot water tap.

They don’t take place at the same time, and the hot water circuit usually takes priority.

Combi boilers do not require any additional tanks or cylinders to store hot water, unlike heat-only and system boilers.


What efficiency rating does a Combi Boiler have?

Most new gas condensing combi boilers will come with an efficiency rating of up to 98%. This can decrease over time due to the boiler’s age, but on average the efficiency rating is higher than conventional systems.

Combi boilers only heat up water when it’s needed, which helps to minimise wastage.

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How does a thermostat work with a Combi Boiler?

Thermostats use sensors connected to a combi boiler to track what your boiler is doing. If your thermostat room sensor senses that your room has reached its optimum temperature, it will switch the central heating off. Likewise, if it senses the room’s temperature has dropped, it will fire up the boiler again.

Without a thermostat, your boiler would continue to pump hot water through your radiators without knowing when it should stop.

What is the difference between a Combi Boiler and a Condensing Boiler?

First of all, a combi boiler is a type of boiler, whereas a condensing boiler refers to the process a boiler utilises to increase efficiency.

All combi boilers in the UK must be condensing boilers but not all condensing boilers are combi boilers; they can be conventional boilers too.

All boilers used to be non-condensing, meaning that the waste gases and heat were released outside the home.

Condensing boilers are fitted with two internal heat exchangers to ensure that less heat is lost. This means that the boiler doesn’t have to work as hard and in turn, uses less energy.

When a boiler works to heat your home, it will produce waste flue gases which are dispersed into the atmosphere via a flue.

How much does a Combi Boiler cost?

The price of a combi boiler will depend on a few different factors, including the brand and model and the size too.

Typically, you can expect to pay anywhere between £1500 to £2,500 to have a combi boiler installed.

If you are worried about paying a large upfront cost, we can help by providing you with our new boiler finance plans. Contact us today to see if you are eligible for boiler finance and we can arrange an interest free boiler installation today.


What is involved in changing to a Combi Boiler?

There are a few steps necessary for changing a combi boiler that require specialist help.

The hot water cylinder of the old boiler needs to be removed, as well as the tanks which feed the cold water which is usually fitted in the loft.

Upgrades need to be made to the gas run, as well as the heating controls. We will also advise on the installation of a heating system filter and a condense pipe.

If you’re looking to upgrade or change your boiler, contact Lacey Plumbing & Heating today – our expert team of boiler specialists have a wealth of experience in installing all types of boiler systems.


How much does it cost to change from a water tank to a Combi Boiler?

The cost of changing and removing your water tank largely depends on where it is and how easy it is to access. Water tanks tend to be heavy and bulky, so it can be tricky to remove them, especially if they are in the loft.

To replace the water tank and existing regular boiler (conventional system) with a combi boiler, you can expect to pay around £3500, if not more, depending on the make and model of the boiler.

What are the Pros & Cons of a Combi Boiler?

Before deciding on installing a combi boiler, it’s important you are aware of the pros and cons.

Benefits of switching to a Combi Boiler


  • Combi boilers are compact in size. Compared to conventional boilers, combi boilers are very small. There is no need to store hot water, and they eliminate the need for large hot water cylinders commonly found in airing cupboards. Combi boilers also free up roof or loft space as there is no need for a cold water storage unit. Combi boilers receive their supply of cold water directly from the mains.
  • Highly efficient. Combi boilers are up to 90%+ efficient, which means they are more affordable compared to conventional boilers.
  • Fast access to hot water. There is no need to wait for water to be heated as combi boilers heat water according to demand. This also means there is no risk of running out of hot water.
  • Saves money. Since water is heated in small quantities, you end up saving money compared to traditional boilers which heat vast amounts of water for just a small period of time in use.


  • Struggle to meet a high demand for water. Since combi boilers draw all its water supply directly from mains, it can struggle to meet a high demand for water.
  • For example, if several people are using the water in different rooms, the water pressure will drastically reduce as it doesn’t have the benefit of a back-up water tank.
  • Breakdowns result in a loss of water and central heating. Combi boilers power your central heating and hot water, so if your boiler malfunctions, you won’t have access to hot water or central heating.
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Common Combi Boiler problems

Just like any other boiler, combi boilers can come with their own problems. We’ve broken down the most common problems and their solutions.

If you come across a complex boiler problem, or are struggling with one of the below issues, don’t hesitate to get in touch. We offer quality boiler repair services for those who need support.


How to bleed radiators with a Combi Boiler

If a radiator needs bleeding, it means you have to get rid of any trapped air bubbles that are produced inside your central heating system. These air bubbles trapped in the system build up over time and rise to the top of radiators which eventually affects your boiler’s performance.

Just like with bleeding radiators connected to any other boiler, you will need a radiator bleed key.

There are a few key steps you will need to follow:

  • First, turn up your combi boiler to its highest heat setting.
  • Check the radiator for cold spots. If you find a cold spot, that radiator will need bleeding.
  • Once you’ve determined which radiators need bleeding, turn off your combi boiler. Wait until the boilers are completely cool before bleeding.
  • Next, you want to set up the area which needs bleeding, with a container underneath the valve to catch the water.
  • Place your radiator bleed key and turn the valve anti-clockwise. A quarter to a half turn should be sufficient. Air will hiss as it escapes from the radiator.
  • Turn the key clockwise once water starts to trickle steadily close to the valve. A steady flow of water means that the air has been released. Repeat this process for every radiator with air bubbles.
  • Turn your heating back on and check the radiator again for any cold spots or odd sounds. Check your pressure gauge and top up the pressure if required.


What temperature should hot water be set at on a Combi Boiler?

Ideally, your hot water should be set to 60 degrees to maximise efficiency. We also recommend setting your radiator dial to about 75 degrees.

You shouldn’t need to adjust these outputs as it may affect your boiler’s efficiency. The higher the temperature is set, the quicker your boiler will be able to heat up your home. However, you may find that your heating bills increase and your boiler’s efficiency may decrease.


Why does my Combi Boiler keep Gaining or losing pressure?

Combi boiler pressure increases when the heating is on as your boiler uses gas or oil to heat the water. The pressure should stabilise again once the system is switched off and cooled down.

However, if your boiler’s water pressure remains too high, this may be caused by a few reasons. If your boiler is losing pressure or it has notably decreased, it could be caused by a leak or potentially from you bleeding your radiators.


How to turn off the hot water supply on a Combi Boiler

If you have a combi boiler, then it means the water is supplied directly. To turn off the water, you simply need to close the mains stopcock.

You can usually find the mains stopcock around the kitchen sink piping, whether that be in the cupboard under the sink or even on an adjacent pipe. Turning the main stopcock off will shut off the water to every outlet in your home.

If you are struggling with any of these issues and are unsure about how to fix them, feel free to contact us today and we will provide you with the best quality services to amend any of your boiler problems.

If you have any other issues, check out our blog post on common boiler problems and how to fix them – otherwise, give us a call.

Which Combi Boiler is best?

If you want to buy the best combi boiler, you need to ensure that it’s suitable for your home and lifestyle. We’ve picked our top four combi boilers in the list below:

Worcester Bosch Combi Boilers

Worcester Bosch is a highly prestigious brand in the boiler industry, and probably one of the most desired boilers. The availability of parts is second to none, and the company boasts excellent customer reviews and ratings.

The brand provides a number of models with different outputs, which is ideal to find the perfect boiler for your home. We are Worcester Accredited Engineers and can offer up to a 12 year manufacturers warranty on Worcester installations.


Baxi Combi Boilers

Baxi is one of the leading boiler brands in the UK. They are highly popular, widely available and each come with a great guarantee.

There are plenty of models and system types to choose from, so you’ll be sure to find a boiler that fits well into your home and with your heating needs.


Ideal Combi Boilers

Ideal combi boilers are compact, highly efficient and come in a range of sizes to suit different homes. They are also regarded as some of the most sustainable boilers on the market today.

We are Ideal Max Installers offering up to a 12 year warranty on certain Ideal boilers.


Alpha Combi boilers

Alpha boilers are a relatively new brand to the UK’s domestic boiler market, But they have quickly made a name for themselves with their affordable and high quality products.

Alpha also offers a 7 year warranty on their boilers, which matches the more premium brands.

Combi Boiler installation with Lacey Plumbing & Heating

For information or advice about combi boilers or the type of boiler best suited for your home, please feel free to contact us. Lacey Plumbing & Heating can provide you with a high quality combi boiler and offer expert installation from our Gas Safe registered heating engineers.

We are here to answer all of your boiler related queries, no matter what the issue is.  Get in touch with us today to discuss having a combi boiler installed in your home by our friendly, professional team.


What areas do we cover?

Lacey Plumbing & Heating is based in Bury, so we can cover Manchester and the surrounding areas quickly and effectively. We have great links to the city and its neighbouring towns. We cover areas such as:


We aim to get to you as soon as possible for all jobs. If you are experiencing a heating or boiler emergency, we aim to reach you that same day. If you are based elsewhere but are in need of our help, contact us and we can discuss whether our services will be available to you.

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Eton Hill Road, Radcliffe, M26 2ZS

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