If you currently have a conventional boiler, it’s likely that you have thought about how a more modern, more efficient combi boiler could benefit your household. 

A lot of larger homes require a conventional boiler, but the average UK home can easily be powered by a combi boiler, providing cost-effective  instant hot water.. Learn more about whether you should convert from a conventional boiler to a combi boiler in our detailed guide.

What is a combi boiler?

A combination or ‘combi’ boiler acts as both a water heater and a central heating system in one compact unit. The water you use around your home is heated directly from the mains when a tap is turned on, without the need for a bulky water storage tank.

A combi boiler is a great space saver and fits well into smaller homes or in homes with no additional storage space for a water cylinder. The water is also powered through the mains, so you can have high water pressure in important areas such as the shower.

A combi boiler can be either gas or electric, although the most common combi boiler uses either natural gas or LPG.

What is a conventional boiler?

A conventional boiler, also known as a regular boiler, has a separate hot water cylinder and cold water storage tank. The cylinder system will provide hot water for the central heating system and the boiler will be fed cold water from the tank to heat for daily use in taps and showers.

The cold water storage tank is usually placed as high up as possible to help increase the water pressure, so is often found in the attic. The hot water storage cylinder is usually stored in an airing cupboard. 

If a lot of hot water is used in a home, often when the home is on the larger side, a conventional boiler usually works best. In this case, you need to consider the time taken to reheat your hot water cylinder.

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

The main difference between a combi boiler and a conventional boiler is that a combi boiler provides both heat and hot water through one unit and a conventional boiler has a separate hot water cylinder and cold water storage tank.

With a combi boiler, you have access to unlimited hot water and heating but with a conventional boiler, you’d need to wait for water to re-heat once you use it all. A regular boiler will be able to deal with high water usage better than a combi boiler would and is usually better for properties with more than two bathrooms or where there is low water pressure.

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Can you replace a conventional boiler with a combi boiler?

Yes, you can replace a conventional boiler with a combi boiler. Often, homeowners want to modernise their heating systems to save space and costs and increase efficiency.

What is involved in a combi boiler upgrade?

The process of an upgrade to a combi boiler can vary hugely depending on your current heating system. Factors involved in this process include:

  • The removal of the old hot water cylinder

  • The removal of the old cold water storage tank

  • Draining the water from the old system and pipework

  • Removing the old boiler

  • Upgrading the gas run

  • Converting the full system to a combi system

  • Installing the new combi boiler

  • Configuring the new thermostat controls where applicable

Options that may alter the process include:

  • Whether the boiler is being moved to a new location

  • Whether a new hole for the flue needs to be drilled

  • Whether new heating controls need to be installed

Is it a big job to change a conventional boiler to a combi boiler?

In general, changing a conventional boiler to a combi boiler takes around 2 days as it’s a more complex heating job and requires a lot of expert installation, intricate structural pipework changes and the removal of large equipment.

Despite the fact that this process is a relatively large one when compared to other heating jobs, it’s a process that we are very familiar with and are well equipped to do in a timely, efficient manner. We regularly carry out system conversions.  

Should I convert to a combi boiler?

There are a few factors that you should consider when thinking about upgrading your heating system. One of the main things to take into consideration is the size of your home. Combi boilers come in a range of sizes and capacities to suit most average homes with 1-2 bathrooms. 

Even if you have more bathrooms, use a lot of hot water or have a lot of radiators, there is still likely to be a suitable combi boiler for you. Combi boilers in general are more efficient, cost-saving and space-saving, so it is well worth seriously considering the upgrade.


Let’s take a look at some of the pros and cons of both the combi boiler and the conventional boiler.

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Pros and cons of a combi boiler

For the average small to medium sized home, the combi boiler is a great option.

  • Efficient: Combi boilers provide hot water on demand as opposed to heating then keeping a cylinder of water constantly hot, making it far more efficient.

  • Energy-saving: Having hot water on demand means that less energy is used to heat that small amount each time.

  • Space-saving: Combi boilers consist of just one unit - the standard sized boiler can fit into most sized kitchens and can be concealed behind a cupboard.

  • Cost-effective: Heating water on demand uses less energy and therefore, helps to keep costs down.

  • Easier to Repair: Spare parts are often easier to source for more modern combi boilers, making boiler repair easier and quicker.

A combi boiler however:

  • Isn’t suitable for larger homes that use a significant amount of hot water;

  • Isn’t suitable for homes that have weak mains water pressure.

Pros and cons of a conventional boiler

For the larger home, or home that has a significantly high demand for hot water and heating, a conventional boiler will work more effectively.

  • Meets high demand: The conventional boiler can supply multiple areas of the home with hot water simultaneously, keeping up with high demand for hot water.

  • Well suited to larger homes: If a larger home already has a conventional boiler system installed, it is sometimes best to keep this system in place rather than replacing it. This is because the system is better suited to the size of the home and will be easier and cheaper to maintain.

A conventional boiler however:

  • Requires a lot of storage and installation space;

  • Will mean that you may have to wait for water to heat before having access to it (if you use up all the heated water in the cylinder and still require more);

  • Requires a well insulated space to keep water hot, otherwise heat will be lost.

  • There are a lot more components and controls to maintain and a leak from the tank can cause significant damage.

Costs of a combi boiler vs a conventional boiler

The cost to convert from a conventional to combi would be around £3000 to £4000 - this is when you’re installing a reputable brand of boiler, such as Worcester Bosch.

A good brand of boiler will come with a 10 to 12 year warranty. Although the cost is front-heavy, you will reap the benefits in the long run with a more efficient heating system and boiler which will lower energy bills over time.

Convert a conventional boiler to a combi boiler with Lacey Plumbing & Heating

If you currently have a conventional boiler and are considering upgrading to a combi boiler, getting in touch with our team of expert heating engineers is a great option. We can chat through your requirements, the size of your home and your hot water usage to figure out whether a combi boiler system would work for you.

We can also help you to decide which combi boiler is best for your home by understanding what your priorities are, such as reducing costs or saving space, and using these to ensure that we get the right combi boiler system installed.

We are experts at conventional to combi boiler conversions (which we sometimes refer to as system upgrades). We especially love larger systems and complex heating problems, so please get in touch even if you have a large or unusual system.

Get in touch with us today to discuss your current boiler setup and how you can upgrade this to save costs and boost efficiency.