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Should You Change Your Conventional Boiler for a Combi?

Before thinking about brand and model of boiler, it’s important to compare the pros and cons of Combi boilers and conventional boilers to find the right type of boiler for your home.

As well as these popular choices of boiler type, there are also System boilers, which sit somewhere in between Combi and Conventional boilers in terms of how they operate.

Before we begin comparing the pros and cons of these boilers, Conventional boilers come under many names, including Regular, Heat-Only and Traditional, in addition to Conventional.

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What is a Combi Boiler?

Combi, or Combination, boilers are the most modern type of unit available on the market. Everything the unit needs to heat your home and hot water is within the unit itself, eliminating the need for any tanks or cylinders elsewhere around your home.

Combi boilers provide hot water on demand, as and when you need it, using the mains water supply. Meaning that the unit is able to provide a strong pressure out of taps and showers.

Combi Boilers: Pros & Cons

Best suited to smaller homes where the supply for hot water isn’t so high, Combi boilers are highly efficient units that won’t take up much space.

Combi Boiler Pros
Single compact unit There’s no need for any water tanks or cylinders as everything a Combi needs to heat your home and domestic hot water is within the unit itself.
Simple and quick installation Combi boilers are much simpler to install than other types of boiler and can be fitted just about anywhere.
Highly efficient Since 2018, all new Combi boilers installed across the UK need to have an efficiency rating of at least 92%. Helping to reduce your carbon footprint and your energy bills.
Lower energy bills A minimum efficiency of 92% means that only 8% of the fuel used by the boiler is wasted. So, for every £1 spent on heating your home, only 8p is wasted. A significant saving when compared to the efficiency ratings of older boilers.
Hot water on demand Conventional and System boilers use extra energy keeping the hot water in the cylinder warm, until it’s needed. This also means that you have to wait for the cylinder to fill up.

Combi boilers take water directly from the mains, so it’s there when you need it.

Strong flow of hot water By taking its water supply directly from the main, the flow of water will be much stronger than if it’s being supplied by a tank in the loft.

While there are many benefits to installing a Combi boiler, especially lower energy bills, it’s important to consider the possible downsides too.

Combi Boiler Cons
Unsuitable for larger homes If you live in a large home with more than one bathroom then it’s worth considering a Conventional or System boiler as a Combi won’t be able to supply hot water to several outputs at any one time.
Not the best choice for homes with weak mains pressure Combi boilers take their water supply from the mains so if it’s weak, the water out of your showers and taps will be too.
Might not be compatible with old pipework Combi boilers are a more recent addition to the market than the other boiler types so if you’re replacing an old heating system with a Combi then you might see the installation costs rise.

Looking at the Combi boiler pros and cons, you should consider one if you live in a smaller home with 1 bathroom. If your home has more than 1 bathroom then you’ll want to look into installing a Conventional or System boiler.

What is a Conventional Boiler?

Conventional boilers are the oldest type of boiler and you might think that they’ve been left behind by the more modern Combi and System boilers but they’ve really improved over the years to be more efficient and reliable than ever.

Conventional boilers are made up of several parts:

  • A boiler
  • Heating controls
  • Hot water cylinder
  • Cold water storage tank
  • Feed and expansion cistern

Water from the mains fills the cold water tanks, located in the loft, which is then heated up in the cylinder. The cylinder is then full of hot water (the amount will depend on the size of the cylinder) ready to be sent to taps and showers when needed.

The feed and expansion cistern is there to ensure that there’s the correct amount of water in the heating system. This is very important because water expands when it heats up.

Conventional Boilers: Pros & Cons

For anyone living in a larger property, with several bathrooms, you should be considering a Conventional boiler as their benefits will really help you to get the most out of your heating system.

Conventional Boiler Pros
Meet high demands for heating and hot water Anyone living in a large home with several bathrooms and a high demand for heating and hot water, should consider a Conventional boiler.

They’re able to supply water to several places all at once as the hot water is stored in a cylinder rather than coming straight from the mains (Combi).

Increased efficiency over older models When Conventional boilers were first introduced to the market, they weren’t very efficient but it’s a different story now. So, if your current Conventional boiler was installed several years ago, you should consider replacing it with a newer model to heat your home more efficiently.
Hot water back-up with an immersion heater In the event of a boiler breakdown, it’s good to have a back-up so that you’re not left without hot water. Fitting an immersion heater to the cylinder will provide an alternative way of heating water.
Heat water using solar energy With a solar thermal system, warm water can be generated using renewable energy from the sun – helping to reduce your energy bills.

For homes with more than 1 bathroom, a Conventional boiler is the ideal choice as it will be able to meet the higher demand for hot water. As ever, there are potential downsides to consider.

Conventional Boiler Cons
No instant access to hot water Conventional boilers heat water and store it in a cylinder, ready for use when it’s needed. This means that you might have to wait for the cylinder to fill up before hot water will be sent to the taps or showers.
Heat loss With the water being stored in a cylinder there’s the possibility of heat loss. To keep the water hot for as long as possible, the cylinder will need to be well insulated.
Can be expensive to install Installing a Conventional boiler can be a long and expensive process as various parts (expansion tank and cylinder) need to be installed around the property.

What is a System Boiler?

As we mentioned earlier in the article, there’s a third type of boiler that might be better suited to your home than a Combi or Conventional boiler: the System boiler.

In terms of how they operate, System boilers are somewhere between Combi boilers and conventional boilers.

Most of the components are within the unit itself but System boilers still need a cylinder to store hot water. The difference, when compared to a Conventional boiler, is that System boilers take their supply of water directly from the mains, removing the need to have a tank installed in the loft.

System Boilers: Pros & Cons

The pros of installing a System boiler include a combination of the benefits you could expect to see from a Combi or Conventional boiler.

System Boiler Pros
Simple installation Thanks to the compact design of a System boiler, which means that everything is within the unit itself, the installation is much easier compared to conventional boilers – which will help to save you time and money.
Can meet high hot water demand By storing hot water in a cylinder, System boilers can provide water to several taps or showers at the same time.
Strong water pressure Taking its water supply from the mains, instead of a tank, means that the water pressure is much stronger than that supplied by a Conventional boiler.
Solar thermal compatibility The hot water cylinder can be made compatible with a solar thermal system, so the water is heated using free renewable energy from the sun.

The cons of installing a System boiler are similar to those of a Conventional boiler, as they both need more space around the property, although Systems don’t need quite so much, and heat could be lost from the water as it sits in the cylinder.

System Boiler Cons
Need more space than a Combi System boilers might not need the cold water feed tank that Conventional boilers do but they still need a hot water cylinder.
Hot water isn’t instant In the same way that Conventional boilers don’t supply instant hot water, neither do System boilers. Once the water has been heated, it’s stored in a hot water cylinder. Which means that you’ll need to wait for the cylinder to fill up before you have access to hot water.
Hot water cylinder heat loss As the hot water is stored in a cylinder, it will gradually get cooler. To combat this heat loss, it’s important to ensure that the cylinder is well insulated.

Should You Change Your Conventional Boiler for a Combi?

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Planning to replace your Conventional boiler with a Combi could result in an expensive installation. This is because the heating controls, hot water cylinder, feed and expansion cistern and cold water tank will all need to be removed and then the heating system needs to be made to suit a Combi boiler.

If your property has 2 or more bathrooms and a high demand for heating and hot water, then you will probably be better off arranging a like-for-like boiler replacement.

Should you not need so much hot water, then a Combi boiler is a highly efficient replacement. However, it’s important to check that the water pressure in your property is enough to deliver a strong supply of hot water out of taps and showers. If your home has a weak mains supply then a pump can be fitted to increase the water pressure.

Combi, Conventional or System?

Now that you’ve seen the pros and cons of Conventional and Combi boilers, as well as what a System boiler can offer, you’re a step closer to finding the right type of boiler for your home.

When looking at specific models of boiler, there are a few important things to look out for:


Efficiency shows how efficiently the boiler operates, the higher the efficiency rating, the more efficient the unit is and the cheaper your energy bills will be.


Output, measured in kilowatts (kW), shows how powerful the boiler is, but you shouldn’t just go for the most powerful unit available, you want an output that’s best suited to your home and a fully-qualified heating engineer will be able to help.


A warranty period is a promise from the manufacturer that you’re covered should anything go wrong with the boiler after installation. It’s important to consider the length, they tend to vary from 1-10 years, and what’s covered.


Finally, the most important consideration for most, is how much the unit will cost. We’ve broken down the potential costs of each type of boiler in the table below, before installation. The total you end up paying will depend on the brand and model.

Type of Boiler
Potential Cost
£500 – £1,500
£550 – £4,500
£800 – £2,500

Get Boiler Replacement Quotes

Whether you decide to have a Combi, Conventional or System boiler installed, you’ll need to find a heating engineer to install it. Sourcing multiple quotes that you can compare will help you find the best possible deal. You can get up to 3 free quotes from local installers using Boiler Guide.

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