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Types of boilers explained – combi, heat only & system boilers

types of boilers

You might think you’d have to remember several types of boilers but actually…there are only 3 you need to know:

  1. Regular or heat only boilers
  2. Combi boilers
  3. System boilers

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What type of boiler do i need?

Right, if you have gas in your home like most homes in the UK then our guide can help you out. All three are classified as condensing boilers. Every UK household should have a condensing boiler installed due to them being much more efficient!

But which among the three is the most ideal for your home? Let’s compare boilers now. At a glance take a look at our table, it may answer your question right away!

Type of BoilerCondensingHot Water TankCold Water Tank
Gas Combi BoilerYesNoNo
Gas System BoilerYesYesNo
Gas Regular BoilerYesYesYes

Your current heating system age, water pressure, number of bathrooms, house size, and hot water demands all contribute to the best type of boiler for your home. Let’s dive in!

Regular (Heat Only) Boilers

Regular boilers (also known as conventional boilers) are essentially made of three components:

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  1. Hot water cylinder/storage tank
  2. Cold water storage tank
  3. The boiler itself

A conventional boiler system will have the hot water stored in the hot water cylinder, where you’ll also have access to an on/off switch so you can turn your heat only boiler on when in use and off when not in use.

But how do heat-only boilers work?

How Does a Regular/Heat Only Boiler Work?

  1. Cold water is supplied to your heat-only boiler through the cold water tank (Your cold water tank is usually installed in the loft/attic).
  2. Your heat-only boiler then directly feeds hot water to your central heating system such as your radiators.
  3. Hello, hot water!

This type of system for a regular boiler type is also called an open vented heat system like a Feed and Expansion tank.

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  • A regular boiler is most ideal for a house with old radiators. Older radiators that cannot tolerate high water pressure are ideally suited to regular boilers. Unlike combi or system boilers which would potentially leak if there was high water pressure
  • Cheapest option among all the three types
  • Has the lowest replacement costs


  • You’ll need extra space to install a cold water feed tank either in your loft or attic
  • If your hot water storage tank runs out of water, you’ll have to wait for the water to heat up again.
  • Not as efficient

A Regular/Heat Only Boiler is Best For:

A conventional boiler has its ups and downs. Still, they’re perfect for certain households, specifically:

  1. If you have high hot water demands
  2. You have multiple bathrooms or live in a large household
  3. If your area suffers from low water pressure
  4. If you have an old heat system that can no longer tolerate high water pressure

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Combi Boilers

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Combi boilers are the smallest among all three. They’re also the most common.

But that doesn’t mean they’re an ideal choice for you. With a combi boiler, both your heating and hot water demand are supplied via a single heating unit.

Allow us to explain further below.

How Does a Combi Boiler Work?

A Combi boiler is typically hanged on a wall, giving it a neater, space-saving look:

Here’s how a combi boiler works:

  1. Combi boilers use a heat exchanger (metallic) to heat water on-demand, meaning…no waiting time
  2. Your heat exchanger burns fuel such as gas or oil to achieve combustion and produce hot water.

Unlike system and regular types, gas combi boilers don’t require a tank. A combi boiler doesn’t carry the definition of storing hot water in its book. Instead, combi boilers heat water directly from the cold mains.


  • Compact, neat-looking, and saves a lot of space
  • Worry less about boiler installation since it doesn’t require a hot water cylinder nor a coldwater tank
  • Instantly heats the water so you don’t have to wait
  • The most energy-efficient boiler among all three
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  • Although energy-efficient, combi types become less efficient and effective when installed in a larger home
  • If you have low mains water pressure, it won’t work as well as you expect it to
  • If two or more outlets are used simultaneously, your water flow rate is reduced. For example, showering and washing your face in the sink simultaneously will only lead to at least one having hot water

A combi boiler is best for:

A combi boiler can definitely help you save on heating bills, but it can also increase it if you’re using this boiler system for a home that shouldn’t have it in the first place.

A combination boiler is best for:

  1. Smaller households or areas with limited space
  2. If you have fewer bathrooms
  3. Ideal for areas with good water flow rates and pressure

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System Boilers

A system boiler is an upgraded version of heat-only boilers and without the external hot water system components, making it less bulky.

How do system boilers work? Let’s find out!

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How Does a System Boiler Work?

The first thing you should know about a system boiler is they have a separate hot water cylinder. You also noticed how heat-only types have a hot water storage cylinder.

The difference between a system and heat-only boiler types, in this case, is that system boilers are unvented boiler systems.

This means a system boiler is less bulky.

To explain further, a system boiler has fewer external components, meaning:

  • No hot water tank installed in your loft/attic.
  • Pumps and valves are integrated into a system boiler

The way a system boiler works is similar to a heat-only type of boiler.

Although a heat-only type of boiler requires an expansion tank, a system boiler uses an expansion vessel where water is supplied to your boiler via the mains.


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  • The internal components integrated into its design makes it easier to install while saving space
  • Since it doesn’t require extra components in the attic, this makes it less prone to freezing or potential leaks during winter
  • Great for areas with high hot water demands since its unvented boiler system allows it to deliver good water rates to multiple outlets


  • You still need to allocate some space for the cylinder
  • More expensive than the two types of boiler: combi and heat only
  • Just like traditional boilers, you’re not exactly getting instant hot water. Once the cylinder runs out of hot water, you’ll have to wait for it to heat up so you can get hot tap again

A system boiler is best for:

Here’s who would benefit the most:

  1. If you have multiple bathrooms or hot water outlets
  2. You’re looking to replace your heat only boiler and want an upgraded version
  3. You prefer minimal hassle and struggle during the installation process
  4. You live in an area with lower mains pressure

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Frequently Asked Questions (FAQs)

Still not sure which new boiler type you should get? We’ll answer popularly asked questions below!

Why do i have to install a condensing boiler?

A condensing gas boiler is required in all UK households by law. At the same time, here are some benefits worth noting:

  • A condensing gas boiler helps lower your energy bills and carbon footprint
  • Are energy-efficient boilers – 90% energy efficiency when it comes to heating water

What size boiler is best for my home?

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Typically, your boiler installer will recommend the appropriate size depending on your hot water demands and the number of radiators.

Most use a boiler size calculator such as this one here.

This also depends on the kWh or power of your boiler. The higher the kWh, the greater the capacity of your new boiler.

Generally, a smaller house will need a small-sized combi boiler while a larger house will need a medium-to-large sized new boiler replacement.

What boiler should I get if I am expanding my property?

If you’re considering adding more bathrooms and expand your total space, we suggest going for a system boiler, granted you have good water pressure.

If your pipes are buried deep with no hope of digging out or you have old radiators installed, go for heat only types.

Boiler types: Our conclusion

We really hope tho article has helped you work out which type of boiler your home requires. If you are still unsure you can speak to one of our boiler installation experts online or over the phone.

If you have any questions relating to installation and work to be carried out we can also help! If you think you know, then you can also use our simple boiler comparison tool and buy a boiler online and get it installed the next day!

If you can’t afford to buy a new boiler outright, we are also here to help. You can spread the cost with boiler finance and pay nothing upfront. Simply choose this option at checkout. Happy boiler buying!

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