A Guide To Combi Boiler Systems

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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: March 28, 2019. Updated: February 8, 2024

A Guide To Combi Boiler Systems

A combi boiler heating setup does not require attic tanks or hot water cylinders. It produces hot water and powers the central heating from within the single boiler unit.

What do we mean by a Combi Boiler?

This may be confusing because there are three main types of boiler configuration:

  1. Open Vent/Conventional/Regular Boiler – Requires tanks in the attic for storing cold water, and cylinders (normally in an airing cupboard) to store hot water.
  2. System Boiler – Requires a cylinder to store hot water. Doesn’t require attic tanks.
  3. Combination (or Combi) Boiler – Does not require any additional tanks or cylinders.

Condensing or Combi Boiler?

This is a question we sometimes get, especially if someone needs to upgrade from an old back boiler.

Any modern boilers manufactured today, or since 2005 condensing boilers. This includes combi boilers. This is due to a government change to building regulations as condensing boilers are so much more efficient than non-condensing boilers.

Condensing technology is simply a way to extract more heat from exhaust gases that used to be wasted in older non-condensing boilers.

You can see how the technology works in this diagram below:

condensing combi boiler system

How a combi boiler works diagram.

Essentially, the hot gases created by burning natural gas, oil or LPG is used to heat incoming water. Essentially it allows the main natural gas burn to do less work overall.

How does a combi boiler work?

Combi is short for combination.  A combi boiler system combines central heating and hot water within the same compact unit. They house two heat exchangers:

  • A primary heat exchanger, for central heating. The primary heat exchanger heats and recycles the water that goes through the pipes into the radiators.
  • A Secondary plate heat exchanger, for hot water. The secondary plate heat exchanger heats up the hot water that goes into your taps.

For a more detailed look at combi boiler pipes read our quick guide >

What’s inside a Combi Boiler

combi boiler system, how does it work

Primary Heat Exchanger for Central Heating

The radiators in your house get warmed up by the hot water coming from the boiler, heated by the primary heat exchanger inside a combi boiler system.  Once this hot water has warmed up the radiators, it travels and recycles back into the boiler.

The boiler keeps firing to reheat that water and keeps it at your preset temperature, normally around 60 to 85 degrees Celsius.  This water can get dirty over time because it picks up dirt and debris from the radiators and pipes.

Secondary Plate Heat Exchanger for Hot Water

The water that travels through the radiators and recycles back to the combi boiler can be dirty so it’s best not to use it for hot water taps or showers.

To give you clean hot water every time you turn on a hot water tap in your house, the combi boiler system pauses your central heating (if it’s running) and then opens a diverter valve which lets the heating water enter the secondary plate heat exchanger.

This hot central heating water runs through a heat exchanger to warm up your clean water for hot water taps. This is why you might hear the combi boiler clicking when you open the taps.  It’s the diverter valve.

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Reliability of a Combi Boiler System

Heat Exchanger

The primary heat exchanger in a boiler is like an engine in a car.  The efficiency and longevity of your central heating depends on the quality of its internal components.

Your combi boiler system blasts its primary heat exchanger with flames at around 700 to 900 degrees to provide central heating to the whole house.

While your radiators get gradually warm over time, inside the boiler it’s like a furnace, i.e. constant, extreme thermal stress on the heat exchanger on a daily basis, and almost 24/7 during winters.

This is why it’s absolutely crucial that the primary heat exchanger is made of a reliable, durable metal of the highest quality, or else it will corrode and lose its efficiency over time.

Best Metal Type for a Heat Exchanger

A stainless steel and chromium alloy is extremely resistant to corrosion, which makes it the best possible metal for a heat exchanger.

A lot of boiler manufacturers, including market leaders Worcester Bosch use aluminium for their heat exchangers.  While aluminium has slightly higher thermal conductivity initially, and is lower in weight, it’s nowhere near as resistant to potentially dirty central heating water as stainless steel.

Furthermore, if special manufacturer-recommended heat transfer fluids and inhibitors are not used to produce and maintain a narrow pH range, it corrodes at a rapid rate and gets damaged.

If you damage your heat exchanger, you may void your warranty, because the manufacturer could cite improper use of fluids.

A replacement heat exchanger costs around £600-£1,200.

The graph below compares a stainless steel heat exchanger (in blue) with an aluminium heat exchanger (in red) for a combi boiler system:

combi boiler reliability, aluminium vs stainless steel heat exchanger

This graph shows that aluminium heat exchangers offer a small initial increase in efficiency but quickly degrade over time. Unlike a stainless steel heat exchanger.

Furthermore, an aluminium heat exchanger may be porous, which has often resulted in many clients complaining about aluminium heat exchangers getting blocked due to sludge, debris and dirt.  Stainless steel doesn’t have these problems.

The only advantage aluminium has over stainless steel is that it’s lighter in weight and cheaper to produce.

If you can get a combi boiler system with a stainless steel heat exchanger in the same price range as one with an aluminium heat exchanger, we would always recommend the stainless steel option.

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Is it Easy to Detect Heat Exchanger Corrosion?

combi boiler reliability, aluminium heat exchanger corrosion

You have no way of knowing if your aluminium heat exchanger has corroded unless you can open the boiler casing, which we don’t recommend. You will need a Gas Safe registered heating engineer to do this for you.

You will get no warning of corrosion on a heat exchanger.  For example, it won’t make a different noise.  Your combi boiler system will still provide central heating and hot water. But there will be a subtle rise in your energy bills over time because the boiler’s energy efficiency has gone down due to corrosion.

Now, you may say who cares about paying a few extra pence every day, week or month?  But just like the effects of good or bad habits, small things compound over time to become big things and it will only get worse over time. For example, the Viessmann Vitodens 200-W is 98% efficient when set up and looked after correctly.

How does Corrosion Equate to Money?

A new combi boiler system starts at around 90% efficiency and gets up to 98% with smart controls and weather compensation. In monetary terms, 92% energy efficiency means for every £100 you spend on your energy bills, you lose about £8 through exhaust and inefficiency.  A high 98% efficiency means, you only lose about £2 for every £100 spent on energy.

So, if your boiler drops down to 80% energy efficiency due to corrosion, you’re now losing £20 for every £100 you spend on energy.

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This is why investing in high-efficiency, high-quality boilers to begin with, saves you and your family a tonne of your hard-earned cash over many years to come.

A premium combi boiler system like the Viessmann Vitodens 200-W may cost you more initially, but not only will it pay for itself in just a few years, it will keep saving energy and your money because of almost 100% energy efficiency with the help of smart thermostats, advanced heating curves and weather compensation.

Frequently asked questions about combi boilers

Yes, depending on the make and model.

Combi boilers come in all different sizes and powers and are good for the smallest flats and homes to quite large homes – depending on the hot water requirement. These boilers provide hot water and heating on-demand to the property. They don’t require hot water cylinders and can be kept in a kitchen cupboard.

However, there are different variations available. For example, Viessmann offer a storage combi boiler which does store a bit of hot water to help with slightly higher demand.

In short; they’re great and can handle most properties.

It all depends on your hot water usage. A combi boiler doesn’t use energy to heat water that may not be used, as a system boiler does. But, a combi boiler will need to run water along pipes to your (for example) shower whilst it’s still warming up and this probably won’t be used…then when the shower is turned off the hot water will sit in the pipes from the boiler to the shower.

It all depends on your hot water usage. A combi boiler doesn’t use energy to heat water that may not be used, as a system boiler does. But, a combi boiler will need to run water along pipes to your (for example) shower whilst it’s still warming up and this probably won’t be used…then when the shower is turned off the hot water will sit in the pipes from the boiler to the shower.

No. The government have a boiler upgrade scheme that allows certain people to receive a grant of £7,500 for a new air source heat pump. But these heat pumps are not a viable option for everyone so we don’t believe natural gas will be phased out in the next 15-25 years. Plus, most boiler brands are now 20% hydrogen ready which will help reduce reliance on gas.

The most popular combi boilers in the UK are gas-burning. Electric combi boilers are available, but they’re very expensive and most require an upgraded 3-phase electrical supply into your home.

Yes, if it’s powerful enough and you have good enough cold water flow rate to the boiler. If you don’t have good cold water flow you could consider a storage combi boiler. These are a mix of combi and system boilers. You can read more about storage combi boilers here >

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