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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: January 23, 2024. Updated: January 29, 2024

Combi Boiler Reliability

Combi boiler reliability is important. Especially when some boiler manufacturers have been accused of not covering warranties. Or designing boilers to fail after a certain length of time.

Reliability is not dependent on brand name, reputation, service, how many engineers the brand has, or national pride. Combi boiler reliability depends on the following factors:

  • Quality of internal components
  • Metal type and quality of the heat exchanger
  • Boiler manufacturer’s experience in central heating

Keeping those things in mind, let’s examine the most reliable combi boiler brands in the UK in 2024.

To understand condensing combi boiler reliability, you must first understand how a combi boiler works (link to full guide).

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In summary, the cross-section diagram below offers a simpler explanation of a combi boiler and its components:

combi boiler reliability

In brief, a combi boiler has two heat exchangers:

  1. Primary Heat Exchanger: For Central Heating
  2. Secondary Plate Heat Exchanger: For Hot Drinking Water

A combi boiler heats water to between 50 and 70 degrees celsius in the primary exchanger. This is then pumped around your home through pipes and radiators. This water is in a closed and pressurised loop.

When you turn on your shower or a hot water tap both heat exchangers are used. The water in the closed loop is diverted to the secondary heat exchanger after being heated to a higher temperature than for heating.

This diverting is done through a diverter valve. Next time you’re near your combi boiler, and you hear a click when you open the tap to call for hot water, that’s the diverter valve.

This hot water is then put into close, but not direct, contact with fresh cold water from your mains supply. You wouldn’t want to drink water from your radiators!

So what is condensing? Condensing is the process of extracting heat from the waste gases to help pre-heat the closed-loop water system. It allows the boiler to be more efficient. All boilers in the UK are now condensing boilers.

The diagram below explains how condensing boiler technology works:

combi boiler reliability, condensing combi boiler diagram

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Combi Boiler Reliability and Efficiency

Heat Exchanger

A primary heat exchanger to a boiler is what an engine is to a car. As we explained above, the primary heat exchanger of a combi boiler is responsible for heating your home. It’s like a furnace inside, which means continuous, extreme thermal stresses. Because it is under such high stress, it’s crucial that the primary heat exchanger is made of reliable, durable and high-quality metal. If not, it will corrode and lose its efficiency over time.

Best Metal Type for a Heat Exchanger

A stainless steel and chromium alloy is the best possible metal for a heat exchanger. Stainless steel is highly corrosion-resistant in a wide range of pH levels. With aluminium, specific chemicals and inhibitors need to be used. If not, the pH can vary corroding aluminium at a rapid rate.

If your boiler’s aluminium heat exchanger is damaged, you will void your warranty. This is because the manufacturer can cite improper use of fluids in your system. A replacement heat exchanger may cost anywhere between £400 and £1,200.

In direct comparison, stainless steel heat exchangers do not require special fluids. They are compatible with clean water.

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

combi boiler reliability, aluminium vs stainless steel heat exchanger

As you can see above aluminium starts off with a slightly higher output compared to stainless steel. But between the first 1000 to 1500 operating hours, aluminium starts to decline rapidly. This is the case no matter what chemicals you put in your system. In some extremely acidic or base environments, aluminium may corrode rapidly with catastrophic results.

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Can you Detect Heat Exchanger Corrosion?

combi boiler reliability, aluminium heat exchanger corrosion

If your aluminium heat exchanger has corroded heavily and lost efficiency, you have no way of knowing. You would have to open the casing inside the boiler. This is not recommended unless you are a Gas Safe registered heating engineer. A corroded heat exchanger will give no warnings, and make no noise.  Heating and hot water will still work like normal.

However, due to the corrosion, the boiler’s energy efficiency will decrease, and your energy bills will start going up ever so slightly. That slight increase compounds over time.

combi boiler reliability, money lost

How does Corrosion Equate to Money?

Brand new combi condensing boilers start at around 92% efficiency. The Viessmann Vitodens 200-W starts at 98% efficiency, rising to almost 100% with smart thermostats and weather compensation.

In monetary terms, 92% energy efficiency means for every £100 you spend on your energy bills, you’re losing about £8. 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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Which Combi Boiler Brands Have the Best Heat Exchanger?

A lot of people in the UK don’t even research combi boiler reliability. They either look for a cheap boiler to slap on the wall or go with an established brand name without researching if there’s anything else better.

Furthermore, many boiler brands in the UK use aluminium heat exchangers manufactured by a third party based in France. Very, very few boiler brands have their own patented designs made from the highest quality stainless steel:

  • Viessmann
  • ATAG

It is no secret that the consumer survey magazine Which? awarded “Best Buy Boiler” to three brands in the UK in 2017 and 2018, which are Viessmann, Worcester Bosch and Vaillant.

The list below is based on each brand’s highest rating in terms of combi boiler reliability.

1. Viessmann Combi Boiler Reliability

Patented Inox-Radial Stainless Steel Heat Exchanger (Manufactured in-house)

combi boiler reliability, stainless steel heat exchanger

Viessmann scored highest in terms of combi boiler reliability in Which? magazine’s “Best Buy Boiler” report for 2017 and 2018, beating both Worcester Bosch and Vaillant.

And that is largely thanks to their patented Inox-Radial Stainless Steel Heat Exchanger, manufactured exclusively in-house.

Every boiler from Viessmann’s range has its patented Inox-Radial stainless steel heat exchanger. It’s the best heat exchanger in the industry in terms of reliability, durability and energy efficiency.

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2. Worcester Bosch Combi Boiler Reliability

Composite Aluminium/Silicon Heat Exchanger

combi boiler reliability, worcester bosch heat exchanger

Market leaders in the UK and comfortably outselling every other boiler brand in this list, but they’ve got 2nd place when it comes to reliability.

With a rich history and a dependable brand name, Worcester, Bosch has built a great brand in the UK. They are especially known for their excellent after-sales service and customer support.

Worcester Bosch continues to receive Which? Best Buy awards even though they have an aluminium heat exchanger. We regularly receive calls from customers wanting to switch from Worcester Bosch to Viessmann after only a few years of use.

Reliability doesn’t mean your boiler keeps working while losing energy efficiency. It should mean your boiler works trouble-free for many years, whilst retaining high energy efficiency.

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3. Vaillant Combi Boiler Reliability

Stainless Steel Heat Exchanger on Some Models Aluminium for the rest

With Viessmann getting the highest reliability rating in the Which? report, many other boiler brands in the UK have switched to stainless steel heat exchangers, including Vaillant, Baxi, and Alpha.

While Vaillant’s entire combi boiler range doesn’t have a stainless steel heat exchanger, some models do. For example, while their ecoTEC plus combi has a stainless steel heat exchanger, the ecoFIT pure and ecoTEC exclusive Gree iQ do not.

Other Brands with High Combi Boiler Reliability

ATAG Heating

Unique Heat Exchanger

combi boiler reliability, atag heat exchanger

ATAG are a Dutch manufacturer, making waves in the UK with one of the highest efficiency boilers (at 96%) in the market right behind the Viessmann 200-W (at 98%). Like Viessmann, their heat exchanger is made of stainless steel and has a unique, patented design.

All the best condensing boilers recycle flue gases, recovering more heat. ATAG’s heat exchanger works because it recycles the flue gases for a second time, recovering even more energy. On top of that, ATAG boilers are known for their solid build quality.

The only reason they’re not a Which? Best Buy Boiler is because they are very complicated boilers, and not everyone can install or service them.

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The Which? Verdict

Which? magazine gave Worcester Bosch their “Best Buy” award alongside Viessmann and Vaillant a few years ago. Viessmann boilers had a higher reliability score and are also more energy-efficient.

As we’ve mentioned several times before, Which? magazine continues to omit certain facts about Worcester Bosch. For example, the performance of their aluminium heat exchanger versus stainless steel over years of daily use.

If you Don’t Care about Saving Money, Think about the Planet

High energy efficiency for saving money may not appeal to you. But by using more energy than needed, you increase your carbon footprint. This contributes to the planet’s already alarming and very real climate change problem.

High combi boiler reliability = higher energy efficiency for longer. With small tweaks, you can save your hard-earned money and leave a cleaner, greener planet behind for future generations.

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