Condensing Boilers – The Good, The Bad, and the Ugly

Since arriving on the domestic UK market more than 20 years ago, condensing boilers have proven themselves as the most highly efficient boilers available, offering much lower running and fuel costs than non-condensing types.

When they were first introduced onto the market, because of a large number of myths and misconceptions about their reliability, they weren’t that popular.

Unfortunately their popularity suffered because of early teething problems that condensing boilers suffered from. However 20-odd years on, these have been fully resolved.

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Two decade ago, condensing boilers suffered from a reputation of being over-complicated, and therefore less reliable than other domestic boiler types. But nowadays this is simply not the case.

The only major differences between a condensing boiler and a non-condensing boiler is the much more efficient heat exchanger and the fact that water condenses inside the boiler and is required to be drained away periodically. This is quick and simple process, and, if your condensing boiler has been fitted correctly by a qualified technician, easy to maintain.

Standard non-condensing boilers have a single combustion chamber and single heat exchanger through which exhaust gases pass, before being expelled through the flue, at a temperature of around 180°C.

Condensing Boilers - The Good, The Bad, and the Ugly

A condensing boiler, on the other hand, employs one or more larger heat exchangers and cools the products of combustion to a temperature of around 55°C.

This causes the water vapour contained in the exhaust gases to liquify, forming the “condensate” which collects at the base of the flue manifold and must be drained away into an existing waste water outlet or a purpose-built soakaway.

The design of condensing boilers makes them the most energy efficient domestic central heating boilers available. Indeed a boiler replacement of the condensing type could result in a reduction of 875kg in carbon dioxide (CO2) emissions per annum in a typical domestic dwelling in the United Kingdom.

Boilers generally account for some 60% of domestic CO2 emissions and, therefore, the effect that condensing boilers can have on your overall “carbon footprint” should not be underestimated.

The reduction in temperature of exhaust gases means that they are not naturally expelled through the flues of condensing boilers, so a fan must be incorporated to drive them out.

Similarly larger or more numerous heat exchangers are most costly to manufacture. These factors mean that condensing boilers are typically more expensive at the outset than their standard non-condensing counterparts.

However, efficient design equally means that reductions in fuel costs, in the long term, more than compensate for the extra initial cost of condensing boilers.

Is my Boiler a Condensing Boiler?

Is my Boiler a Condensing Boiler? Most new boilers on the market today can be classed a highly efficient, but what makes a boiler a high efficiency condensing boiler?

If your boiler is new, it is likely it could be classed as such, and the boiler can be a combination boiler type, a system boiler or even the traditional type of open vent boiler. Back boilers are not highly efficient, and many back boilers are not longer allowed to be sold in the UK, as they do not meet the minimum efficiency required by the Government in the new SEDBUK scheme, meaning they are definitely not high efficiency condensing boilers. An old back boiler can be replaced with a new back boiler in only rare circumstance, and special permission has to be obtained by the consumer from their local council to be eligible.

A high efficiency condensing boiler is named as such based on the technology it uses – with more heat being absorbed into the system to heat hot water, rather than being lost up the flue on an older boiler type. Because of this, the gas in the flue is at a lower temperature than in older boilers, and causes condensation in the form of as small amount of visible “pluming”.

If a condensing boiler is placed close to a cold surface, water droplets will form. Because of this, is it recommended that high efficiency central heating boilers are placed away from windows, cold surfaces and corners in households due to it causing aesthetic damage over time, but the pluming is not harmful, merely a slight inconvenience.

Because the heat is recovered from the boiler that would normally escape up the flue, a high efficiency condensing boiler can operate in efficiencies, which vary depending on the season, of up to 90%, making them very economical.

These means a high efficiency condensing boiler can even save its owner up to 35p out of every £1 spent on gas, in comparison to an existing boiler over 15 years old.

The SEDBUK Bands and High Efficiency Boilers

The SEDBUK scheme stands for ‘Seasonal Efficiency of a Domestic Boiler in the UK’, and provides an accurate and fair comparison of the performance of different boilers, based on their consumption and loss of energy in comparison to energy used effectively for purpose.

This rating is calculated by measuring the average annual efficiency of a boiler, that has been achieved in average domestic conditions, factoring issues such as the UK climate, fuel used boiler type and the internal store size.

Once a boiler has been assessed to see what percentage of fuel it converts to heat, it is given a rating using the SEBUK scale – from ‘A’ for the best performing boiler at the highest rate of efficiency, to ‘G’ for the worst.

All new boiler replacements are recommended to be rated in either the A or B SEDBUK band, with only few specific instances where they may be lower.

How to Check if Your Boiler is a High Efficiency Condensing Boiler

If you are unaware of the SEDBUK rating your boiler has been classed as; check the manual and any documentation that came with your boiler. This should let you know the SEDBUK rating, and make you aware of how efficient your boiler is.

If you cannot find your boilers paperwork, look on the boiler itself for a brand name and a model name or number and use the internet to search for your boiler using these. Most modern manufacturer’s websites are proud to display the credentials of their boilers, especially if they are energy efficient.

Remember, to be classed as a high efficiency condensing boiler, a boiler must be rated in the ‘A’ band of the SEDBUK energy efficiency scale.

What are the benefits of a condensing boiler?

When it comes to what is so good about condensing boilers, other types of central heating boilers simply cannot compete.

The large list of advantages include:

Environmentally Friendly

Condensing boilers have much lower CO2 emissions than other boilers available, and because of this are much more environmentally friendly. Every year a condensing boiler could save up to 1.7 tonnes of carbon dioxide that would otherwise escape into the Earth’s atmosphere.

For this reason condensing boilers must now be fitted by law.

Energy Efficient

Condensing boilers are from 15-30% more energy efficient than older boilers, carrying an A or B scale SEDBUK rating – the highest possible, and are even more efficient than other modern new non-condensing boilers on the market today.

A condensing boiler with modern heating controls fitted could save you over £300 a year in your energy bills.

If you already have heating controls fitted in your home to a regular boiler, replacing this boiler with a condensing boiler could save you up to £130.

Lower Fuel Bills

Because condensing boilers are so energy efficient, they need to burn much less fuel to extract the same amount of energy to supply to your home. This means considerably lower fuel bills, which is a relief in today’s unstable fuel market with prices rising all the time.

Although condensing boilers are expensive to purchase and install, numerous studies into fuel bills and the energy efficient nature of a condensing boiler in the modern home have proven that they will easily return your investment in money saved on fuel bills in their average 15 year lifespan.

Modern Design

The extra-large heat exchanger, or double heat exchanger, is unique to condensing boilers, and maximised the amount of heat that will be transferred from the burner of the boiler, saving energy.

As well as this, the heat transfer technology in condensing boiler traps and recovers heat which would be lost up the flue along with gases in non-condensing boilers.


Because of the modern nature of their design and the new types of materials used in condensing boilers, they come in a variety of compact sizes, depending on the needs of the individual customer.

Next to a clunky older boiler type, a condensing boiler is sleek and will often fit into convenient spaces the size of an average kitchen cupboard.


The bid to tackle climate change is one also adopted by homeowners. But did you know that heating our homes and water for domestic use accounts for a substantial percentage of the UK’s carbon emission?

In fact it amounts to a staggering 35 million tonnes of CO2 per year.

Around 77 per cent of the emissions come from heating the home, with 23 per cent from water heating.

One of the main ways for consumers to help reduce these figures can be attributed to the energy efficiency of a boiler. From April 1, 2005 it has been a requirement of the Building Regulations Part L1 (Conservation of Fuel and Power) that any replacement or new gas fired boiler be of the condensing boiler type unless there are exceptional circumstances.

By installing a new high efficiency boiler, homeowners can reduce both their heating costs and reduce their carbon footprint.

A boiler replacement is recommended if your boiler 10-15 years-old. The older it is, the more inefficient it becomes.

The new regulations mean that all new gas boilers installed in England and Wales must be high efficiency combi boilers or condensing boilers with either a percentage of 90 or over on the SEDBUK table.

Further advice on energy savings:

  • Install heating controls – so you can adjust the heating to your needs and prevent wasteful heating.
  • Powerflush – this removes sludge and build up from the heating pipework, which improves the system’s efficiency and reduces wasted energy.
  • Service your boiler – annual boiler servicing ensures the boiler is in good operating state, and ensures it operates within its most efficient range.

Are Condensing Boilers Unreliable?


The condensing boiler and its reliability have improved dramatically in the years since they were first released on the UK market – of course, condensing boilers were beset with small problems in their early years, as with most major advances in technology in appliances.

Unfortunately due to all the bad press created by their early faults, now-repaired issues and persistent myths about the boilers still lingering in the minds of the general public, which are, in most part, completely false or resolved completely with new technology and designs.

Dispelling the Myths

Many of the early condensing boiler issues stemmed from the very first models of condensing boilers and their launching faults, and are now completely untrue for the modern condensing boilers on the market today. All new condensing boilers have an excellent reliability and safety record, and are known to be even more reliable than many brands of modern central heating boilers on the market today.

Here are a few of the most persistent rumours which are leading the public to think “are condensing boilers unreliable?”

1. Condensing boilers corrode quickly

In the earliest models of condensing boilers, the heat exchanger which was used had a tendency to corrode if the flue gases created by the boiler would condense upon it. This issue was quickly resolved, and new boilers all should be fitted with a heat exchanger which is of a non-ferrous metal.

2. Condensing boilers are over-priced

Because non-ferrous metals are often more expensive, the price of a condensing boiler is higher than with other types of boiler.

It’s a common myth that condensing boilers are over-priced because of their higher energy efficiency, that the public is paying an extra premium because they are the highest technology for the best environmental benefit.

This is simply not true, and, as mentioned above, the price of a condensing boiler is influenced by its more expensive essential metal parts.

3. Water gets ‘trapped’ inside a condensing boiler, and they are damaged

All the cool water created as a by-product of a condensing boiler is easily drained away, and should pose no issue at all to the health of your boiler if the boiler has been correctly fitted by a qualified and registered installer and the boiler is serviced regularly every 12 months.

4. Condensing boilers use unreliable new technology

Now 20 years after its launch, the condensing boilers ‘new’ technology has been proven to be highly reliable, and in those 20 years any bugs have been resolved and tweaks to the design have been made to ensure nothing can go wrong.

5. The pluming from the condensing boilers flue is dangerous

This is simply not the case – the small amount pluming you see from the flue of a condensing boiler is merely water vapour, and will turn into small droplets of harmless water when it comes into contact with a cold surface.

6. The pluming will make my house damp

Condensing boilers only make a small amount of pluming, and its highly recommended that you place a boiler in a well ventilated area, or site the flue to guide the plume to outside of your residence. If a condensing boiler is placed close to a window or inner corner, then some droplets of water may be seen, but all qualified boiler fitters will site a condensing boiler in an area where pluming will never be an issue.

7. Condensing boilers are only more efficient than other boilers when they are condensing

Whether running in condensing mode or not, a condensing boiler is up to 30% more efficient than older boilers.

8. Condensing boilers will kill my plants

Your plants will love the small amount of carbon dioxide that a condensing boiler produces!

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