What are Condensing Boilers?

Condensing boilers are water heaters fuelled by gas or oil.

These boilers achieve a high-efficiency level, typically greater than 90% on the higher heating value).  They do this by condensing water vapour in the exhaust gases and so recovering its latent heat of vaporisation.  This would otherwise have been wasted.

This condensed vapour leaves the system in liquid form, via a drain.

In many countries, the use of condensing boilers is compulsory or encouraged with financial incentives.

So, why purchase a new condensing boiler?

The main reason to purchase a new condensing boiler is the large saving in fuel and therefore money.

Most standard efficiency boilers will operate at 70%-80% efficiency (some older boilers will operate at levels as low as 50% efficiency).

A condensing boiler will run at an optimum efficiency of 95% sometimes rising to 98% in certain conditions.

The reduced boiler operation time reduces your carbon emissions too.

How does a condensing boiler achieve such high efficiency?

The first thing to remember about conventional boilers and condensing boilers is the heat input from the burner is the same. Roughly 250-300 degrees celsius.

It is important to understand how a conventional boiler works in order to compare the two different types of boilers.

A conventional boiler has one heat exchanger. Water enters the heat exchanger at the coldest (and generally lowest) point and collects heat leaving the heat exchanger and flowing off to the radiators and hot water tanks.

The flue temperature can be anything up to 250 degrees celsius. That heat is expelled into the atmosphere never to be used again, so is wasted.

A condensing boiler has two main heat exchangers. This time the water enters the secondary (and condensing) chamber first. Picking up latent heat from the hot flue gases as they cool.

The moisture in the gases condenses into droplets that form at the bottom of the heat exchanger. The condense is mildly acidic (about the same pH as tomato juice) and is expelled to drain.

From there the water enters the primary heat exchanger to collect more heat before flowing to the radiators and hot water tanks.

The flue temperature will be around 55 degrees celsius. That means up to 200 degrees of heat is being transferred to your heating water, and not just being expelled into the atmosphere and wasted.

This means that you save loads of money in operating costs.

Condensing boilers do not condense all of the time!

The condensing boilers only condense when the return water temperature is low enough.

However you are always saving money because of the greater heat exchanging surface area offered by an extra heat exchanger and the improvements in modern design.

Expelling the Myths about condensing boilers

Condensing boilers cannot be installed to existing systems.

This is not true.

A condensing boiler can be fitted to any system that previously had a conventional boiler installed.

Sometimes the increased efficiency means a feed and expansion system is not suitable.

This is easily overcome however by pressurising the system.  Many engineers will do this as a matter of course.

Condensing Boilers are not as reliable.

Again this is not true.

In their infancy boiler manufacturers did have problems dealing with the slightly acidic condensate attacking the boiler heat exchanger but like all things they have improved this.

This is now no longer an issue.

All components are now just as reliable as conventional boilers.

Condensing Boilers are only more efficient when fully condensing.

Not true. All condensing boilers are more efficient then conventional boilers at all times of operation.

The British Research Establishment confirmed this in a paper published in 2003 after research going back to 1980.

With Condensing Boilers the condense is a problem.

The condense can be easily installed by running to a safe drain or lime chipping soak away to neutralise the pH. Manufacturers even produce an inline acidic neutraliser that has puch fit fittings for easy installation to 3/4″ condense pipe.

What are the best Condensing Boilers to choose?

For the best Condensing Boilers of 2021, 2022 and Beyond please see our Best Condensing Boilers of 2021, 2022 and Beyond article.

Here at Let’s Heat we supply and install Condensing Boilers also know as the most popular selling combi boilers.

If you want a quote for supply and installation of a Condensing Boiler, please click here.

Other Boiler Types:

In addition to Condensing boilers also known as combi boilers, we also supply and install these other boiler types:

  1. The Common, Heat Only Boiler
  2. A System Boiler
  3. Combi Boiler
  4. The new Heat Pump!

The Let’s Heat boiler installation & repair team are all fully qualified to the highest standards. We take pride in producing exemplary work standards.  This  includes completing all boiler installation requirement no matter what time or day this is required!

Our team of high quality professional engineers have an extremely high 4.9 out of 5, Trustpilot score given by our previous happy customers and we are fully accredited by Gas Safe and Corgi registered, as well as FCA approved for all of our financial transactions.

Full range of heating & plumbing services provided by Let’s Heat:

Areas covered by our heating engineer services:

The Let’s Heat engineers cover all of Wales & England


Let’s Heat provide Heat Pump installations throughout Wales.

This includes areas like; South East Wales, South Wales, South West Wales, Mid Wales, East Wales, West Wales, and North Wales.

Welsh Cities:

This includes all of the Welsh cities like; Cardiff, Newport, Swansea, and all other towns and cities in Wales.


Let’s Heat provide Heat Pump installations  throughout England.

This includes areas like; South West England, South East England, East Midlands, West Midlands, North West England, and North East England.

English Cities:

Southern Cities

We provide our plumbing & heating services to cities all offer England like; London, Reading, Southampton, Portsmouth, Bristol, Exeter & Plymouth in the South.

Midlands Cities

As well as cities like; Gloucester, Swindon, Oxford, Leicester, Nottingham, Derby, Birmingham, Worcester, Wolverhampton, Walsall in the Midlands.

Northern Cities

And cities like; Liverpool, Manchester, Leeds, York, Sheffield, Newcastle, Sunderland and Middlesbrough in the North.

Any more questions?

If you have any further questions about boiler efficiency, other boilers, or anything else, please see our FAQ Page because you might find the answers there.

Alternatively you could try our News Blog for lots of news and article on all aspects of plumbing and heating.

Check out our Gas Safe Registration and if you want to see our near 5 star Trustpilot rating.

Please click the relevant link because you can then find what you are looking for.

Get a quote today because you don’t want to miss out on our exceptional value for money offering!