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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: March 8, 2019. Updated: February 15, 2024

What is a Condensing Boiler

Viessmann 200-W Cross-Section - What are condensing boilers | BoilerhutWhat is a condensing boiler? A condensing boiler is a type of gas, LPG or oil boiler that utilises the heat in waste gas to help pre-heat incoming water. They are the most efficient boilers currently available.

Condensing boilers have been the primary type of boiler installed across the UK since 2005 for gas and in 2007 for oil. This was when the government changed the law to ensure all boilers were as efficient as possible.

These changes were made as a study found that 40% of UK emissions came from households.

By using a condensing boiler in your home you will benefit from reduced energy bills. This is because less energy is required to heat your home compared to non-condensing boilers.

All combi, system and regular boilers sold in the UK must be condensing.

How does a condensing boiler work?

All boilers manufactured since 2005 are condensing boilers.

A condensing boiler recovers heat by recycling exhaust gases (mostly hot steam with some acidic content). Older non-condensing boilers used to waste this heat.

The hot gas contains energy. The hot gases go through a heat exchanger in a modern condensing boiler. This transfers the heat back into the central heating water. Transferring the heat back to the water reduces the amount of gas, LPG or oil needed as the water is hotter.

Once the heat is transferred, the majority of the gases condense and turn back into a liquid. This is condensate.

This condensate needs to be drained off. The condensate pipe is usually a white plastic pipe that exits your boiler from the bottom. This is one way you can tell if your boiler is a condensing boiler or not.

The condensate pipe will either head towards an internal drain, like a sink waste pipe or head outside to a downpipe.

If there is no waste pipe near a soakaway can be put into the ground. These are filled with alkaline chips to counteract the acid in the waste condensate. If a soakaway isn’t possible, a condensate pump could be placed below the boiler which will pump the condensate to any part of the home.

We have a helpful “what is a condensate pipe” guide here >

The diagram below shows how a condensing boiler works:

condensing combi boiler

Every new boiler installation needs to be a condensing boiler, whether it’s a combi, open vent or sealed system.

So, whether you buy a Worcester Bosch or Viessmann, Vaillant or another brand you know you are buying a condensing boiler.

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Condensing boiler efficiency

combi boiler efficiency

55% of your energy bills in a year is spent on heating.

According to the Energy Saving Trust: “Heating accounts for about 55% of what you spend in a year on energy bills, so an efficient boiler makes a big difference.”

The biggest reason for modern boiler efficiency in 2024 is down to condensing boiler technology.

How is efficiency for a condensing boiler calculated?

Boilers are measured on how good they are at converting fuel to heat.

ErP Rating: ErP uses letters from A to G to rank the efficiency of a boiler or a central heating system. A-rated boilers have the highest efficiency and G the lowest. Almost all modern boilers have an A rating these days.

The table below shows average annual energy costs based on boiler energy efficiency:

Type Of BoilerEfficiencySemi-DetachedDetachedSmall Home/Flat
Modern Condensing Boiler89% – 98%£1,025£1,455£713
Non-Condensing Boiler75% – 78%£1,169£1,673£802
Old Back Boiler (light weight)60% – 65%£1,415£2,008£952
Old Back Boiler (heavy weight)50% – 60%£1,681£2,371£1,124
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Which condensing boiler is best for your property?

Before installation, you must choose the best boiler for the size of your property. Whether that’s a combi, system, or regular boiler, different installations will be required. The power, however, will remain consistent for heating.

If you want to learn about the best boilers available right now, our new guide will help >

The best condensing boiler currently available is the Viessmann Vitodens 100-W. It offers fantastic efficiency and warranty, all at an affordable price.

Viessmann Vitodens 100-W 35kW Combi Boiler

The Viessmann Vitodens 100-W 35kW Combi Boiler is one of Boilerhut’s most installed boilers.

  • Price: From £1,519 (boiler-only, correct February 2024)
  • 94% efficient
  • 12-year warranty
  • 14 litres/minute hot water flow rate
  • Integrated WiFi
  • Stainless steel heat exchanger

Read our full guide here >

  • Flat/1-2 Bedroom House (1 Bathroom)
  • Combi Boiler
  • Storage Combi Boiler
  • Conventional Boiler
  • Sealed System Boiler
  • Semi Detached House (3 Bedrooms & 2 Bathrooms)
  • Combi Boiler
  • Storage Combi Boiler
  • Conventional Boiler
  • Sealed System Boiler
  • Mid or End Terraced House (3 Bedrooms & 2 Bathrooms)
  • High Output Combi Boiler
  • Storage Combi Boiler
  • Conventional Boiler
  • Sealed System Boiler
  • Large Detached House/Bungalow (4 Bedrooms & Multiple Bathrooms)
  • High Output Storage Combi Boiler
  • Sealed System Boiler
  • Conventional Boiler
  • Sealed System Boiler

Disadvantages of condensing boilers

There are a couple of disadvantages of condensing boilers.

  • Complicated: Condensing boilers are more complicated than older types of boilers. They need to be serviced annually to ensure proper operation.
  • Expensive: Because they have more internal components and are more complicated, they are expensive to manufacture and take longer to maintain and service. However, very old boilers will require many more repairs and are more likely to breakdown.
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Condensing Boiler Reliability

The primary heat exchanger of a boiler is responsible for the central heating in your home. To heat all the radiators quickly, the boiler heats the primary heat exchanger with flames at around 700 to 900 degrees.

It’s like a furnace inside. This means continuous, extreme thermal stresses on the internal components of a boiler.

Because of this, it is crucial that the primary heat exchanger is made of a reliable, durable and highest-quality metal. If not, it will corrode and lose its efficiency over time.

Best metal for a heat exchanger

A stainless steel and chromium alloy is the best possible metal for a heat exchanger. Stainless steel is resistant to corrosion in a wide range of pH levels.

Aluminium is not as robust. If manufacturer-recommended heat transfer fluids and inhibitors are not used issues can occur. The main issue is that aluminium heat exchangers can corrode and lose efficiency or stop working.

If your boiler’s aluminium heat exchanger is damaged, you will probably void your warranty. This is because the manufacturer will cite improper use of fluids.

A replacement heat exchanger may cost anywhere between £400 and £1,000 depending on the model.

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

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 at a higher output compared to stainless steel. This is due to better thermal conductivity. However, aluminium starts to decline rapidly during the first 1000 to 1500 operating hours.

In some extremely acidic environments, aluminium may corrode more rapidly with catastrophic results.

Furthermore, the aluminium surface of your heat exchanger may be porous. This means aluminium heat exchangers can get blocked with sludge, debris and dirt.

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

combi boiler reliability, aluminium heat exchanger corrosionCan you detect heat exchanger corrosion?

If your aluminium heat exchanger has corroded heavily and lost efficiency, you have no way of knowing, unless the boiler breaks down. A heating engineer could open the boiler to have a look for you. This is something they can check during the annual service.

A corroded heat exchanger will give no warnings and make no noise.  Heating and hot water will still work but the boiler will have to burn more gas to get up to temperature. This reduces efficiency and wastes money.

Looking for a new condensing boiler?

If you need a new or replacement condensing boiler or are looking at having a central heating system installed, get an online quote (it takes less than 1 minute), and you’ll get an instant price. But why choose Boilerhut:

  • Next day boiler installation. Quick central heating system installation.
  • Fixed prices. No hidden extras.
  • ‘Excellent’ 5* customer satisfaction on Trustpilot.
  • 10 to 12-year warranty on boilers.
  • Finance options available for central heating or boiler installation.
  • Get up to three quotes for different-tier boilers.
  • Instant, no-obligation quote in less than 1 minute.

Should you try to install a condensing boiler yourself?

NO. No, no, no.

Please DO NOT try to install a boiler yourself.

New gas connections will have to be made, which means only Gas Safe Registered Engineers can legally do it.  If you try to do it yourself or get a plumber without the proper certification to do it, without the required tools, you can face heavy fines or even prison time.

Here are a few reasons for why you shouldn’t do it yourself:

  • Gas Safe Registered Engineers spend 5 years studying, and hit their true level of experience in 5-10 years.
  • Flue Gas Analysers, which cost between £600 & £1000, are required by law.
  • Flue Gas Analysers need to be calibrated annually and hold a certificate.
  • Suitability of the combi boiler for the property needs to be checked.
  • Boiler siting needs to be safe and follow Gas Safe & Building Regulations.
  • Manufacturer specified appropriate materials and fittings have to be used.
  • Heavy fines or prison.
  • Equipment like manometers test the gas supply and make sure the pressures are safe.
  • Gas integrity needs to be tested before connecting the boiler. And again, once connected.
  • Let-by test & tightness test.
  • Flow rates need to be checked and adjusted.
  • Gas Safe Engineers need to gas rate the boiler, and check gas pressures and flue integrity.
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