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Condensing Boilers: How They Work

It can be hard knowing which boiler is the right one for you and your family, especially when you want to save money whilst also being energy efficient. At Lacey Plumbing and Heating, we can install a new condensing boiler, allowing you to heat your home efficiently whilst saving money on your heating bills. Continue reading below to see how you could benefit from a condensing boiler.

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What is a condensing boiler?

A condensing boiler is one which runs on gas or oil, designed to help with energy efficiency. 

Condensing boilers are typically at least 93% energy efficient, and some models even go up to an outstanding 97% efficiency rating. All modern boilers are now condensing boilers as non-condensing boilers have not been allowed to be installed since 2005.

What is a non-condensing boiler?

Unlike condensing boilers, non-condensing boilers do not make efficient use of all the heat they make, largely down to them only having one heat exchanger. 

The heat escapes through the flue pipe as it does not get recycled in the same way condensing boilers recycle the air.

This means they are less energy efficient, with up to 30% of the heat generated able to be lost.

If your boiler was installed before 2005 it will be a non-condensing boiler. Aside from it being over 17 years old, it is also going to be very inefficient compared to modern condensing boilers. 

Efficiency of your boiler is particularly important at the moment with the large increase in gas prices. 


Are there any regulations on condensing boilers?

The UK government introduced a series of regulations surrounding condensing boilers to help reduce the CO2 emissions produced by non-condensing boilers. The regulations include:

  • After 1st April 2005, all gas-fired boilers must be condensing, including all brand new installations. 
  • After 1st April 2005, all condensing boilers must have a SEDBUK efficiency rating A or B.
  • After 1st April 2007, all oil-fired boilers have to be condensing boilers. This applied to all new installations and replacements.

How do condensing boilers work?

Your condensing boiler will either use gas or oil as a fuel, and it will begin to burn when lit and input the heat from the burner into a primary heat exchanger. 

The hot air travels through the heat exchanger and is stored there as long as possible to bump up the temperature. This heat is then taken to radiators. 

The heat also travels through a secondary condensing area. The warth condenses, which causes droplets (water vapour) to occur, which are collected and taken away via a drain. 

Condensing boilers are great at recycling heat, and are designed to recover more heat before it’s lost. 

How efficient is a condensing boiler?

Condensing boilers can ensure an energy efficiency rating of 90% at the lowest, with some of the newer top range models reaching a desirable 97-99% energy efficiency rating.

Types of condensing boilers

Whether your boiler runs on gas or oil will depend on whether you’re connected to the gas network. Most UK properties do have a gas boiler, however, those who aren’t connected to the gas network will have an oil or LPG boiler.

There are three main types of boilers that can use both natural gas, LPG or oil. This includes:

  • Combi boilers are the newest type of boiler, and they’ve quickly become the most popular. Combi boilers are single cost-effective units that deliver heating and hot water on demand as they don’t need any external tanks or cylinders.
  • System boilers are similar to combi boilers, but they do not supply hot water on demand. Instead it is stored in a water cylinder. This means system boilers can reach much higher demand for domestic hot water compared to combi boilers.

Regular boilers are the most traditional type of boiler, usually found in older, larger properties. Instead of taking water directly from the mains, it is taken from a cold water cistern in the loft. The hot water is stored in a cylinder and the central heating is heated directly by the boiler.

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Pros and cons of condensing boilers

What are the advantages of condensing boilers?

Higher efficiency ratings 

Condensing boilers typically have a 15-30% higher efficiency rating compared to non-condensing boilers. 

Uses less fuel

Higher efficiency means the boiler can deliver the same level of heating, but using less fuel. The less fuel used will result in lower energy bills.

Lower carbon footprint 

As condensing boilers use less fuel, they can help to reduce households’ total carbon footprint by up to 3 tonnes of CO2 per year. 

Space efficient 

Condensing boilers don’t require any specific form of ventilation like non-condensing boilers do, which helps to save space. 


What are the disadvantages of condensing boilers? 

Frozen condensate pipes 

Many condensing boilers have the same issues as every other boiler. One unique issue is a frozen or blocked condensate pipe. 

Condensate pipes are fitted outside of your home, and when it comes to winter, they can become frozen. This can prevent your boiler from working properly. During the installation, your condense pipe should be fitted to current standards to avoid freezing. However, sometimes in very extreme cold snaps, they can still freeze.

During the ‘Beast from the East’, we found pipes that were as wide as outside drain pipes were freezing because the weather was so extreme. We can help you with a frozen condensate pipe if this happens during the winter.

If you have an older model of boiler, it might be worth considering upgrading your condensate pipework as a preventative measure.



Overtime, limescale can build up on the heat exchanger which can restrict the flow of water. This can lead to the water overheating and steaming, which resembles the sound of a kettle. Annual and proper boiler servicing will help with this. 

If you hear this sound coming from your heating system, it’s best to call a heating engineer who will look for you.


Leaking and dripping 

Water coming from any part of the heating system is always a concern. A number of reasons could cause leaking or dripping, and a heating engineer will need to be contacted to run a diagnosis. 

If you find that water is coming from your heating system, turn the boiler off and contact a heating engineer


Condensing vs combi boiler

There are a lot of questions surrounding condensing vs combi boilers and how they are different. 


What is the difference between a combi boiler and a condensing boiler?

Ultimately, a condensing boiler and a combi boiler are the same thing. All modern boilers are condensing, including combi boilers. 

So, in simple terms, a condensing boiler describes the technology the boiler uses and a combi boiler is the type of boiler that is installed. 

Best condensing boilers

All brands of boiler are condensing boilers, but the brands that we recommend are Worcester Bosch, Baxi, Ideal or Vokera. 

It really depends on your budget and the length of warranty you are looking for when you’re considering a new boiler. We can advise you on the right boiler for your home.