Boiler reviews: Condensing boilers explained
Why is a condensing boiler greener than a non-condensing one?
A condensing boiler makes better use of the heat that it generates from burning fuels such as gas or oil. In a conventional boiler some of this heat is wasted because the boiler releases very hot waste gases from its flue. 
A condensing boiler uses some of the heat from these waste gases to heat water returning from your central heating system, so it requires less heat from the burner. This makes your condensing boiler more efficient.
The efficiency of a boiler is normally expressed as a percentage - some new condensing boilers can be up to 92% efficient compared to new non-condensing ones that are around 78% efficient and older boilers that are only 55 to 65% efficient.

How does a condensing boiler reduce carbon dioxide emissions?

Reduce your emissions by burning less fuel for heating 
Carbon dioxide savings come from burning less fuel to meet your heating needs.
Imagine that one unit of fuel potentially contains enough energy to heat your home for an hour. Burning that fuel in a boiler that is 100% efficient would heat your home for an hour.
Burning that unit of fuel in a boiler that is 90% efficient would only give you enough to heat your home for 54 minutes, and if it’s 60% efficient you'd only get 34 minutes per unit.
So, the lower the efficiency of your boiler the more units of fuel you need to burn to keep your home at the right temperature. 
The more fuel you burn, the more carbon dioxide you emi
t.
How does a condensing boiler work?

Inside a typical condensing boiler
Like conventional boilers, a condensing boiler burns fuel to heat the water in a metal heat exchanger.
A condensing boiler uses an extra-large heat exchanger (or sometimes two) to maximize heat transfer from the burner and recover useful heat from the flue gases.
In condensing mode the flue gases give up their ‘latent heat’ and exit the flue in a visible plume of water vapour at 50-60°C - they’re usually 120-180°C in a non-condensing boiler.
At the same time water or ‘condensate’ is produced which must be drained away.

Are they only efficient when in condensing mode?
No, a condensing boiler is always more efficient than a conventional non-condensing one, due to its larger and more efficient heat exchanger.
Every new boiler has an efficiency figure called a Sedbuk value which stands for Seasonal Efficiency of Domestic Boilers in the UK. 

Boiler reviews: FAQs

What are the advantages and disadvantages of a combi boiler?
The combi or combination boiler is now the bestselling boiler type for gas central heating in Britain, but they are not for everyone. What are their advantages and disadvantages? 
Pros of combi boilers
Instant, unlimited hot water supply – you never have to wait for your hot water tank supply to be replenished
An economical choice for smaller households with lower hot water demands. More efficient and cheaper to run for hot water supplies
Smaller systems – combi boilers eliminate the need for a cold water tank and hot water cylinder
Drinkable water at all taps because none of the cold water is stored in a tank
Some smaller capacity combi boilers can be fit into cupboards
Modern combi boilers can produce up to 18 litres of water a minute at 35°C
Storage combi boilers available - if you want to have a small store of hot water
No airing cupboard for the cylinder, so more space
You can run a great shower off the bath taps with a combi boiler

Cons of combi boilers 
Standard combi boilers provide maximum pressure through only one tap at a time. If you have two taps running the flow rate diminishes; flow can be interrupted if someone turns on a tap elsewhere in your home
The flow rate for all the appliances in the house is limited by the capacity of the rising water main. Flushing a toilet may thus lead to the hot water temperature fluctuating, or even going cold.
There is a delay in getting hot water out of the tap – you need to wait for the combi boiler’s burner to warm up enough to get the water to the right temperature
A combi boiler can’t be used with a power shower because they can’t heat hot water fast enough to supply large volumes of water; also slower to run a bath than hot water systems using stored hot water.
Lower water pressure than some other types of boiler
Not suitable for big homes where multiple sources of water might be used simultaneously
Delayed hot water in summer months with some models (not storage combis)
No airing cupboard for storage or drying clothes
No immersion heater, so no back up if there is a problem with the boiler or gas supply

Could I save money on my fuel bill?
It's estimated that the average efficiency of boilers in UK homes is about 60%. This means they waste up to 40% of the heat they generate in the form of hot flue gases.
Using more of this escaping heat to warm your water, rather than the outside, would reduce the amount of gas you need to burn to keep your central heating piping hot.
Using less gas will obviously save you money on your gas bill. 
Our table below illustrates just how much a modern condensing boiler could cut your gas bills, suggesting savings of between £160 and £360 per year depending on the size of your house.

Cost-effective and reliable boilers
Don't let your new boiler burn a hole in your pocket. The Which? report on boilers reveals that choosing the right boiler could cut your gas bills, saving you a considerable amount each year. Our tests assess boilers for efficiency, durability and more - bringing you the best on the market.