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 emit.
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.
Sedbuk's energy efficiency chart
The rating represents the average annual efficiency the boiler can achieve in typical domestic conditions, making reasonable assumptions about how it's used, the climate, controls, and other influences.
This indicates how efficient your boiler will be taking into account its efficiency in condensing and non condensing mode.
Can I ensure it is always condensing?
Not entirely, but there are some things you can do to help by ensuring that the water returning to the boiler is at the right temperature to encourage condensing - around the mid 50s degrees centigrade.
A boiler is sized to suit your home's hot water demands
It is important to make sure that any heat-only boiler is not too big for your central heating demand or it won’t operate efficiently.
Make sure that all efficiency measures such as insulation and double-glazing are in place, then size the boiler to meet the remaining space heating demand (you can work this out yourself using the calculators at the Sedbuk or Energy Savings Trust websites, and fit good heating controls.
A basic room thermostat and fixed boiler temperature isn’t flexible enough because the set temperature doesn’t take account of how much heat is actually needed. Best is an outdoor weather sensor (known as 'weather compensation'), which enables the boiler to run the central heating only as hot as is necessary, and to be in condensing mode for most of the time it is operating.
You can achieve a similar effect by turning your central heating output temperature down as the weather gets warmer and up as it gets colder.
Do I need to oversize my radiators?
Having larger radiators will increase the efficiency of most heating systems by a small amount, because the circulating water loses more heat moving through your system and returns at the right temperature for condensing.
Deliberately choosing larger radiators than you need means that you can operate the whole heating system at lower temperatures (again encouraging condensing) but still get the right amount of heating for your room.
But it will often be neither cost-effective nor practical to change your radiators just to achieve this small gain. If you’ve had double-glazing installed since your radiators were fitted they are likely oversized anyway.
Is it worth changing a five year old conventional boiler for a new condensing one?
To comply with building regulations in England, Wales and Scotland, new or replacement boilers should be rated as Sedbuk band A or B, that is more than 86% efficient.
The Sedbuk boiler efficiency database lists all of the boilers in production, their efficiencies and estimated running costs in examples of typical houses.
Rising gas prices are a worry
These calculations were made when gas cost just 1.63 pence per kWh.
With current gas prices of two to five times more these days, it’s clear that the more gas you use, the more you’ll save now and in the future by going for the most efficient boiler you can afford.
But there is a little financial point in paying much extra for a Sedbuk band A boiler of 91.5% versus 90%, unless doing as much as you can to curb carbon dioxide emissions is vital to you.
Quick facts about condensing boilers
- They produce a visible plume of vapour when they are operating so the flue terminal needs to be sited carefully to avoid the steam creating a nuisance
- They produce an acidic liquid that must be plumbed into a suitable drain
- Early condensing boilers had durability problems caused by the acidic condensate but modern heat exchangers use non-ferrous metal to reduce rusting