Heatline, Intergas and Chaffoteaux specialist
- What is a condensing Boiler ?
- Are Condensing Boilers more expensive ?
- But won't it only be more efficient when in condensing mode ?
- Don't I need to run my heating on full or put in bigger radiators ?
- Are they unreliable ?
- What about the steam coming out of the flue ?
- So what should I look for ?
- Further Information
What is a condensing Boiler ?
These boilers increase efficiency by recovering the heat that is normally wasted in the hot flue gases given off by a conventional boiler. The flue gas from a conventional boiler will be at a temperature of about 150 degrees Centigrade, and sometimes much more than this, but a condensing boiler will reclaim most of this heat, and give off flue gases at about 50 degrees Centigrade.
Around 20% of the UK's carbon dioxide emissions come from domestic heating systems. Depending on the efficiency of your old boiler, a replacement condensing boiler will reduce carbon dioxide emissions from your central heating system by 15 to 30%. Building Regulations now require that all new boilers must have an efficiency rating of A or B, and at the moment only condensing boilers achieve this. Installing only high-efficiency boilers will help the UK to achieve reductions in carbon emissions and so reduce the impact of climate change. If everyone in the UK had a condensing boiler our annual carbon dioxide emissions would be reduced by 17.5 million tonnes, and it would also result in a saving of £1.3 billion on energy bills every year.
Are Condensing Boilers more expensive ?
Heating your home with a condensing boiler is cheaper than with a non-condensing model. The condensing boiler itself may cost a little more (between £100 and £300) to install, but will use less fuel.
The highest-rated condensing boilers are about 15% more efficient than the best non-condensing units. This means that if your fuel bill is about £400 per year, you will save about £60 per year. A condensing boiler that cost £250 extra will therefore recover this amount after 4 years, and over a 10 or 15 year period it will be much cheaper. As gas prices are currently rising above inflation, payback could come much sooner.
There may be local grants available to reduce the cost still further, especially if you are on benefits. Contact your local Energy Efficiency Advice Centre (see below) to find out what you are entitled to.
But won't it only be more efficient when in condensing mode ?
No - a condensing boiler will always be more efficient than a non-condensing, as even when it is not in condensing mode it will still give about 85% efficiency - going up to well over 90% when condensing. A brand new conventional boiler will be at the very most 78% efficient, and old ones will be between 55 and 65% efficient. This is why only condensing boilers meet the criteria for grade A or B efficiency that are required by the latest version of the building regulations.
In order to gain maximum efficiency from a condensing boiler, a good control system must also be in place. A basic room thermostat and fixed boiler temperature setting will not get the very best out of a boiler. A better approach is to have an outdoor weather sensor (known as 'weather compensation') which enables the boiler to run the central heating only as hot as is necessary, rather than going for the set temperature each time, irrespective of whether or not it is a milder day outside. When water at a higher temperature is needed for the hot water cylinder, the controls will take this into account and produce hotter water until the demand is met.
By using such a control system, the water returning to the boiler will more often be in the range needed for it to condense - around the mid 50s degrees centigrade (just below the 'dew point' of the flue - 58 degrees). The boiler will then be in condensing mode for most of the time it is operating, so will run at it's maximum efficiency almost all the time.
Don't I need to run my heating on full or put in bigger radiators ?
If an installer tells you this, then either the boiler is not being installed properly, or is oversized for your house. As mentioned above, a condensing boiler will always be more efficient, so does not have to be run at full pelt all the time to operate well. It is important to make sure that any boiler is not too big - they are often overspecified. Make sure that all efficiency measures (insulation, windows, etc) are taken first, then size the boiler to meet remaining heat demand, and fit good heating controls (see above). If a boiler is too big it will be operating inefficiently. Having larger radiators will increase the efficiency of most heating systems by a small amount (3%), but it will often be neither cost-effective nor practical to change them just to achieve this small gain.
Are they unreliable ?
In the early days of the technology (over 20 years ago), condensing boilers did have some teething problems, and this impression has persisted. But the technology is now well developed, and modern units are as reliable as any other new boiler. Most of the components in either type will be identical.
One initial problem was with the type of heat exchanger used. If flue gases condense within a ferrous metal heat exchanger, instead just after leaving it, they can cause corrosion. New boilers should all have a heat exchanger made of a non-ferrous material such as stainless steel (this is what accounts for most of the extra cost of the appliance).
What about the steam coming out of the flue ?
Because they leave a condensing boiler at a lower temperature, flue gases will condense as they exit the flue. This can be unattractive or a nuisance, so take car to put the flue in a place where it will not cause inconvenience. If you don't have much space directly outside the building, then an internal vertical flue could be used, so that the gases exit near or through your roof.
So what should I look for ?
Before thinking about getting a new boiler, make sure that you have maximised the efficiency of your home. Then if your boiler does need replacing in the future, you'll be able to get a smaller one to meet your reduced energy demand.
See CAT's free information sheet on energy efficiency in the home.
If your old boiler needs replacing, then look for a boiler that is rated 'A' for efficiency. Also, look for 'Energy Saving Recommended' logos on both boiler and associated heating controls. A warranty gives an indication that the manufacturer has faith in their product. Make sure that the heat exchanger is made of a non-corrosive material such as stainless steel (see above). Getting your pipes and radiators flushed out (to remove any built-up gunge) will cost a few hundred pounds, and should be done while fitting any type of replacement boiler.
All modern boilers are complex pieces of equipment and so it is worthwhile spending some time choosing the right one for your application. As with most appliances, annual servicing will ensure that the boiler remains operating efficiently. It is therefore also worthwhile ensuring you use a reliable installer. If an installer does not think much of condensing boilers, then it would probably be pointless asking them to install one - they are unlikely to put in place the heating controls that will get the most from the system. Instead, look for an installer who is knowledgeable about the technology and confident that they can give you an efficient and convenient heating system.
Anyone working on domestic gas systems must be Corgi registered (or OFTEC for oil systems). If you are unable to get a personal recommendation from a friend (often the best way), then look for a tradesperson who belongs to one of the bodies listed below. Members of these will have signed up to a code of conduct.
Remember also that if you are thinking about getting a solar water heating panel, you will need a hot water cylinder. A combi boiler does not need water storage, so cannot be easily combined with a solar panel.
For more information solar water heating.
Further Information
All of the above information was supplied by :-
CATs Free information Service - Centre for alternative technology
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