Domestic Condensing Boilers • The Benefits and the Myths

Condensing boilers are highly efficient boilers that have much lower fuel and running costs than conventional boilers.

Benefits of Condensing Boilers

Condensing boilers offer tangible benefits by:

• Reducing carbon dioxide emissions and helping to combat global warming.
• Improving efficiency thus reducing fuel bills.

Condensing boilers work on the principle of recovering as much as possible of the waste heat which is normally rejected to the atmosphere from the flue of a conventional (non-condensing) boiler.
This is accomplished by using an extra large heat exchanger or sometimes two heat exchangers within the boiler which maximises heat transfer from the burner as well as recovering useful heat which would normally be lost with the flue gases.When in condensing mode (for condensing boilers do not condense all the time) the flue gases give up their 'latent heat' which is then recovered by the heat exchanger in the boiler.
As a result the temperature of the gases exiting the flue of a condensing boiler are typically 50-60°C compared with 120-180°C in a current non-condensing boiler. At the same time an amount of water or 'condense' is produced.
A condensing boiler will have a better operating efficiency than a conventional non-condensing one, due to its larger and more efficient heat exchanger. The benefits of condensing boilers are therefore quite clear, and in order to encourage greater take-up of these benefits we now need to
address the myths surrounding them.

Myths Surrounding Condensing Boilers

Myth: They are only efficient when fully condensing - Not true.
Due to its larger heat exchanger, a condensing boiler does not have to condense in order to be more efficient. Typically a new gas condensing boiler will have a seasonal efficiency of between 84-92% compared with a new non-condensing boiler at 78% or an older boiler at 55-65%. See the
table on page 4 for typical annual fuel costs for condensing and non-condensing boilers.
The SEDBUK (Seasonal Efficiency of Domestic Boiler in the UK) rating is the annual average efficiency achieved in typical domestic conditions and is quoted in a series of bands (from 'A' at 90% or over, to 'G' at below 70%).
The SEDBUK is linked to current building regulations in England and some boilers which are rated A-B are endorsed as an 'Energy Efficiency Recommended'.
Myth: They are too expensive - Not true.
In the past condensing boilers were more expensive, mainly due to the materials used in the construction of the heat exchanger. However, as with all new things, the cost of technology reduces over time, as some regular condensing boilers can now be obtained for less than a regular
non-condensing boiler.
In some cases the difference can be offset by grants that are available from local authorities, energy companies or via Energy Efficiency Advice Centres (EEAC's). The payback period will then be minimal (0-4 years) and the net savings in fuel costs will be greater in the long term.
Myth: They need larger radiators - No change necessary.
In most systems, radiators are already oversized for all but the severest weather. A marginal benefit of approximately 3% may be obtained from oversizing radiators for a new system, as this will allow slightly cooler return water to the boiler and maximise the time spent in condensing mode, but this is usually uneconomic and impractical.
Myth: They are less reliable - Not true.
This was true with the earlier models of condensing boilers, but through improvements introduced as a result of this earlier experience, the components in modern condensing boilers are as reliable as those in equivalent non-condensing models.

Myth: They are difficult to install - Not true.
The only difference compared with non-condensing boilers is the requirement for a condensate drain.
As competent installers we have no problem in deciding on a suitable location for the drain and fitting a normal plastic overflow pipe. Flue locations will also need to be a consideration - see myth on pluming.
Myth: They are harder to maintain - Not true.
The only minor difference is the need to ensure that the condensate drain is clear when servicing.
Myth: They cannot be fitted to existing systems - Not true.
Condensing boilers are suitable for most existing boilers. As with any replacement boiler the effectiveness of the control system and type of hot water cylinder should be assessed when conducting a site survey.
Consideration should also be given to cleaning and flushing the system before fitting a new boiler.
Wall hung condensing boilers are readily available, with extended fluing options if required.
Myth: They have limited availability - Not true.
The number of manufacturers offering both regular and condensing boilers is increasing all the time, with major players offering dozens of different models.
Myth: The plume is a nuisance - Not true.
Because the flue gases leaving a condensing boiler are cool, they tend to produce a noticeable mist or plume of water vapour around the terminal itself (as they condense upon contact with the atmosphere), especially under cold conditions. This is not a problem and in fact indicates that the
boiler is working as intended. However, consideration should be given to boiler and flue location prior to installation so that the plume will not be too close to neighbouring properties or windows and doors or paths regularly used in the winter.
Myth: The condensate is a problem - Not true.
With modern systems, only about one litre per hour of condensate is produced, which has a pH in the range of 3.5 to 5 - about the same as tomato juice. As previously discussed, this is carried to a normal drain by means of a simple plastic overflow pipe.