What is the difference between conventional and condensing boilers?

Conventional boilers burn fuel in a combustion chamber surrounded by a water jacket. This heats the water, which is then circulated around the heating system.

Unfortunately, not all the heat produced by the burning fuel is used to heat the water. Much of it is lost to the surrounding air, and more disappears up the flue.

To overcome these inefficiencies, condensing boilers are designed to extract more heat from a given quantity of fuel than is possible from a conventional boiler.

This increase in efficiency is obtained by reducing the temperature of the flue gases to a point where water vapour produced during combustion is condensed out, releasing latent heat that would otherwise escape with the flue gases.

To achieve this objective, condensing boilers use either an extra large heat exchanger or a second heat exchanger to collect the heat from the flue gases. In effect they scavenge the heat not recovered by the main heat exchanger and make use of it.

The net result is that less heat is lost up the flue or wasted by heating the air that surrounds the boiler.

The boilers can be fuelled by natural gas, LPG and oil.

Are condensing boilers more energy efficient all the time?

It is important to note that condensing boilers do not operate in the condensing mode all the time. However even when not in the condensing mode they are significantly more efficient than conventional boilers. To check the efficiency rating of conventional boilers and compare them with condensing boilers please visit the DETR/BRE Boiler Efficiency Database site at - http://www.sedbuk.com/

Who should use condensing boilers?

Condensing boilers are ideal for large properties, properties with extensive heating systems or large heat load requirements, e.g. swimming pools. If there is a large difference between the boiler flow and return temperatures, then a condensing boiler should be considered. It would be working at its peak efficiency for long operating periods.

Condensing boilers cannot and should not be used for every application. The Building Research Establishment Conservation Support Unit (BRECSU) has produced a number of good practise guides under the Environmental Best Practise Programme. Many of these give advice on condensing boilers.

What are the typical efficiencies of boilers?

Boilers over ten years old has nominal operating efficiencies of between 50 and 65%, floor mounted boilers are usually worse than wall mounted combi-type boilers. Boilers that have been serviced regularly should be more efficient.

Modern conventional boilers can have operating efficiencies of between 75 and 85%. Modern condensing boilers can have operating efficiencies of between 85 and 95% which should only drop by 3% when not condensing or on part load. Obviously as any boiler gets older its efficiency will reduce.

What do I need to consider before buying a condensing boiler?

There is a lot of negativity in the heating trade towards condensing boilers, although it is not clear why. When condensing boilers first appeared in the UK 10-15 years ago there were a lot of mistakes made with regard to the installation of the boilers, this resulted in a lot of return maintenance visits and bad press.

The history of boiler installations has replicated the situation in mainland Europe, particularly Holland, who are about ten years ahead of the UK in their use of condensing boilers. Condensing boiler installations now make up over half of the total market for domestic boilers in Holland.

All the large traditional manufacturers of conventional boilers now offer condensing boiler models to the UK market..

Most condensing boilers are of the balanced flue type and as such they need to be sited on an external wall.

Due to the condensate produced from the flue gases they must be permanently connected to a foul drain, this should run inside the property to prevent the condensate freezing..

The main feature of condensing boilers is that they sometimes produce a plume of water vapour around the flue terminal. Although this is in no way harmful it can be perceived as a nuisance. Therefore the siting of the flue terminal is important so that the plume does not discharge over neighbouring properties and preferably not within full view of windows.

Upgrading Systems

With regard to upgrading of old, gravity central heating systems and replacement of conventional boilers there is always the risk that by pumping the system, old joints and valves come under stress. To efficiently control heating, the systems must be pumped and occasionally this cause radiator valves and joints to leak. This is not the fault of the condensing boiler and would happen no matter what boiler was installed. It is a relatively simple matter to renew radiators or replace valves.

How Much Do Condensing Boilers Cost?

Condensing boilers do cost slightly more than conventional boilers but the price differential is narrowing all the time.