High efficiency central heating

Although not a renewable technology, high efficiency condensing gas and oil boilers are now being specified for a much wider range of applications due to changes to the Building Regulations and recognition of their ability to extract more heat from the combustion process than conventional boilers.

They are also better placed than any other available energy efficient technology because of the huge installed base of standard efficiency boilers, which can be replaced on an almost like-for-like basis with no major, expensive changes to the existing heat distribution system.

There are still five million permanent pilot low efficiency gas boilers installed in UK homes and if these were all replaced by high efficiency condensing alternatives, the overall efficiency of our central heating market would be transformed quickly and for a relatively small investment.

Condensing boilers
Condensing boilers make full use of heat from the exhaust gas created by the combustion process. With conventional boilers, a lot of usable heat energy disappears out of the flue with the waste gases from the combustion process. A condensing boiler has additional heat exchanger surfaces, which extract more heat from the exhaust gases converting that energy to pre-heat the water in the boiler system. The water vapour is then at a lower temperature than it would be in a conventional boiler and so condenses back into liquid form releasing further ‘latent’ heat. 

At peak load this means they will extract 90% or more of the available heat energy, which means that overall they should work out 30% more cost-effective to run than conventional boilers of ten years old or more and are 10% more efficient than even the most up-to-date conventional alternatives.
Condensing systems are now being regularly twinned with solar water heating systems to deliver a highly efficient way of producing the complete needs of a home or commercial premises.
This technology should also be twinned with improved controls to ensure maximum benefit is extracted and waste is minimised. As well as upgrading programmers and fitting thermostatic radiator valves, installers could also consider using weather compensation controls to ensure the system always runs at the optimum temperature for the prevailing conditions and that the boiler remains in condensing mode for most of its operating time.
Key points:The radiators and pipework may need to be re-designed to ensure that water returns to the boiler at as low a temperature as possible to deliver maximum efficiency by keeping the system in condensing mode.
The condensate produced by the boiler must be removed by an extra pipe or condensate drain and then disposed of sensitively via a ‘sink’ system.
Condensing boilers will produce a fairly dramatic plume of steam from their flues so the position of the flue is important to avoid nuisance to neighbours and passers-by as well as preventing this ‘mist’ from drifting through windows.
The flues must also be resistant to corrosion, which might be caused by the condensing process, so it is usually not possible to use an existing flue when upgrading to condensing.