CONDENSING BOILERS

A condensing boiler burns fuel-gas or oil- to heat water and recovers some of the wasted heat from the hot flue gases to improve its efficiency.

In a conventional boiler approximately 30% of the heat is lost because very hot gases are discharged from the flue. A condensing boiler uses a heat exchanger to heat the water in  the usual way, but also recovers heat fromthe flue gases. This can increase the efficiency from around 70% in a conventional boiler to around 90% efficiency. This means it costs 20% less to do the same job.

The hot flue gases are passed over a secondary heat exchanger. This reduces the temperature of the flue gases from around 180C to about 60C.

The flue gases contain water vapour, and because the temperature in the flue is reduced in a condensing boiler, the flue gases are  able to hold less water vapour. The water condenses out into water droplets, which are drained away.

Watchpoint 1:
The flue gases pre-heat the return flow of water from the central heating circuit after it has given off its heat in the radiators and hot water cylinder. If the temperature of the return water is to high, less heat will be transferred from the flue gases to the water. The optimum temperature for the returning water is around 55C. Tip: increase the boiler temperature in cold weather and reduce it to maintain efficiency as the weather gets warmer so that the return temperature is not too high
Watchpoint 2:
Flues from condensing boilers produce a plume of vapour, so the flue should be sited where it is not a  nuisance.

To check the efficiency of your boiler, have a look at the boiler database and other useful information at www.sedbuk.com