Condensing Boilers

This is a design of boiler which can have an increased efficiency over the more traditional boiler. The efficiency of a typical non-condensing boiler is around 75%, whereas with condensing boilers it can be over 87%. This increased efficiency is due to the extraction of heat from the otherwise wasted flue gases. Most boilers have a single combustion chamber enclosed by the waterways of the heat exchanger through which the hot gases can pass. These gases are eventually expelled through the flue, located at the top of the boiler, at a temperature of around 180C.

Condensing boilers, on the other hand, are designed first to allow the heat to rise upwards through the primary heat exchanger; when at the top the gases are rerouted and diverted over a secondary heat exchanger. These can reduce the flue gas temperature to about 55C. This reduction of temperature causes the water vapour (formed during the combustion process) to condense and, as the droplets of water form, fall by gravity to collect at the base of the flue manifold. The remaining gases are expelled to the outside environment through a fan-assisted balanced flue. The condensation produced within the appliance should be drained as necessary into the waste discharge pipework or externally into a purpose-made soakaway.

It is only possible for a condensing boiler to work to these very high efficiencies if the flow and return pipework is also kept below 55C. The flow & return temperatures need to be maintained for the heat transference to occur from the flue to the water (i.e. heat transference goes from hotter to cooler materials).

Many people are installing condensing boilers in homes which are fitted with radiators and a primary flow and return to the hot water. Some of these people may be under the impression they are getting more for their money; unfortunately, as stated above, low flow and return connections are essential; therefore they are not making the vast savings they are led to expect. For a central heating system to work with radiators and hot water primary circuits, flow temperatures need to be around 82C, so in fact the installer has put in an expensive condensing boiler which gives only slightly improved efficiency over the more traditional boiler. The appliance basically only works in its condensing mode during initial heat-up.

To achieve a system which will function in its condensing mode the installer needs to consider a suitable system of radiant heating.

Get a Quote