Condensing Boilers

It is easy to be mislead by advertisers' claims for environment advantages of their products - in this case condensing boiler. We will look at the environmental claim and see what parts are true and which parts are misleading.

The following claim was made for a condensing boiler:

If your boiler is over 10 years old, simply by replacing it with a new high efficiency condensing boiler, you could cut your gas bill and CO2 emission by 32%, and by adding the right heading controls you could bump that figure up to a dramatic 40%.

To make the figures more believable, the Energy Saving Trust was cited as the source of the percentage savings in April 2006.

A condensing gas-fired boiler is a new type of boiler which makes maximum use of the heat energy produced by the gas combustion process. Using its "super-exchanger", combining the functions of exchanger and condenser, the boiler first recovers the sensible heat from the combustion products, with an efficiency of about 12% greater than that of a traditional boiler even without condensing. If the flue gases are evacuated at this stage of combustion, they are at temperatures of 200 °C to 300 °C. These flue gases still contain some of the sensible heat and in particular appreciable amounts of latent heat in the form of water vapour. By routing the heating return through the bottom of the exchanger/condenser at a temperature of less than 53°C, the flue gases will condense on the walls of the last rows of tubes. This condensation allied to the high performance of the exchanger results in an energy saving of up to 30% compared to traditional equipment. As the phenomenon of condensation only occurs for heating return temperatures of below 53 °C, the operating efficiency of the heating installation will increase as the average annual heating return temperature decreases.

What this means is that having a high boiler temperature reduces the efficiency of a condensing boiler. As a boiler is likely to heat water, it will have a high return temperature as the return temperature has to be higher than the desired hot water temperature. Most people have their hot water in excess of 53°C and it should be even higher to prevent Legionnaires Disease. Legionnaires' disease is a form of atypical pneumonia caused by any species of Legionella bacteria. Legionella bacterium thrives in water at temperature between 20°C and 50°C. So, when heating water, the return temperature may start below 53 °C but as the water heats up, so does the return temperature.

Old style condensing boilers would have run at maximum output temperature and this would entail a high return temperature which results in the inefficiencies mentioned above. The newer modulating boilers overcome some of these inefficiencies as they monitor the return temperature and run at a level to only have the necessary output temperature. The amount of heat that can be transferred from the boiler to the hot water storage tank is related to the difference between the boiler output temperature and the current water temperature. But, if the temperature is greatly different between the boiler and water storage tank there are more losses in the pipework and the boiler. So, the modulated boiler will limit the boiler temperature to optimise the heat transfer to the water storage tank and the input energy used. It typically does this by having an optimum difference is temperature between the boiler and the return temperature. This means that it will limit the boiler temperature to keep the return temperatures to below 53 °C for longer and only increase the boiler output temperature when necessary to raise the water temperature above 53 °C.