Question from Brian Warburton via e-mail:
I’ve got a non condensing boiler. The flow temperature is 65-70ºC and the return is 55-60ºC. What advantage would I get from fitting a condensing boiler? Will it be condensing at these temperatures?
Answer from Martyn Bridges, director of marketing and technical support at Worcester, Bosch Group:
Condensing boilers always condense, even at high flow and return temperatures. They obviously condense more and are therefore more efficient when the returning water is below the dew point of the flue gases, around 53-54°C and will certainly condense even more when you get down to say 30 or 40°C.
My advice would be to fit a condensing boiler and to fit a weather or load compensation device. This sophisticated control system will adjust the boiler flow and return temperature to the lowest it needs to be and still provide the desired room temperature levels, all based upon the external air temperature.
Under normal design considerations the radiators are sized upon the assumption that it is -1 or -3°C outside with a 75°C mean water temperature. Clearly when the external temperature is at say 12°C, the radiators would then be oversized for the rooms and therefore a weather compensation system will effectively take over the control of the boiler thermostat and reduce the flow and return temperatures to a lower level ensuring condensing takes place. It is claimed that savings of between 10 and 20% can be achieved (depending on what the previous control system was) with this type of arrangement.