Condensing boilers increase efficiency by extracting heat from the outgoing flue gas by condensing steam onto the heat exchanger, releasing the latent heat of the phase change from gas to liquid (steam to water). Condensation can only occur if the heat exchanger temperature is low enough: lower than the dew point, which is in the high 50s (about 57°C). In general, the lower the temperature the more efficiently the boiler will run.
Keeping the heat exchanger temperature low requires the temperature of the water returning from your radiators to be low. This can be achieved by:
Keeping the heat exchanger temperature low requires the temperature of the water returning from your radiators to be low. This can be achieved by:
- Having large enough radiators, so that they can be operated at a lower temperature, whilst maintaining the same room temperature. We need to provide information on radiator sizing.
- Running the boiler output flow at a lower temperature. You can do this by turning down the temperature on the boiler, especially during the autumn and spring when less output is required. It can also be achieved automatically by modulating thermostats.
- Maintaining a more constant temperature in your rooms. Rapid increases in temperature require higher radiator temperatures, leading to higher return temperatures. This can stop the boiler condensing and reduce efficiency. It can be less costly to run at a lower boiler temperature and allow more time for rooms to heat up. Of course, better insulation helps with this.