Condensing Technology Explained – How do condensing boilers work?
When gas burns in a conventional boiler, the heat generated is available in two forms:
Around 89% of it is ‘sensible’ heat, i.e. it can be transferred through the boiler heat exchanger to the system water. In practice conventional boilers are limited to 80% efficiency to ensure that condensate does not occur and corrode the inside
The balance of 11% is ‘latent’ heat, i.e. it is locked in the water vapor discharged through the flue
Condensing boilers derive their additional efficiency from their ability to allow the water vapor to condense and pass most of the latent heat into the heating system. In practice a conventional boiler would have an efficiency of 81% to 84% because as well as the latent heat, further flue gas heat is lost. A condensing boiler will give between 96% to 98% as all the latent heat is captured and flue losses are smaller. But there is a catch.
This only happens when the boiler is condensing.
Boilers can only condense when the flue gases within the boiler are at their ‘dew point’ of 134.6°F or 57°C. So to ensure continuous condensing operation the heat exchange surfaces and therefore the return water must be at this temperature. There are two ways to achieve this. Design the heating system so that its return temperature is low (underfloor systems for example). Alternatively control the boiler flow temperature so that it runs as low as possible for the longest time. Letting the boiler automatically ‘float’ in line with weather demand is the proven approach. Does it matter if you run the boiler at low temperatures? Not for most of the winter.