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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 vapour discharged through the flue

Condensing boilers derive their additional efficiency from their ability to allow the water vapour 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 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. In an average British winter only perhaps 30 days out of 200 are anywhere near the temperature (0°C) that the system was designed to provide for.

An average winter temperature is around 10°C, so it pays to keep the boiler running at a low figure as long as possible.

Weather compensation can do this automatically.

Weather Compensation Explained!
Weather compensation controls enable a condensing boiler to work at its optimum efficiency. The controls allow the boiler to vary its operating flow temperature automatically, day by day to suit the weather outside and the temperatures inside the house.

Weather compensation works with a sensor outside and another in the boiler communicating with each other, all the time and varying the boiler's water flow temperature accordingly, rather than the boiler turning on and off which wastes energy.

The aim is to keep the temperature as low as possible and so the boiler condenses for virtually all of its operating period.

Fitting weather compensation can increase the efficiency gain of your boiler by 10% and this can all be achieved from as little as £100 extra. 

Viessmann boilers come with the connections and control on the boiler to make weather compensation a "plug and play" option.

Click to view the Vitotronic 200 control with weather compensation

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Vitotronic 200


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