How do gas condensing boilers work?

When gas burns in a conventional boiler, around 89% of heat is transferred
through the gas boiler heat exchanger into the water system. However, in
practice conventional boilers have to reduce this figure to 80% to avoid
corrosive condensation. The balance of 11% latent heat is in waste water
vapour discharged through the flue.


Condensing boilers allow the water vapour to condense and put most of the
latent heat back into the gas heating system. This increases efficiency from
81%- 84% in conventional boilers to 96%-98% in condensing boilers when
the boiler is condensing.

Gas condensing boilers only condense when the flue gases are at a
‘dew point’ of 57°C. So to ensure continuous condensing operation the return
water must be below this temperature. To achieve this, heating systems can
be designed so the return temperature is low (e.g. under-floor systems) or
temperature controlled so the boiler flow temperature runs as low as possible
for the longest time. The latter is the proven approach as in the UK matter
if you run the boiler at low temperatures. This is because in an average
British winter the temperature is approximately 10°C and only approximately
30 days out of 200 are close to 0°C - the temperature most gas condensing
boilers are designed for.

How does ‘weather compensation’ work?*
Weather compensation technology monitors and responds to the flow
temperature of the heating circuit in relation to outside weather conditions.
Weather compensation systems are intelligent: they automatically switch the
boiler on and off at optimum times; equalising room temperatures, and
factor in seasons to avoid autumn / spring over or under heating. They
minimise discomfort and the need to crank up the heat - or ventilate and
'dump' heat. The aim of weather compensation technology is to keep the
interior temperature as low as possible and so the boiler condenses for
virtually all of its operating period.

Weather compensation uses a sensor outside communicating with another
sensor in the boiler, all the time and varying the boiler's water flow
temperature accordingly, rather than the boiler wastefully turning ‘on and off’
at peak temperatures.

* Based on information supplied by Viessmann Ltd.

The main points...