"dennis@home" wrote in message
...


"Mark" wrote in message
...
On Wed, 29 Oct 2008 10:39:13 -0000, "dennis@home"
wrote:



"Mark" wrote in message
...
Hi,

My boiler shows the return temperature of the CH on its display.
Earlier today it was showing 75(C) which seems very hot. As it is a
condensing boiler won't this mean it is operating outside its
condensing mode?

Steam starts to condense at 100C.
Most of it will have condensed at 75C.

I had heard that the flue gasses had to be below 55C to get the full
benefit of the condensing mode.

I read the following on several web sites:
"It is only possible for a condensing boiler to work to these very
high efficiencies if the flow and return pipework is also kept below
55°C."

You can treat that with a pinch of salt.
Most of the waste heat saved is the latent heat of vapourisation.
If you condense most of the steam you get nearly all the heat back.
Cooling the flu gases down to 55-60C will only get you a very small amount
of energy back.
You could say that the return temp should be kept to 40c as then it is
even more efficient.
However it comes at a cost..
bigger radiators and making (and running) them wastes energy.

58C is the theoretical temperature in which flue gasses start to condense.
In doing so they give off latent heat. The return water temperature needs
to be about 54-55C to make the flue gasses condense.

The energy clawed back is about 12% to the best non-condensing boiler.