The message
from Mike Clarke contains these words:

roger wrote:

Now consider the actual boiler output. The plus whatever it takes to get
it back to temperature may push teh boiler into continuous flat out
mode. How much less efficient is it then?

You have your condensing boiler set so it always condenses. If it is a
non condensing boiler then the difference would not be significant.

For our non-condensing Worcester Bosch 350 the difference is the other way
round:

71% efficient at 9 Kw
79% efficient at 25 Kw
80% efficient at 35 Kw

Interesting. I wonder how much of that lack of efficiency at low output
is due to heat losses through the casing which presumably is not counted
as useful output.

--
Roger Chapman

On Tue, 23 Dec 2008 16:26:03 +0000, roger wrote:

The message from Mike
Clarke contains these words:

roger wrote:

Now consider the actual boiler output. The plus whatever it takes to
get it back to temperature may push teh boiler into continuous flat
out mode. How much less efficient is it then?

You have your condensing boiler set so it always condenses. If it is
a non condensing boiler then the difference would not be significant.

For our non-condensing Worcester Bosch 350 the difference is the other
way round:

71% efficient at 9 Kw
79% efficient at 25 Kw
80% efficient at 35 Kw

Interesting. I wonder how much of that lack of efficiency at low output
is due to heat losses through the casing which presumably is not counted
as useful output.

....very little I would think.

Part of the loss of efficiency is due to the fact that there will be
increased excess air going through the boiler at part load.
Typically on these type of boilers the fan is a fixed speed device and
moves a more or less fixed air flow. Since the boiler won't cool the flue
gases below about 100C (give or take) the energy lost in the flue gases is
more or less independent of gas rate.

With a condensing boiler typically the air flow is proportional to the gas
flow and the gasses are cooled further as well.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

TheOldFellow wrote:
On Mon, 22 Dec 2008 08:51:35 +0000
The Natural Philosopher wrote:

Andy Champ wrote:
The Natural Philosopher wrote:
Well the healthiests time of my life was at college with a wonderful
free and totally overspecced central heatings system.

That's not the heating, that's because ethanol is an antiseptic.

Andy

For a moment there, I thought Andy Hall had written that. (I am very
tired at the moment and susceptible to suggestion)

God rest his soul. He is so missed.

Dave