"YAPH" wrote in message
...
On Sun, 26 Oct 2008 22:23:45 +0000, asalcedo wrote:

I have installed two new Worcester 30 CDi system boilers at home.
(They work in tandem, at the same time, as if they were one unit)

According to the user's manual, under "Tips for Energy
Conservation", if the property has TRVs and room thermostats (which
is my case), the boilers should be set at the maximum temperature
setting and use the thermostats and TRVs to regulate the temperature
as needed in each room.

I certainly agree that that arrangement gives the most flexibility.
However, based on my reading of condensing boilers, the most
condensing and thus, the highest efficiency is achieved at 55
degrees Celsius.

If I set the boilers to the maximum the boiler's water reaches about
92 degrees.

If I use a 5 setting, 7 being the maximum, the boilers' water is
about 62 degrees and still gets the house warm enough (at least now
that it is not too cold outside)

What is then the most energy efficient setting?

I think the user's manual is wrong in this respect.
As you percieve, the lowest possible flow temperatures
which actually keep the house warm will result
in /marginally/ greater efficiency. Note that
condensing boilers are more efficient than
non-condensing types, even when they aren't
condensing: because they /can/ safely condense
the designers can get more heat out of the
combustion than in non-condensing designs
which have to allow a margin of safety (and
inefficiency) to avoid any possibility of
prolonged condensing in normal operation.

Condensing boiler heat exchangers are bigger so even when not condensing
they are still extracting more heat.

There are boilers which wring the last drops of efficiency out of
their designs by modulating their flow temperature to achieve the
necessary heat output to the building, using controls which "tell" the
boiler the actual room temperatures rather than giving a simple
stop/go signal as a normal thermostat (programmable or otherwise)
does. Unfortunately such boilers cost half as much again, or more,
than an ordinary good quality condensing boiler costs[1],

That is wrong. The Glow Worm xi range can have a proprietary controller and
integrated weather compensation as standard. Look at the Broag boilers with
OpenTherm controllers and weather compensation integrated too.
They will modulate the burner to the room and outside temperatures. Broag
Remeda are cheap are excellent.