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.

So in practice I'd say set the boiler temperature control about halfway to
start with and experiment. If it doesn't get the house up to temperature
in very cold weather turn it up until it does. Not ideal in this age of
programmable, automatic controls, I know.

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], and the extra money can usually be better spent on
improving draught-proofing, insulation etc rather than on very marginal
improvements in boiler efficiency.

[1] there doesn't seem any good reason why this should be, except "because
they can".

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