Thought some of you might like to know the best buys in Which?
Vaillant ecoTEC(Larger model)
The Baxi and Vaillant were the only two boilers that met the claim of
Bottom of list were:
Worcester Junior (Always thought these boilers had an over- inflated
Forgot the rest, but these boilers did not meet efficiency claim.
No open vent boilers were best buys but the top of list was the
Vaillant and at the bottom was the Potterton promax
The next boiler after the 4 best buys was a Boulter Buderus
Having just had a glance at the non subscription bits, I get the
impression that they are not too on the ball anyway:
"Nearly all new boilers are now 'condensing' types. Condensing boilers
are far more energy-efficient than traditional boilers, as they
cunningly reuse heat that would otherwise be wasted.
Some heat generated by traditional boilers disappears up the flue in the
form of hot waste gases. But condensing boilers use some of the heat
from these flue gases to heat water, making the boiler far more efficient."
That would be a "woosh" then ;-)
One also wonders how they made the efficiency measurements; especially
when SEDBUK says:
"Statistical analysis suggests that if two boilers have SEDBUK values 3
percentage points apart then there is 95% confidence that the boiler
with the higher value is more efficient."
What they don't say is what the confidence level is when the figures
are only 1 - 1.5% apart. Also, one wonders what the accuracy of
measurement is. Better than 1%? That would be surprising.
Which? being able to make the measurements? Even more surprising.
From the online version they appear to have made no comment on or
allowance for (i) the effect of the return temperature on efficiency or
(ii) the cost of upgrading radiators in order to make it practicable to
reduce the return temperature. Is it naughty of them to show potential
savings on heating bills based on 90% efficiency and costs for boilers
For me (with 20+ year old albeit unvented system and separate multipoint
water heater) in a small Victorian terrace with suspended floors the
need to upgrade radiators in order to get anywhere near 90% has been a
big factor in doing nowt. Or (as is increasingly likely) have I
misunderstood what wd be involved?
The range of efficiencies of modern condensing boilers is in the range
from 90 - 91.5%.
SEDBUK say that where there are two boilers with 3% difference there is
a 95% confidence that one will be better than the other. They don't
say what the confidence is when the difference is only 0.5% or 1%, but
can measurements be made consistently to a fraction of a percent?
Seems unlikely considering the many factors involved.
Consider also the energy cost saving between 90 and 91% (assuming the
figures are reached).
A much more important set of criteria at this point becomes build
quality and servicability.
The issue is a comparative one.
Increasing radiator sizes does allow the return temperature to be
reduced for a given heat output. However in terms of what is needed
and achieved, there are a number of factors:
- Assuming that the radiators were only just adequately sized for 82/70
conventional operation in coldest weather, then the boiler will still
run at relatively low temperatures during the 6-8 months of the year
when only small amounts of heat are needed - i.e. max operation is only
usually for 1-2 months of the year.
- Some or all of the radiators may well be oversized anyway.
I found that I was able to redesign for 70/50 operation by leaving most
radiators alone, moving three to new positions and replacing those.
In any case, even if the 90% figures are not met if the existing boiler
is one of the older wall mounted natural ventilation types like I had,
the starting point was 65%, and there would easily be an improvement in
Well AIUI in science if you're using a statistical test to determine
whether A is different from B using whatever measurement or assessment,
then if there is less than a 95% (tytpically) chance of the measurements
being different then you cannot claim that there is any difference.
If so, then the above would indicate that there is actually no
statistically significant difference between the efficiencies of any
modern condensing boilers.
Does that sound about right?
I think that at least it puts it into the context of being a second
However, if the manufacturer was claiming 90.3% and the boiler only
does 85% it would be a different matter, but I think that that would be
In article ,
At University, in the department of Physics and Astronomy,
the business of being within 3 standard deviations was often
quoted. The Physicists were required 3 SD's in the mantissa,
but the Astronomers were allowed their 3 SD's in the exponent ;-)
That's probably right for the basic combustion -- I suspect
they all eject the flue gasses at around only 5C hotter than
the return water temperature..
However, there's much scope for the control system to optimse
e.g. inside the boiler by accurately setting the flow and return
temperatures to their optimim values (low as possible whilst
getting exactly the right amount of heat into the house).
I don't think many of the condensing boilers (particularly the
cheaper ones) even attempt this. I see lots of condensing
boilers with the water temperature still set high when it's
not necessary -- I suspect few owners actually understand what
the controls do in any detail.
Then there are the external control systems -- timeswitches and
crude thermostats can be bettered with control systems which
implement setbacks, and know about the occupancy of the house
and even occupancy of separate zones. Mine is integrated with
my burglar alarm (as it also knows things like occupancy of
areas of the house), but that kind of system design is still
I would have to say to you that doing nothing because of trying to
upgrade to a perfect installation is unwise. There are real and
substantial saving to be made with a modern boiler and controls.
Most of the boiler improvement comes not from the condensing itself but
form a better heat exchanger in which condensing can and does happen.
The condensing is a bonus and increasing the emitters to reduce the
return temp is a further bonus.
Thanks. Understood and fully accepted. (A fuller account would have
gone on to say that I would expect to be able to get around 80% which
will be worth doing now we do not get the benefit of trogging off to
heated offices - but only after we have sorted out the roof, which comes
after sorting out the subsidence recently discovered..... )
In passing, I have now also read the printed report in Which? and still
think they are naughty not to explain what is required to get to 90%