Andy Hall wrote:
On 2007-12-23 07:02:50 +0000, Edward W. Thompson
said:

On Sat, 22 Dec 2007 10:48:22 +0000, Andy Hall
wrote:

snip
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
obviously less.
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 discussion on boiler efficiency may be interesting but really is
irrelevant if you accept that the difference of efficiency of modern
boilers will fall within a 10% envelope. For most, I suggest this
equates to something like �50-�60 in their annual gas bill.

On the most recent models it's in about a 3% window allowing fo
rmeasurement uncertainties and the difference in gas bill perhaps around
�15.

As you suggest, in the scheme of things, almost negligible.



If we can
agree that a call out charge for servicing is of the same order, more
if it requires part replacement,cost of repair and servicing are the
more important criteria. I know this has been raised by many but 'we'
still persist with focusing on boiler efficiency.

A call out charge for a service is probably closer to �100.




Further, the cost to replace a boiler, non DIY, is what, something of
the order of �1500 plus? If the reason is purely to benefit from
gains in efficiency and we agree the efficiency of a non condensing
boiler is 60%, repayment on cost will be 15 years plus, hardly worth
considering.



In my judgement it is economically advantageous to keep a non
condensing boiler in service for as long as economically possible. How
long that is will obviously vary from case to case but to replace a
boiler to benefit from gains in efficiency is not a factor.

I know there is nothing new here but I would really benefit from
changing the focus of the discussion from efficiency to reliabilty and
cost of ownership.

It depends on the point from which one is measuring and projecting and
isn't a simple linear discussion.

On the one hand, if one has a an existing old boiler that is 15 -20
years old and has 60% efficiency then the capital cost is long written
down and the discussion is about the cost in parts of keeping it
running. That one is a fairly simple game of costing. If a failure
is from a thermocouple or perhaps a thermostat, it is probably economic
to continue with it. If a failure is from the heat exchanger or gas
valve, probably not.

On the other hand, if the boiler is a more recent model, it may already
have a 70-75% efficiency and may also not be written down. Then the
replacement of even the more expensive parts may be viable.

One also has to factor in the increasing cost of gas (say 10-15% pa);
although at this point that is still a second order factor - being a
percentage of a percentage.

On that first level, I would agree with you. If a boiler is already
running well and has some lifetime left, and remains servicable
economically, there is no point on a purely financial basis to change it.


There can be other triggers for changing a boiler:

- Change in usage pattern - e.g. more people in the house, building of
an extension

- Original was never up to the job

- Need to relocate the boiler


For me it was a mixture of criteria:

- The old boiler was 15 years old and had had a new thermostat.
There were signs of deterioration from the heat exchanger.

- It was a 65% efficient model and had been written down

- It was necessary to move it owing to a kitchen remodeling

- Additions were being made to the property in terms of a conservatory
and heating for the workshop and it would not have been adequate.

On the basis of these, I felt that there was a fairly clear decision to
replace. Having made that decision, the choice of replacement was
based on build quality, test reports (in my case from Germany), research
of different models and on servicability. I did not decide based on
whether a product was 90 or 91% efficient. As it happened, the model
chosen was at the top end of efficiency at the time (there are others
than are more efficient now but only by about 1%). There was a 5
year parts and labour warranty.

All of that put the purchase price into the top decile of gas boiler
prices among condensing models.

The other thing that I do is to maintain a slush fund for replacement of
domestic equipment. My philosophy for that is to choose only high
quality products - e.g. Miele for white goods and to write the cost down
on a more aggressive basis than the likely lifetime. So for example,
boiler, washing machine, tumble dryer is 10 years. I know that I am
safe with the Miele appliances because the warranty is 10 years. I am
taking a reasonable punt with the boiler. For each year's
contribution to the slush fund, I put in an amount to replace with
something from the top decile of products of the type and add 10% to
that to compensate inflation.
This works pretty well and always remains in "profit".


It would be very hard to find TCO for a boiler based on repair costs.
Manufacturers know what they are funding in terms of spares during
warranty. They also know what they are selling out in terms of spares
after that, plus they know the size of the installed base. Of course,
they won't tell you that. Then there is the issue of generic parts
(although those are generally the low cost ones).

Given all of that, there is not much to go on, other than the common
sense things of looking at build quality - a Viessmann or Vaillant is
better built than a Ravenheat - something obvious to anybody taking the
trouble to research.

Then you can talk to those who actually fix the things independently but
don't have an axe to grind commercially.

In those senses, we do have several people here who give honest opinions
based on their experiences.

I don't think that it gets more scientific than that.



Its so nice to see someone else applying ruthless cost benefit analysis
to home economics.

One point that should be made,and that is the ratio of energy usage to
capital cost.

If energy usage is very high, quite expensive solutions that only
increase efficiency a smidegeun are economically effective.


Another one is reliability: consistently unreliable kit costs money in
terms of the time needed to deal with the problems ..


There is an old axiom of production engineering: What leads to low
productivity is largely *wasted time*. Instead of worrying about making
people work harder, or more effectively,simply watch how often they are
standing idle waiting for something to happen etc, then work out why
thats so and fix it.

If you have to take a day off work to fix a boiler, thats maybe £300 of
holiday time you lost.

Regular servicing is not about the lowest cost of ownership directly,
its about spending fixed predictable sums of money to achieve 100%
uptime :-)