On Sun, 19 Sep 2004 20:19:21 +0100, Andy Hall
wrote:

The operating range possible with a conventional boiler is so narrow
anyway that this makes very little difference. The heat exchangers
used are relatively small.

Perhaps we are talking about gas (you) and oil (me). Condensing
boilers do have a larger heat exchanger. However, if you look at the
sedbuk figures you'll see best standard is 85% (capped), condensing
range 90-95%. So the efficiency difference is only 5-10% and with a
lower return temp on a standard boiler, or a high return on a
condensing, the difference is less again.

The SEDBUK figures really confuse the issue because of the factors
used. For the purposes of looking at boiler behaviour it is much
more useful to use the measuring methods used in the rest of Europe.

These specify certain temperatures and firing rates.

If the figures are available. The only database I know of with all
the uk boiler. If you have a URL for another database please post it.
Also, for oil, isn't it a problem that UK use kerosene and europe uses
gas oil so europe figures will be different anyway.

So what you are saying is that a larger difference than 82/70 does
improve efficiency. Like I said.

I don't think you did, but the point is that a larger temperature
difference together with a larger heat exchanger certainly does help.

A = Large Heat Exchanger + Large difference
B = Large Heat Exchanger + Small difference
C = Small Heat Exchanger + Large difference
D = Small Heat Exchanger + Small difference

A is better than B. C is better than D. A is better than C (but
boiler cost is more). B is better than D (but boiler cost is more).


The difference in cost is becoming less and less, and from next year
the whole issue will be academic since changes to the Building
Regulations will mandate condensing boilers anyway.

Not clear if that will cover oil, but just in case I'm putting mine in
now.

In terms of cost saving, I have found it to be in the predicted band
of 25-30%

For oil that flies in the face of the sedbuk figures which suggest
5%-10% for floor mounted. 5% for wall mounted (which I need).

You must be spending very little on energy if £20 represents 25% of
your energy cost.

GBP20 is 5% of GBP400 my estimated fuel cost.

There are no real problems with modern condensing models in terms of
pluming. Much improvement has been made over early designs. Where
you get the idea that there are smells, I have no idea.

Oil smells. I suspect you are talking more about gas.

A condensing boiler with its lower flue gas temperature is much more
likely to accumulate fumes at low level (that's why they plume after
all). The advice on siting says to take account of the plume
possibility. My flue will face my neighbour. Do I want to risk the
possibility I might be dropping smelly fumes into his garden (and over
his washing line). For, perhaps GBP20 pa when I won't break even on
the boiler costs for 10yrs. NO.

If the TRVs are even partially closed then the flow is reduced. An
alpha will keep the head constant-ish resulting in less flow and thus
lower return temperature.

No because when this happens the heat emission will have dropped.
The purpose is to reduce the power to the pump as the flow resistance
increases.

We are talking about non-modulating boilers aren't we? I am since 99%
of oil boilers are not modulating. I don't know how common modulation
is on gas. So on a non modulating boiler how does the heat emission
drop? Come to that since we were talking about short cycling then how
does a modulating boiler short cycle since they would modulate before
that and thus very unlikely to short cycle.

A standard + bypass will result in constant
head (the setting the bypass is set to), and a constant-ish flow

No it won't because the bypass opens in effect digitally at a certain
head. After that, it has a pressure/flow characteristic.

It's a valve with a spring. It will have some slope to its
characteristic. See http://www.sunvic.co.uk/bypass.html which repeats
what I said.

I would say the sunvic curve is soft. For my house I have calculated
a head of about 1m (about 0.1 bar) and a flow of 0.8 m3/hr. So I'd
set my valve at #3 (this assumes the pump matches this, in practice
the head would be higher to get the flow as I can only restrict the
flow). The valve would start to open at 0.15 bar (1.5m) and by 2 bar
(2m) all my flow would be via the bypass. The head range is thus
1.5-2m. Not exactly digital.


--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
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