On Thu, 23 Sep 2004 08:42:08 +0100, Malcolm Reeves
wrote:
On Thu, 23 Sep 2004 00:21:08 +0100, "G&M"
wrote:
I don't have any direct experience of this, but I was speculating that -
with an Alpha pump - it may be difficult (or perhaps even impossible?) to
adjust everything so that the by-pass *only* operates during pump over-run
conditions.
Oh you can adjust an Alpha to do almost anything. You can even make it run
like a normal pump if you want though it gets a lot louder then. But I
found that by setting it a 'bit of a turn' more clockwise than is really
required then I can make the by-pass operate.
At last an interesting comment of practical experience. However, I
think the alpha is now operating in almost standard pump mode. The
alpha data sheet curves show for a normal pump change of flow from Q1
DOWN to Q2 gives INCREASE of head from A1 to A2. As expected. But
for alpha Q1 down to Q2 gives DECREASE of head from A2 DOWN to A3.
Which is what you want if don't have an auto bypass.
This ignores the issue of the effect of short circuiting the flow back
to the return if a bypass is used for the low flow rate case.
With an alpha and an auto bypass you have to adjust the alpha to
behave like a normal pump for the bypass to operate, that is flow
down, head up. OK, an alpha has more adjustment than a normal pump
but is the extra cost worth it?
Obviously. The bypass is simply set to the appropriate opening point,
if one is used at all.
For standard boiler I'd say no as you don't want flow reduction and
low return temperatures with a standard due to the risk of
condensation.
That's a red herring, because if the flow has reduced (which it will
have done), then relatively little low temperature water will be being
introduced into the heat exchanger. In a conventional boiler, the
heat exchanger is not designed to allow condensation anyway and so the
low flow will simply cause the boiler to cycle off as the heat load
falls.
For a condensing boiler I would be tempted by an alpha
as with the better adjustment you are more likely to be able to delay
the operation of the bypass and so work sometimes with a lower flow,
lower return temperature, which makes a condenser more efficient.
This misses the point. A condensing boiler typically modulates as
well, so there is much less of an issue of mismatch between the rate
of heat production and use.
However I wouldn't like to say for sure that you would recover the
extra GBP35 and alpha costs in the lifetime of the pump.
That depends on the lifetime of the pump, but it certainly improves
system behaviour as regards noise.
If it saves the cost of an ABV, then it's a no-brainer, although the
latter may be useful as a better alternative than a simple lockshield.
..andy
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