"Malcolm Reeves" wrote in message
...
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.

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?

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. 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.

The efficiency of a condenser is not determined by the return flow, only the
temperature. You can get a lower return temp by having the flow faster, or
in some instances slower.

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.

They are quieter. Nevertheless, if a system is installed properly and the
odd bit of vibration absorbing flexible pipe or plastic pipe is used, a pump
should not be noisy, even if the TRVs are nearly all closed.

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
electronics. More details plus freeware, Win95/98 DUN and Pspice tips,
see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

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