"Andy Hall" wrote in message
...
On Wed, 22 Sep 2004 13:38:42 +0100, "FrancisJK"
wrote:


"Andy Hall" wrote in message
.. .
On Wed, 22 Sep 2004 09:57:06 +0100, Malcolm Reeves
wrote:


"FrancisJK" wrote in message
...

Has anyone any opinions (silly me, of course you have) about alpha
pumps?

With an alpha, as TRVs close the flow falls but the head does not rise
as in normal pumps.

Not exactly.

Between a certain range of flow/pressure, the Alpha behaves
conventionally. When the flow falls below a certain point, that is
detected and the power is reduced to reduce the head.


With a normal pump you need a bypass. So normal
pump+bypass gives constant-ish flow rate and return temperature rises
as TRVs close. Alpha gives reducing flow and a lower return
temperature.

Only close to the point of total closure.


Hence Alpha's are a good idea if you have a condensing boiler since
those are more efficient with lower return temperature. But normal +
bypass is probably better for a standard boiler since they do NOT want
a lower return temperature due to the risk of condensing causing
boiler corrosion.

it's suitable for a normal boiler as well, since if the bypass opens,
the flow increases anyway.




.andy

To email, substitute .nospam with .gl

Andy,

I'm now puzzled.

Is it a choice between

Standard pump AND automativ by-pass

against

Alpha Pump AND no by-pass


Francis

Not as far as I am concerned.

You can use an Alpha pump either way.

The point comes back to the earlier one of what do you want the boiler
to do when the flow becomes low as the result of the TRVs beginning to
close.

If it;s a conventional boiler, you want it to be locked off by the
room thermostat when the flow becomes so low that the boiler begins to
short cycle as a result of the high boiler output vs. low demand..
That will happen if if the flow rate is low because the TRVs have
reduced it or if the return temperature is high because there is a
substantial amount of bypass. Therefore you have the room
thermostat in a room without TRV to put that additional control in
place and stop the boiler shortly after the TRVs have reduced the
load.

With a condensing boiler, you still want to detect this case, but
because the boiler will have modulated down adjustment will be easier.

It's useful to have a bypass anyway, to cover the case where the
boiler has been in full burn and the heat demand is suddenly satisfied
- this is the pump over-run case. Some boilers have very low water
content and if they have an integral pump can live with an internal
bypass as well. If you have an external pump, then an external
bypass is normally used anyway - in the form of a lockshield valve
across flow and return. Using an automatic bypass instead is a
better idea just for that since you are not shunting the flow back to
the return until either the CH has reached low demand or turned off
altogether.


.andy

To email, substitute .nospam with .gl


Well, I'll read all that later. I have to go and paint a door now.

I'm leaning towards the alpha and it's installation team!

Which one is you and wich one is Ed?! ;-)


Francis