"Andy Hall" wrote in message
...
On Wed, 22 Sep 2004 21:52:50 +0100, Malcolm Reeves
wrote:

On Wed, 22 Sep 2004 14:00:09 +0100, Andy Hall
wrote:

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.

OK. Nothing is perfect but the TRVs will close enough to reduce the
flow otherwise what are they doing, nothing. The flow has to reduce
to lower the average radiator temperature and so lower its output. So
at the bit of the alpha curve where the head rises to keep the flow up
then all that will happen is the TRVs close more. They must reduce
the flow.

The flow reduces naturally as a result of the increased restriction.
The point is to reduce the power to the pump to stop it from screaming
its nuts off.



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.

The heat flow from the boiler is the flow, kg/s, and the difference
across the boiler.

Only while the boiler is firing. At the point where the heat used is
less than that produced it will begin to cycle.


The heat output from the rads is the temperature
difference.

It is proportional to the temperature difference and flow.

Since the input water temperature is fixed

It isn't. It will move up and down as the boiler cycles.

then TRVs must
slow the flow down so the output temperature is lower and thus the
average water temperature lower. Ergo TRVs must reduce the flow AT
any time they are having an effect. NOT just when they are about to
shut the rad off completely.

That wasn't my point.



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.

So what is the point of an alpha? If the bypass is kicking in before
the alpha "effect" why not use a normal cheaper pump! If the alpha
"effect" kicks in the bypass won't open.

That would depend on how the bypass is set.

Have you read through the Alpha datasheet?

Both effects are driven by
pressure, ones got to happen first then the other won't.

You're missing the point. It is far better to have the pump reduce
output as the heat flow reduces rather than shortcircuiting it back to
the boiler with the pump on full power.



An alpha would suit something like a coal fired room heater where the
best way to run it is slow continuous release of heat, not
super-burn/off/super-burn/off as you get with normal pump + stat (no
bypass). Also a condensing modulating gas boiler (as long as it could
cope with low flow (coal fires can).


This misses the point completely.



.andy

To email, substitute .nospam with .gl



Well, the door is painted, a nice grey. Gunmetal, actually.

You guys seemed to be enjoying yourselves while I was away.

I have upstairs on a separate zone, with TRV's on all but one Rad. I've just
installed a wireless stat in that room.

Don't know about the pump yet. I'll probably go for the alpha and i can
always close the abv fully if necessary.


Francis