On Thu, 23 Sep 2004 09:18:51 +0100, Malcolm Reeves
wrote:
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.
Not according to the alpha data sheet which says:
Electronically controlled, the pump can, by means of a
selector switch located on the terminal box, be set to:
• 2 constant pressure curves
• 2 proportional pressure curves
• 3 fixed speed curves.
So either constant head (I think I said that), proportional head (head
goes down with lower flow), or 3 normal pump modes.
You need to look at the technical manual for the Alpha.
They have some other products which have different selectable
operating modes.
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.
I'm glad we agree it seemed to me before that you said the flow did
not reduce until the TRVs almost closed, which is of course not the
case.
No I didn't say that.
Protection of standard pump nuts can be achieved by a bypass.
Possibly, but there's no point.
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.
No. If alpha is in constant or proportion head mode flow decreases.
Otherwise it is in normal pump mode.
Have a look at the technical manual.
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.
Of course but what is the relevance of that. It will always cycle
when Watts in Watts out. We are discussing TRVs and pumps.
I know. The point was that the behaviour is not simple because the
burner is being fired on and off. This has a substantial effect on
the temperatures.
The heat output from the rads is the temperature
difference.
It is proportional to the temperature difference and flow.
No it isn't. Temperature output from a rad is proportional to the
average rad temperature (or rather difference to the room). The flow
does not come into the rad output.
From the perspective of heat output to the room, it is approximately
proportional to the Mean Water to Air Temperature.
From the perspective of heat delivery to the radiator (which will be
the same since none of it disappears) it will be proportional to the
flow and temperature difference across the radiator.
What happens is that as you restrict the flow the output temperature
drops so the average rad temperature making its output less.
That is because the rate of heat delivery to the radiator reduces.
Since the input water temperature is fixed
It isn't. It will move up and down as the boiler cycles.
We aren't talking about the boiler cycling but in any case at any
point in time the temperature in to the rads is fixed. It is the
output temperature that decreases, as the TRVS control it, in order to
lower the rad output.
You can't treat them in isolation though. If the boiler is cycling,
the output temperature is far from being fixed,
My point is that the flow varies since you seem to be saying that TRVs
only reduce the flow as the close completely.
I didn't say that at all.
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.
I'm glad we agree on that then as I read your post differently.
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?
Yes, have you?
Yep, and the technical manual.
Plus I have one of these and know how it behaves.
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.
I'd agree that the fine adjustment on the alpha is nice. But you
still have to operate the alpha in normal pump mode to use it with a
bypass. If the fine adjustment necessary, or worth the money? I
think we'll have to disagree on that.
Up to you. You can effectively replace the need for a bypass and
have quieter and better controlled operation.
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.
Why? The alpha's modes of constant and proportional head are designed
for system that work best with a variable flow. That has to be
boilers that modulate their output. Also, those boilers that are
happy with a low return temperature, which is another effect of low
flow.
Set it any differently and it's just a fine adjustment standard pump.
Even a fixed boiler has varying output temperature as the burner
cycles. There isn't going to be an issue with the return
temperature being low at low flow rates anyway.
..andy
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