"Andy Hall" wrote in message
...
On Wed, 30 Jul 2003 18:25:49 +0100, "IMM" wrote:


"Andy Hall" wrote in message
.. .

BTW, how does the Alpha know when to restart? Does it "poll" at a
certain
frequency to check the resistance?

The Alpha doesn't stop and start of
its own accord, it simply winds
down to a lower power with increasing resistance.

I was informed by a Grundfos rep when they first came out that is
stopped.
Maybe he didn't know too much as it was a brand new product.

Possibly it did to begin with. I have one for my workshop circuit and
it definitely doesn't stop, although it it does drop power level
considerably. I tested it using the ball valves that I have to
isolate it.

How slow does it get to? If it is very slow pumping nothing it is probably
OK.

When the Alpha detects pressure it reduced the speed
thinking TRVs are cloosed down.

I believe it will still deliver enough to open
an auto bypass on its bottom end setting.

If that is the case then all soved by inserting an auto by-pass. The pump
just keeps turning until a TVR opens up, the the auyto by-pass will slowly
close.

A permanent by-pass is the best option.
The flow switch detecting if the TRVs are closed should not be in the
circuit of the by-pass.

The trouble is that flow switches (apart from expensive industrial
process control ones) are not that accurate in terms of operating at a
certain rate. They tend to be either a gravity or spring assisted
mechanism.

The unit sold by DPS uses a magnetic switch. They are quite accurate in
detecting 2 litres flow per min. I have used them and they are fine.

I have one on the secondary side of my workshop
circuit. The pump is located in the workshop and operated by a
thermostat.

A pipe stat detecting heat?

When water flows, the flow switch detects it and
operates a 2 port zone valve to the primary side of the heat
exchanger, and of course fires up the boiler. This way, I don't need
any electrical control signals between the locations.



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