On Wed, 30 Jul 2003 23:31:20 +0100, "IMM" wrote:



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.

I haven't measured it, but the pump certainly backs right off as the
flow is restricted by closing the valve. The data sheet says that
the minimum level is around 0.4 cu.m/hr






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.

This is what I'm using, although I did have an issue with DPS. They
originally sent a duff one that had obviously been used before in
something. Admittedly they did immediately send a new replacement,
but I don't appreciate receiving secondhand items. I am not sure
that I would do business with them again.

The switch is certainly sensitive so does its job as I require it.
However, it has no form of adjustment, so in an application where the
need was to control something based on a specific flow rate I am not
sure how successful it would be. Also, if the switch were used to
control the pump, you would create positive feedback. The switch
operates to stop the pump when the flow stops. However when the flow
does stop the switch operates again and the pump starts. Some form
of logic using relays such that the flow switch triggers the pump
going off and it stays off until some other event starts it again.
Something like the thermostat causing heat demand might have to be the
means to reset everything.



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?

No, the logic is the other way round. The thermostat in the
workshop is a room type . In fact I did rather well. I happened to
be in Homebase one day (I don't normally use them because of their
poor customer service) and they had a table of clearance items. I
picked up a Landis & Staefa REV100 controller with time setting, set
back, proportional control and LCD touch screen for £12. List price
is nearly £100. There's even a set of auxiliary switch contacts so
that you can override the heating to off. Apparently the Germans use
this feature connected to a window switch so that when they take the
bedclothes and hang them over the balcony rail of the apartment in the
morning, leaving the window open, the heating is cut off.
For me it's ideal because I can automatically turn off the heating
when I open the door of the workshop for any length of time.

The controller simply turns the pump in the workshop on and off as
required. This causes water to flow in the secondary circuit.
This is picked up by the flow switch in the house and that in turn
operates the motorised valve controlling flow from the primary circuit
through the heat exchanger.

It works quite effectively. Ultimately I'll tie it in to the rest of
the house system probably using some form of RF telemetry.




..andy

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