I have to agree completely with jet1749 with the exception that the pump is enormously oversized and the primary problem with this installation.I also drew out the pump performance curve and overlayed it with a system curve to give an idea of what is happening. I made a couple of assumptions by using 1" steel pipe for the friction loss calculation, didn't allow for any valves or the heater and you have not clearly defined the total static head on the pump so this is not included.For a flow of 12 l/m you need less than 0.5 metre head at the pump not allowing for, the heater, valves and controller or static head etc. You are therefore throttling or trying to somewhere near 19m metres of head across the  regulating valve - I would suggest that such a turn-down is not within the capabilties of the RV and is the secondary problem with the system, the first being the oversized pump. For this application all you need is a hot water circulator ie, Grundfos or Danfoss etc capable of 12 l/m  at 2 -3 meter head.
As we don't have access to all the detail on this pump unit, a number of assumptions need to be made, one of which is the stability of the pump at 12l/m, one other is the capability of the RV to control the flow at such a massive "turn-down" coupled with the pumps performance at near to shut-head. Just because the supplier has given a flow at 20l/m doesn't mean the pump is stable at or beyond this point- anything could be happening, stall, massive recirculation -a complete collapse of H/Q. However, I know little about RV valves but suspect that trying to control the Magnetic pump output from the RV from 20 + meters to a couple metres head with what could be a very unstable pump output may be well beyond the control of the valve. A well sized pump operating somewhere near to its BEP in conjunction with a RV - should be able to give a stable controlled output.

2011-08-11

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