I had a UPS2 fitted about a year ago and it's running fine on fixed speed 1. I have been thinking of setting it to a variable speed but am reluctant to do this as I can't work out what the effect will be. One problem is that there is very little difference between the three speeds. This chart shows the current working point, obtained from data provided by my Remeha Avanta 18V boiler - downrated to 15kW.
The pump is producing a head of 3m with a flow of 0.8m³/hr (equivalent to 15kW with a 16C differential).
I reckon that based on your above info that you would now have a flow rate of 10.05 LPM (0.6 M3/hr) @ 1.7 M head and a boiler deltaT of 21.4C.
Unfortunately I cannot upload the S.sheet (my own) to show the method that I used to arrive at the above but its pretty accurate based on my own findings.
I agree that the UPS PP speeds are far too close, they should certainly have a PP setting of 4 & 5 IMO.
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What I would like to know is: what will be the operating point if I select one of the variable speeds? If the flow rate increases, that's no problem as the rads will produce more heat. But if the flow rate decreases could the flow/return differential exceed the recommended maximum 20C (o.65m³/hr)?