Just in case you didn't follow that, and I'm not sure I did, here is another very rough explanation of PP as explained to me by a Grundfos engineer.
Very basically, a pump could run faster and slower to maintain the same available head as rad valves are open and shut. When a valve is opened, more water can flow so the pump runs faster to maintain the same pressure. This would be a constant pressure setting.
Proportional pressure slightly over-compensates, so as the flow increases when you (or a TRV) open the flow to a rad, the pressure at the pump is not only maintained by increasing the pump rotation speed, but the pressure is actually raised a little. The reason for this is that the primary circuit (the pipe from the boiler to the motorised valve and the common return to the boiler) creates more drag when the water velocity is higher. So if the pump merely maintained a constant pressure at the pump, the pressure at each radiator would drop when all valves were open and each radiator would still receive a little less water than if only half were turned on. By over-compensating, the pump makes up for the increased drag on the primary circuit as velocity increases.
On the UPS2 (and I would imagine most pumps are similar), the PP1,2, and 3, differ in that the PP3 does the most overcompensation, and the PP1 does the least.
Just in case you didn't follow that, and I'm not sure I did, here is another very rough explanation of PP as explained to me by a Grundfos engineer.
Very basically, a pump could run faster and slower to maintain the same available head as rad valves are open and shut. When a valve is opened, more water can flow so the pump runs faster to maintain the same pressure. This would be a constant pressure setting.
Proportional pressure slightly over-compensates, so as the flow increases when you (or a TRV) open the flow to a rad, the pressure at the pump is not only maintained by increasing the pump rotation speed, but the pressure is actually raised a little. The reason for this is that the primary circuit (the pipe from the boiler to the motorised valve and the common return to the boiler) creates more drag when the water velocity is higher. So if the pump merely maintained a constant pressure at the pump, the pressure at each radiator would drop when all valves were open and each radiator would still receive a little less water than if only half were turned on. By over-compensating, the pump makes up for the increased drag on the primary circuit as velocity increases.
On the UPS2 (and I would imagine most pumps are similar), the PP1,2, and 3, differ in that the PP3 does the most overcompensation, and the PP1 does the least.
I would like to ask that grundfos engineer why in a 6M pump that they only allow a max 3M PP head, even the "cheap" pumps allow a 5 M PP head. I think that even the UPS3 only has a max PP setting of 3.5M.
If you take the above example that I used and assume a perfectly normal CH system. Ideally to set up the PP control, one should open up all the zone valves including HW and open all or any TRVs fully to get the maximum flow, you can then calculate the head and flow rate, I did mine by measuring the boiler deltaT and because I knew its output I was able to calculate the flow rate, I then looked at the (fixed) speed pump curve and was able to read off the head.
So if the head & flow required are 2.9M @ 15 LPM, you should then be able to set the PP control high enough to give you this but because the UPS2 can only be set to a max of 3M the head and flow will fall to 1.8M & 11.6 LPM so you are getting 77% of the max flow. IF you could increase that PP setting to 4M (3.8M to be exact) then you would have the required head & flow of 2.9M & 15 LPM on change over to PP mode. I know this is exactly how it works because I can set my Wilo pump any where between 0.5 M & 5.5 M PP head in increments of 0.1 M so if I set it to the 3.8 M above I will get that 2.9M head & 15 LPM, it will never go any higher but will ramp down as I described on reducing heat demand.