Originally Posted by
therealfronty
Hmmm this is interesting, new user here and just seen this thread, I also have a Grundfos Alpha2 pump like this, also a Viessmann 26kw boiler, except mine is y-plan open vented, and I have 13 radiators. I also have struggled with the pump settings, for years I have fiddled with the different settings and found that the middle fixed speed setting "II" (pump shows around 20w consumption) is the only one that provides a satisfactory amount of heat with an acceptable level of noise.
I just recently had another play, reading the instructions it suggests that auto-adapt or PP2 are the best settings for a system like mine, and I would love for the pump to auto-adjust based on how many TRVs are open, but with both settings the pump only draws between 5-8w and the boiler will shut down after a few minutes because the flow temperature overshoots the set point, you can see the flow temp slowly drop as the pump continues to run until eventually the boiler fires and overheats the water again and so on - I end up waking up to a cold house in the mornings. I think the water simply isn't moving fast enough through the hex. I tried CP2 as well with similar results, it's just not running fast enough. On fixed speed 2 everything runs fine, the boiler stays lit and the flow temp stabilises around 68-69 degrees.
It's a shame there isn't a way to adjust these PP settings, because I have this fancy pump with all this "intelligence" built in but end up ignoring all that and just running it like a standard fixed speed pump. There is a slight noise from some of the radiator return valves but it's bearable, what makes more noise are the motors in the HR92, the one in our bedroom wakes me up, but that's another topic!