Hi,
I have a pellet heating connected to solar panels. The installer first put a Wilo Pico Plus pump in the solar.  (long story about a missing part from Oekofen)
Over the first summer I had about 15 events of stagantion (140°+)
After I insisted he replace it with the intended modulating pump I have had no cases of stagantion.
Any technitions out there that can comment on this. 
The installer insists that I suffered no dissadvantage from the origonal pump, even said it was better, as it was cheaper to run in electrical terms.
Be very grateful for comments