Hi, I have a Grundfos UP15 42RUC7 circulator which doesn't appear to be working. the impeller moves freely when I spin it manually but when there is power tot he device all I hear is humming but the impeller doesn't turn even though it does not appear to be stuck. Also, oddly I can not find my model # anywhere for this Grundfos. The closest I have found is UP15-42BUC7 but you can see clearly in the picture I have attached that my model is an "R" and not a "B". Any help would be appreciated. As mentioned - power appears to be at the unit and the impeller is not turning even though I can spin it freely by hand - thanks