I swapped out my Taco 007 primary circ pump over the summer with a Grundfos 15-58 three speed pump. I've been toying around with the speed settings this week during good "normal" burns on my EKO. I began having this strange thought that low speed might be best since the water "sits" and heats longer in the HX tubes vs. the higher speeds. But at the same time the lower pump speed means I'm moving less hot water to my tanks. Just this morning I threw in the complication of my Danfoss valve. When my tank bottom temps are below 140 degrees (which is quite common for me) and I'm running on high speed aren't I just running more hot water through my bypass and not my tanks at all? I'm just wondering if anyone else has played with thier primary pump speeds like this and noted any results? I've convinced myself that low speed does seem to produce an increase in boiler temp vs. high speed. But I'm still trying to decide if this is overall providing better system operation, or is it worse. I monitor my tank temps during this "toying" but I don't have flow meters so it's hard to tell exactly the why/when of the temp increases on my tanks. Thanks in advance for any thoughts from the Boiler Room!