You will have a hard time running a solar system on low speed of a two speed pump. In order to do that, you will need to plumb the vacuum release valve at a low elevation right above the pad. However, I would not recommend this as it will also the require the panels to run under a slight vacuum. Plus you will also lose efficiency running the panels at lower flow rates. With a two story house and recommended VRV placement (i.e. top of panels), you will need to run a pump at high speed or a variable at fairly high speed in order to keep the pressure up and the VRV closed. This is why I have such a low HP two speed pump. I use high speed for solar and low speed when I don't need solar.