I started up my solar heating this week and noticed that the energy usage on the Pentair Intelliflo pump was around 1650 Watts when it normally would be around 1500 Watts. The temperature of the water is currently 57F while our normal temperature when it gets fully heated is 86-88F. I thought that the difference might be due to the increased kinematic viscosity of the water at lower temperatures so I plugged the numbers into a spreadsheet I wrote for this (a while ago) here. Sure enough, at the 48 GPM flow rate I have (so using the 50 GPM column in the spreadsheet) I get a pipe head loss of 4.14 feet per 100 feet at 57F and 3.82 feet per 100 feet at 88F so would expect a 1500 Watt at 88F to go to 1500 * 4.14 / 3.82 = 1625 Watts assuming no significant change in pump efficiency (i.e. output power of Head times GPM to input power roughly the same).
At least for me, it's a really great feeling when chemistry and physics work out in a nice way to predict reality. Thank you to all of you experimenters and fellow inquisitive folks out there that have helped us figure out what really goes on in our pools. I reviewed a public draft copy of APSP-11 described here, here, here and here and was very pleased to see that it had information on the chlorine/CYA relationship, at least in a strong qualitative sense. I submitted comments to fill in more detail including a more quantitative sense and to show the true chlorine vs. pH graphs when CYA is present.