** DIY Solar hot water heating to extreme temperatures, 180 degrees is still pumping. below 27 degrees. and this morning it was -5 and do not freeze out even with antifreeze. The tubes are cold as ice and the inner tube hot as hell. I would not use it much, but if I did would be between 90-120 degrees in the winter. Last summer I was using it and it was getting steam to 225 degrees. yes, wright, steam! INCREDIBLE!