Oil in vacuum tubes

[WillyP]

OK, you're varying the volume of hot water, rather than varying the temperature of a fixed volume. That works to store energy too.
Do you recirculate from hot tank to make up for it cooling over time?

With an elevated hot tank, pumping would only occur when the sun shines, and gravity could feed hot water through radiators.
You would prefer to extract most of the heat in a single pass. Reverse-flow water to air heat exchanger?

Basically it is too heaters. One is run through the vacuum tubes. That will be two tanks. The other is the water for the floor. That will be stored underground in an insulated vault. There will be a discharge tank for water coming out of the floor. That will be above ground. It will be heated by the water in the vacuum tube system. The ground around the underground tank will be heated all year. So basically.
One system will be really hot (for the vacuum tubes) The other system will run at 95 degrees F. Oh and yes The underground tanks will be heated as well.