Thanks for the feedback
The current goals for the sand slab is really to act as a moderator. I want to keep it between 100F-86F degrees during the heating season. So my current quest is for generating water up to 110 degrees during the heating season.
Here is how I'm thinking I will use the sand slab. Charge it during late summer with solar thermal drainback system that could also be used during sunny winter days.
Once the sand slab is charged there will be days it can handle the heating alone and other days will it be charged using some sort of heat-pump.
On the coldest days the sandslab can boost the water temperature with could be a problem with an air source heat-pump.
I have just been doing some calculations to figure out the quantity of btus I can store and looks like I can store 341,100 btus. That I can use for passive heating of the my office or boosting the water temperature of the system or both.
I got the numbers based on these links:
Using Sand to Store Solar Energy | GreenBuildingAdvisor.com
Sand bed storage for solar homes