How does Underfloor Heating get the right temperature water via a Thermal Store?
This isn't a DIY project. You need to have someone who has experience engineering these systems give you input. Steam radiator systems are not something that a DIYer should be designing; there are a lot of design aspects that are not intuitive and need an expert's hand. 

On top of that, the question that you're asking here is one that you need to take up with the manufacturer of the floor heating system that you're considering and the manufacturer of the thermal storage and furnace system.

Again, this is not a DIY project -- it's not something where you can head on off to your local hardware store and they'll have all of these products in stock. You need to at least consult with all of the manufacturers of the components that you're considering, or you need to have an experienced firm design the entire system.