I happened to hit this discussion through scanning the web...
I've been looking for an installed system of this type for some time to see how it would be done.
My idea would be for a new-home application where heat would be needed, but not something that may be absolutely crucial... In my case, a smaller cabin in a forested area near the California coast where temps are typically not much lower than the low 40's with no typical freezing, icing, or snow...
The system would have a sealed (outdoor input air only) thermal-fireplace (say, soapstone) with a water-heating coil inside the the firebox. The heated water from the fireplace would recirculate through an insulated hot-water storage tank by way of a pump. Once the fire was out, and the fireplace was loosing its thermal kick, the stored hot water would then be circulated through a radiant floor heating system (pex in slab) to further warm the house.
This could be a closed system with antifreeze, pressure relief valves, expansion tank, etc....
The obvious concerns are pressure and over-heated water--mostly from pumps failing. But some emergency options have been mentioned:
-- Dousing the fire.
-- If the water in the storage tank gets too hot, then it would sound an "alarm" and the radiant-heat floor pump kicks on to use the house slab as a kind of radiator to cool the tank and let you know to ease up on the fire.
-- Additional water could be added to the tank to cool it.
-- The addition of a small solar water heater panel could be added to the roof to preheat the storage tank (if needed), which could also radiate unwanted heat.
Any ideas or criticisms? All input welcomed.