As I don't know how to post pics. I will try to describe a new design I'm working on.
Burnham Revolution RV3 boiler, heats 120 gal. DHW/Buffer storage tank (called Tank from here on) via flat plate HX.
30* delta aquastat on Tank controls boiler fire. (say 140*-170*) DHW output is tempered down with 3-way thermostatic valve.
On Boiler side of HX (Taco 007) pumps into (HX-side-A-in), through HX, (HX-side-A-out) & back to boiler. A purge delay relay will take left over heat out of boiler at end of fire.
On Tank side of HX, (Taco bronze 006, runs continuously) pumps from bottom of Tank into (HX-side-B-in), heat is gained in HX,
(HX-side-B-out) back into top of Tank.
(Taco 006 is enough pump for 50K BTU heat transfer @ 6 GPM + keeps tank destratified)
System can draw heat during the above Tank heat storage cycle or long after, from the Tank, where it performs the Buffer function of preventing short cycling in shoulder weather.
If the boiler is off, the HX transfers heat in the reverse direction from side-B to side-A & draws down the Tank temp. The water flow in the HX does not change direction on either side.
When hydronic slab system thermostat calls for heat, its (Taco 007) pumps cool system return water (say 80*) ALSO into (HX-side-A-in), heated in HX, (HX-side-A-out) (say 110*) to a 3-way therostatic valve which tempers it down (say 100*) then provided to slab.
This design should provide long infrequent burns with minimal components.