This setup in our barn combines one water heater for hand-washing/dishes/kitchen use and also hydronic heating for the slab in this small, well-insulated room.

A small circulation pump drives hot water through about 100' of PEX in a serpentine loop that we placed before pouring the slab. There's a 3" layer of spray foam beneath the slab to isolate it from the ground and so far it seems to work nicely.

Because this setup is seasonal it is configured to be easy to purge the slab loop with compressed air in the spring so that we can continue to use the (outrageously oversized) water heater for domestic hot water without breeding bacteria in the stagnant loop.