For a building that is centrally heated by local fuel combustion (i.e. heating within the building rather than district heating), one may distribute the heat throughout the building using as a energy carrier either water or air. Which type is more efficient? On what circumstances does it depend? Water carries a lot more energy per unit mass, but with air heating we can dispense of radiators that transfer heat from water to air, and air heating might more easily allow for heat exchangers, on the other hand you'd still need a separate system for hot water. Are there studies comparing the overall efficiency between the two, given typical household use in a single family home?
Of course, there are many technologies more sustainable than either, with passive houses as the gold standard, but my question here is assuming we are still in a phase of central heating by local fuel combustion (such as natural gas).