[quesera]

I think we were talking about furnace efficiency. This is measured without respect to the heating distribution elements. You don't need to have a distribution system connected to measure furnace efficiency.

So when I said that 100% of the air involved in a forced hot water system is used for combustion and exhausted, I mean the air at the furnace, not the air that is indirectly heated in the living space.

Agreed that the goal is to heat the living space though, including the air. A forced hot air system does that directly (pushing heated air into living space), and forced hot water does so indirectly (pushing hot water into radiators in living space, which heat the air by convection).

Radiant floor heat is another example where there's no air at the heat source.