[MagicMoonlight]

Furnaces are almost 100% efficient because you're literally producing heat. There's very few ways to burn fuel and not get that fuel to produce heat.

This kind of argument works for power stations, but it doesn't work for heat because it's obviously not more efficient to heat 13 miles of piping in addition to your building rather than just burning stuff in your building.

[quesera]

An old but common oil-fired furnace for a forced-air system will be about 70% efficient at heating air which will be delivered to the living space. This is measured at the egress point of the heating chamber, which is distinct from the combustion chamber. It is not measured at the living space, because there are other losses in distribution (uninsulated and/or leaking ductwork, etc).

The rest (almost 30%) goes up the chimney, or radiates out from the furnace itself. It's heat, thermodynamically speaking, but it is not useful heat.

(Some is also lost to light energy, and some fuel is not fully-combusted. These account for a small but measurable loss.)