[thaumasiotes]

What does it mean for a heating furnace to be less than 100% efficient? Efficiency is normally measured in terms of how much waste heat you generate. If your goal is to produce heat, it isn't possible to produce any waste heat.

[quesera]

A lot heat is lost as exhaust into the air.

This can be a large portion, e.g. in older systems from the days when fuel was cheap, and pollution wasn't a topic.

Modern systems attempt to recover exhaust heat, and can be 90-ish% efficient.

[thaumasiotes]

Releasing heat into the air is the entire point of the building furnace. For the heat to get lost, it would have to leave the building.

There's a more obvious argument that leaking steam into the streets is a waste of heat, but that still depends on your goals. You might like having warmer streets.

[quesera]

> Releasing heat into the air is the entire point of the building furnace

You're thinking of forced hot air systems, but these do not vent combustion exhaust into living space. There are fuel remnants and combustion byproducts in the exhaust which would smell bad, and kill people.

Combustion exhaust, and all of the heat it contains, goes up the chimney and is lost. The living space air is heated by proximity to the combustion chamber, but circulates in a separate closed loop.

Also, in a forced hot water system, there is no air circulation at all. 100% of the air involved is heated and goes straight up the chimney.

Aside: Some people call FHW systems "boilers" instead of "furnaces", but this seems to vary by region. Where I'm from, "boilers" are steam systems only -- which seems reasonable, since water is not boiled in a FHW system.

[thaumasiotes]

> You're thinking of forced hot air systems

> Also, in a forced hot water system, there is no air circulation at all. 100% of the air involved is heated and goes straight up the chimney.

Without disputing your other points, this isn't right. I'm not thinking of any particular style of system. The point of every heating system is to release heat into the air. The problem people experience is that the air they occupy is too cold, and the solution is to heat that air. The point of a system that circulates hot water through your building, or your apartment, or anywhere, is that the hot water will release heat into the air there. Heating the air is the only goal regardless of whether the system involves circulating any air.

The point that you can lose heat by venting hot combustion waste to the external atmosphere is well taken.

[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.

[elcomet]

And it is leaving the building. Think of a chimney, most of the hot air leaves the building.

[pests]

If the goal is to heat a certain building and the heat is being lost in the building the furnace is in or in the streets that is definitely a waste.