[lukeschlather]

There's an invisible component which is the efficiency of the furnace used to create the heat. Suppose every building uses a small furnace which is 30% efficient, then there's no visible waste heat in the streets. On the other hand, suppose a bunch of buildings use a single furnace which is 50% efficient and pipe the heat with 10% waste from piping. You're still substantially less wasteful to use the central furnace. There's no such thing as a system that doesn't generate waste, and the visibility or invisibility of the waste doesn't tell you how wasteful the system is.

[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.

[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.)