That means the whole thing is incredibly wasteful. I always wondered why nobody seems to care about the huge amounts of energy being wasted in NYC in this way.
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.
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.
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.
> 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.
> 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.
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.
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.)
In NYC the steam is from co-generation facilities which produce electricity first, so the steam is essentially waste product to begin with – in most coal plants for example you'd just have giant steam stacks to condense the water again. But yes, it's all very normalized waste.