bob villa says:
http://www.bobvila.com/ArticleLibrar...WaterHeat.html
"With traditional baseboard heat, water leaves the boiler at
approximately 180°F to 200°F and is sent through pipes throughout the
house via a pump or circulator. The hot water enters various heating
loops and conducts heat to the pipe, or panel to which it is connected.
Once the heat has been transmitted to the surface or fins, the air
within the room is warmed principally by convection. In this system,
cold air is drawn across the warm surface, absorbs warmth as it passes
and then moves upward. Cool air from below the heating unit is
constantly being drawn into the unit as the warm air rises and flows on
to spread warmth throughout the room.

Baseboard Heat
The most common type of hot water heat in the United States is
finned-tube baseboard heat. Located close to the floor, finned-tube
emitters are usually housed in metal enclosures that have reflective
material behind them to encourage heat transfer from the fins or slats
to the air passing over them, rather than being lost via conductivity
through the cold wall behind it. The hot water pipes emerge from the
floor and run directly behind the enclosure with thin, metal slats or
fins, placed close together all along the pipe. Heat is transferred
from the water to the pipe and ultimately to each of these fins. The
fins serve as multiple heat emitters, warming the cold air from below.
Enclosures must be located at least one to two inches above the
finished floor to allow for adequate air circulation and heat transfer.

Piping
There are actually a number of different layouts for hydronic heat, but
they all serve the same principle: Hot water leaves the boiler in a
supply pipe, transmits its heat along the way, and returns to the
boiler as cooler water (typically 150°F to 170°F) , ready for
reheating and redistribution. The basic variable in the equation is how
long the hot water stays out there before returning to the heat source.
Obviously, if there are a number of registers or panels along the way,
the water will steadily lose heat, delivering most of its heat in the
first registers and imparting less warmth to those registers at the end
of the loop.

To account for this strain on the system, different piping layouts are
used and zones are incorporated to provide more even heat throughout
the home. The simplest of all piping layouts is the series or single
loop system. Here, water simply flows through the pipes, into each of
the radiators along the way, without diversion. Since the water will
have cooled about 20°F by the time it reaches the last radiator, this
last emitter is typically sized much larger to take full advantage of
the heat remaining in the water. A one-pipe system is slightly more
elaborate and provides tees to divert water into each emitter along the
way. In this way, radiators can be individually controlled so that heat
need not necessarily always go to the first room on the loop."