HYDRONIC HEAT-DISTRIBUTING UNITS AND RADIATORS
RADIATORS, convectors, and baseboard and finned-tube units are
heat-distributing devices used in steam and low-temperature water
heating systems. They supply heat through a combination of radiation
and convection and maintain the desired air temperature and/or
mean radiant temperature in a space without fans.. In low-temperature
systems, radiant panels are also used.
Units are inherently self- adjusting in the sense that heat output
based on temperature differentials; cold spaces receive more heat
and warmer spaces receive less heat.
The term radiator, while generally confined to sectional cast-iron
column, large-tube, or small-tube units, also includes flat panel
types and fabricated steel sectional types. Small-tube radiators,
with a length of only 1.75 in. per section, occupy less space
than column and large-tube units and are particularly suited to
installation in recesses .Column, wall-type, and large-tube radiators
are no longer manufactured, although many of these units are still
The following are the most common types of radiators:
Sectional radiators are fabricated from welded sheet metal sections
(generally 2, 3, or 4 tubes wide), and resemble freestanding
Panel radiators consist of fabricated flat panels (generally
or 3 deep), with or without an exposed extended fin surface attached
to the rear for increased output. These radiators are most common
Tubular steel radiators consist of supply and return headers
with interconnecting parallel steel tubes in a wide variety of
and heights. They may be specially shaped to coincide with the
building structure. Some are used to heat bathroom towel racks.
Specialty radiators are fabricated of welded steel or extruded
aluminum and are designed for installation in ceiling grids or
floor mounting. An array of unconventional shapes is available.
Pipe coils have largely been replaced by finned tubes.
A convector is a heat-distributing unit that operates with gravity
circulated air (natural convection). It has a heating element
large amount of secondary surface and contains two or more tubes
with headers at both ends. The heating element is surrounded by
enclosure with an air inlet opening below and an air outlet opening
above the heating element.
Convectors are made in a variety of depths, sizes, and lengths
and in enclosure or cabinet types. The heating elements are available
in fabricated ferrous and nonferrous metals. The air enters the
enclosure below the heating element, is heated in passing through
the element, and leaves the enclosure through the outlet grille
located above the heating element. Factory-assembled units comprising
a heating element and an enclosure have been widely used.
These may be freestanding, wall-hung, or recessed and may have
outlet grilles or louvers and arched inlets or inlet grilles or
Baseboard (or baseboard radiation) units are designed for installation
along the bottom of walls in place of the conventional baseboard.
They may be made of cast iron, with a substantial portion of
the front face directly exposed to the room, or with a finned-tube
element in a sheet metal enclosure. They operate with gravity-circulated
Baseboard heat-distributing units are divided into three types:
radiant, radiant convector, and finned tube. The radiant unit,
is made of aluminum, has no openings for air to pass over the
side of the unit. Most of this unit’s heat output is by
The radiant-convector baseboard is made of cast iron or steel.
The units have air openings at the top and bottom to permit circulation
of room air over the wall side of the unit, which has extended
surface to provide increased heat output. A large portion of the
emitted is transferred by convection.
The finned-tube baseboard has a finned-tube heating element
concealed by a long, low sheet metal enclosure or cover. A major
portion of the heat is transferred to the room by convection.
The output varies over a wide range, depending on the physical
and the materials used. A unit with a high relative output per
length compared to overall heat loss (which would result in a
concentration of the heating element over a relatively small area)
should be avoided. Optimum comfort for room occupants is
obtained when units are installed along as much of the exposed
Finned-tube (or fin-tube) units are fabricated from metallic
tubing, with metallic fins bonded to the tube. They operate with
gravity-circulated room air. Finned-tube elements are available
several tube sizes, in either steel or copper—1 to 2 in.
or 3/4 to 1 1/4 in. nominal copper—with various fin sizes,
and materials. The resistance to the flow of steam or water is
same as that through standard distribution piping of equal size