16 Heat emitters/Terminal
units—Modern hydronic
systems can have many types
of radiation to provide heat throughout a
structure, including baseboard, convectors,
radiators, hydro air coils, radiant,
etc.
Heat emitters receive heat from the hot
water circulating through the piping and
deliver heat to the areas where they are
installed.
Types of Heat Emitters
(aka terminal units)
Typical hot water heating systems
deliver heat by convection. They circulate
water through heat emitters, warming
the air around them. As the air is heated, it
expands, becomes less dense and rises. As
the warm air rises, the colder air it displaces
falls, creating heat convection currents.
These convective currents don’t require a
mechanical means to move the air.
Baseboard Heating
Finned-tube baseboard and cast-iron
baseboards use convection to circulate
heat. Cooler air at the floor level enters
the bottom of the baseboard and because
of the heated material in the baseboards,
heat is transferred to the air which then
moves up the wall. Baseboard location is
usually along outside walls to help these
convective currents and to remove the
feeling of a cold wall or window.
Finned- tube baseboard features copper
tubing with aluminum fins attached.
Manufacturers have different types, sizes
and numbers of fins per inch. With these
variables, the manufacturer rates the Btu
output per linear foot based upon a given
supply water temperature. Typical finnedtube
baseboard is ¾" copper tubing with
½" and 1" available. One inch is typically
12 National Oilheat Research Alliance
for commercial applications and has a
higher heat output, ½" and ¾" are similar
in output, but overall length of the loop is
shorter with ½".
Finned-tube baseboard heats up
rapidly and dissipates its heat quickly.
Surface temperatures of the units are not
an issue as they are protected with covers.
This system works well when there
is enough clearance to create convective
currents. Cool air enters the bottom and
the heated air comes out the top front
grates. Furniture placement in front
and proper space under the cabinet are
critical for the finned-tube baseboard to
work properly. See below, Figure 13 for
Ratings Charts.
Cast iron baseboard heats up slowly
and cools off slowly. Surface temperatures
are high and there are no protective
enclosures. Cast iron baseboard
also provides radiant heating because
the large mass of iron stays hot even
after the circulator shuts off, and slowly
releases energy into the space.
Convectors
Convectors are similar to fin tube
baseboard, but tend to be higher, thicker,
and not as long. They heat up quickly
and cool off quickly. Surface temperatures
are not an issue as they are protected
with covers. Like finned-tube baseboard,
they need enough room in front to
create convective currents.
Radiators
Cast iron radiators deliver heat in a way
that is similar to cast iron baseboard; they
heat up slowly and cool off slowly, but surface
temperatures are high and there are no
protective enclosures. They are relatively
large and have higher water content than
baseboard heating.
Figure 13
Hot Water Ratings: BTU/HR. per linear ft. with 65°F entering air
Element
Water
Flow
Pressure
drop+
110°
F
120°F 130°F 140°F 150°F 160°F 170°F 180°F 190°F 200°F 210°F 215°F 220°F
NO. 30-75
baseboard with
3/4'' E-75 element
1GPM 47 160* 210* 260* 320* 360 450 510 580 640 710 770 810 840
4 GPM 525 160* 220* 270* 340* 400 480 540 610 680 750 810 860 890