small supply ducts to the areas to be heated. The air is
then returned through return ducts to the furnace for
reheating. Outside air can be supplied to the return ducts
for a continual supply of fresh air.
Forced-air furnaces are controlled by two thermo-
stats: a room thermostat to control the burner and
another thermostat to control the blower. Most of these
furnaces have filters that eliminate any solid particles in
the air before it is heated. These furnaces are also
frequently equipped with humidifiers to replace
moisture that has been removed from the heated air.
Ducts for forced-air furnace systems can be round,
square, or rectangular in shape and can be fabricated
from tin-plated steel, fiberglass, or more commonly,
galvanized sheet metal using methods discussed in
chapter 11 of Steelworker 3 & 2, NAVEDTRA 10653-G.
Insulation for the ducts usually consist of 1/2-inch to
2-inch-thick fiberglass or rock-wool blankets wrapped
around the ducts.
Supply and return outlets may be located in walls,
ceilings, or floors. The cover for the outlet may be a
decorative grill that covers the end of the duct opening,
or it can be a register that can be adjusted to vary the
amount of airflow. Diffusers are used to direct the flow
of air. They can be either adjustable or nonadjustable
and can also include a register. Supply outlets carrying
only hot air are best located in or near the floor. That
way, the hot air is introduced to the coolest part of the
room, and the cold air is returned through return outlets
located near or in the ceiling. When the ducts are used
also for supplying cooled air, then the opposite location
arrangement is best. A small building, such as a
residence, may have a single return air grill located in a
central hallway. In this case, doors leading to the hall are
undercut by about 1 or 2 inches.
For a more thorough discussion of warm-airheating
systems and equipment, you should read chapter 9 of
Utilitiesman 2, NAVEDTRA 10662.
out of the system through thermostatically controlled air
wolves at the radiators. When the air has been expelled
and steam reaches the valve, the valve closes
automatically. As the steam gives up heat through the
radiators, it condenses and runs back to the boiler
through the bottom of the supply piping. In the one-pipe
system, the mains must be large and sloped to allow the
condensate to flow back to the boiler without interfering
with the flow of steam.
In a two-pipe system, the steam flows into one end
of the radiator and out the opposite end through a
thermostatically controlled drip trap that is set to open
automatically when the temperature drops below 180°F.
When enough condensate has collected in the radiator
to cool it, the drip trap opens, allowing the condensate
to flow into return lines where it is carried to a collecting
A radiator used in a steam- (or hot water) heating
system usually consists of a series of interconnected,
vertical cast-iron sections. As the steam flows through
the radiator, the surface of the sections radiates heat to
the surrounding walls, objects, and the surrounding air.
As the surrounding air is heated, it rises towards the
ceiling, setting into motion a convection current that
transfers heat throughout the room.
Convectors usually consist of iron or copper pipes
surrounded by metal fins and are most often placed near
the floor. Openings at the top and bottom of the
convector unit allow circulation of air over the fins. That
movement of air over the fins transfers heat to the
surrounding area. Small connectors placed around the
base of the wall are termed baseboard heaters.
For a more thorough discussion of steam-heating
systems and equipment, you should read chapter 7 of
Utilitiesman 2, NAVEDTRA 10662
Steam-heating systems consist of a boiler, a piping
system, and radiators or connectors. The boiler is fired
by oil, gas, coal, or electricity. Although there are many
variations and combinations of steam-heating systems,
they are all basically either one-pipe or two-pipe
The one-pipe system uses the same pipe to convey
the steam to the radiator and to return the condensate to
the boils. When the unit is started, the steam pushes air
A water-heating system includes a boiler, a piping
system, radiators or connectors (discussed above), and
a water-circulating pump that is used to force the water
to the radiators or connectors and back to the boiler. For
water heating, three types of piping systems are used.
The one-pipe system (fig. 4-6) consists of a single
supply main that carries hot water to each radiator in
turn. To overcome a loss of water temperature at each
successive radiator, you must balance the size of the
piping or the orifice at the radiator.