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 tank. 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 Water-Heating  Systems Steam-Heating  Systems 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 systems. 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. 4-6