Heat Distribution Systems
Heat Distribution systems are categorized according the medium used to transport heat. The two most common types are:
1. Forced Air systems that use ducts and registers to distribute heated or cooled air, and
2.Hot Water and Steam systems that use pipes and radiators to distribute heat.
Some systems need no distribution. These include space heaters, room air conditioners and room heat pumps that are located where heating or cooling is needed.
Problems/Solutions with Air Distribution and Steam Heat Distribution
Air Duct System:
Air systems use ductwork to distribute heated or cooled air throughout the home. The duct system is a collection of tubes that distribute conditioned air to the home’s rooms. This system can make a big difference in both the cost and the effectiveness of heating and cooling the home. It also has an important effect on the comfort and health of the occupants.
Problems/Solutions with Air Distribution
If you are having comfort or air circulation problems in your home, investigate the ductwork to see if it is contributing to or causing the problem. Begin by locating the ductwork, tracing it from the heating or cooling unit to the rooms it serves. Now that you know where each branch duct leads, you are in a better position to ask whether your system is likely to be a problem or a big energy loser.
The Problems . . .
Blind-Alley Ducts
No Return-Side Ductwork
Disconnected, Torn, or Damaged Ducts
Uninsulated Ducts in Unconditioned Spaces Poor Duct Layout
Unbalanced Air Flow
Supply- and Return-Side Leakage
Health Hazards with Duct Systems
The Solutions . .
Inspection
Blower Door Tests
Finding Leaks
Areas to Seal
Duct Sealing Materials
Duct Systems in New Homes
.Hot Water Distribution
Hot water distribution, known as hydronic systems, use hot water to carry heat from a boiler where it is heated to roughly 200°F, through pipes to the rooms where it is needed. An advantage of hydronic systems is that there are no drafts or chilling effects from air moving over the body, as are characteristic with forced air systems. Another advantage is controllability. By using zoned valves or circulating pumps, different temperatures can be maintained in each area of the home, thereby improving comfort.
Hydronic systems distribute water by either forced circulation or by gravity. The two systems are easy to distinguish by following the flow of hot water from the boiler through the distribution system. In a gravity system, the pipe will slop downward toward the boiler. In a forced circulation system, a circulating pump forces water into the boiler and then through the distribution system. Also, pipes used in forced systems are smaller than those used in gravity systems.
In a one-pipe hot water system, both the supply pipe and the return pipe of each radiator are connected to a single main line running from the boiler. As the water flows in a one pipe system from radiator to radiator down the line away from the boiler, the temperature of the water supplied is successively lower.
The size of the radiators must therefore be increased successively, in order to produce a balanced system. This is not necessary in two pipe systems where the supply pipe and return pipe are separate.
Because they both use pipes to distribute heat, hot water and steam systems may be confused. You can distinguish between them by looking for the air relief valve and traps on the radiator, which will be present in steam, but not in a hot water distribution system.
Hot water systems need some way of adjusting to the expansion of the water when it is heated, and of protecting the system if steam is accidentally produced. An expansion tank partially filled with water is installed in the system to meet this need.
Steam Heat Distribution
Steam is a good heat distribution medium because it moves easily through the system and it has an extremely high heat-holding capacity. Steam’s heat-holding capacity is much greater than that of water. Consider that when one pound of water moves heat, it gives up one BTU for every degree Fahrenheit it changes in temperature. That means if water in a distribution system dropped from 180°F to 100°F, it would give up only 80 BTUs. One pound of steam on the other hand gives up 970 BTUs when it condenses. This is known as its latent heat of vaporization. So you can see how steam is able to carry such an enormous amount of heat.
Generally found in homes built before World War II, steam systems are similar to hot water systems. Steam systems operate at very low pressures and generally under 220°F. No circulating pump is required to distribute the steam throughout the home. It rises naturally, then as it gives up its heat to the surrounding air in the home, it begins to condense and must be either gravity fed or pumped back to the boiler.
The steam system’s heat exchanger is a metal element that separates a combustion chamber from a water tank. The tank can be located above the combustion chamber, or wrapped partially or completely around it.
Enough space is provided above the water line in the tank to allow for evaporation. This space is called the “steam dome.” The supply pipe, located well above the water line, carries steam out of the boiler and up to radiators in the conditioned portion of the home.
The steam’s heat is transferred into the living area through a metal radiator or convector, which is heated by the steam and transfers that heat into the room both by radiation to people and objects in the room and by conduction and convection to the air that contacts the radiator. The heated air then forms convection currents, which carry the heat throughout the room. Steam heat can be distributed by one or two pipe systems. You can tell which is which by determining how many pipes enter and leave the radiator.
And Furthermore . . .
1. ONE PIPE SYSTEMS 2.TWO PIPE SYSTEMS
ONE PIPE SYSTEMS
One pipe systems are the simplest and easiest hydronic systems to understand and install. As the name implies, one pipe systems have a single pipe to the radiators, which serves as both a steam supply and a condensate return line. As water in the boiler is heated and transformed into steam, it rises and travels upward through the supply pipe to the radiators. The supply/return pipe is located at the base of the radiator to allow for condensate return. As the steam enters the radiator it displaces trapped air. Each radiator is equipped with an air vent to allow release of this air. Air vents close when the steam reaches them ensuring that only the air escapes and the steam stays in the radiator. 
As heat is released from the radiator, the steam condenses and water accumulates. Radiators on one-pipe systems are angled down-ward to allow condensate water to flow back to the boiler, and their pipes are usually greater than 2 inches in diameter to allow the water and steam to pass simultaneously. Since the condensate flows opposite to the flow of steam, these systems can sometimes be noisy.
TWO PIPE SYSTEMS
Two pipe systems allow steam to rise from the boiler to the radiators just as in the one pipe system. However, the condensate returns to the boiler through a second pipe.
Two pipe systems have temperature activate traps located at the end of the flow through the radiator, at the start of the return pipe. When the radiator is cold the trap is open. As steam enters the radiator it pushes the cold air in the radiator out through an air vent. When steam reaches the trap, the high temperature causes the trap to close thus keeping the steam in the radiator. When the room reaches the thermostat setting, the boiler shuts off and steam flow stops. As the radiator cools, the water vapor condenses and starts to fill the radiator with water. When it cools sufficiently, the trap opens and allows the condensate to flow back down through the return pipe.
Steam traps should be checked frequently. If they stick open, steam will flow through the radiator and down the return pipe reducing the efficiency of the heat transfer and perhaps unbalancing the entire distribution system.
Source:Home Energy Library
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