Classifying Hot-Water Heating Systems
Hot-water heating systems can be classified in a number of different ways, depending on the criteria used. Three broad classification categories based on the following criteria are generally recognized:
1. Type of water circulation
2. Piping arrangement
3. Supply water temperature
In all hot-water heating systems, the water is circulated either by forcing it through the line or by allowing it to flow naturally. The latter is referred to as a gravity hot-water heating system because circulation results from the difference in weight (specific gravity) of the water caused by temperature differences (heavy when cold, light when hot). In a forced-hot-water heating system, the accelerated circulation of the water can result from several commonly employed methods, including: (1) using high pressures, (2) superheating the circulating water and condensing the steam, (3) introducing steam or air into the main riser pipe, (4) using a combination of pumps and local boosters, and (5) using pumps alone.
The four principal piping arrangements used in hot-water systems are:
1. One-pipe system
2. Two-pipe direct-return system
3. Two-pipe reverse-return system
4. Series-loop system
These piping arrangements are described and illustrated in several sections of this chapter (see, for example, the sections, One — Pipe System and Two-Pipe Direct-Return System).
If a hot-water heating system uses supply water temperatures above 250oF, it is classified as a high-temperature system. High — temperature systems are used in large heating installations such as commercial or industrial buildings. A low-temperature system is one having a supply water temperature below 250OF and is used in residences and small buildings.