Hot-water heating systems use water as the medium for conveying and transmitting the heat to the various rooms and spaces within a structure. The motive force for the water in these systems is based either on the gravity flow principle or forced circulation. The latter type (referred to as hydronic, or forced-hot-water heating) is the one currently used in residential and commercial heating system. In a typical hot-water heating system, the water is heated in a boiler or water heater and circulated through pipes to baseboard convectors or radiators located in various rooms. The energy source used to heat the water may be oil, natural gas, propane, electricity, or a solid fuel, depending on the heating unit.
The hot water is circulated through pipes to baseboard convectors or radiators located along the walls of the rooms, or through radiant panels installed in the floors or ceilings. A centrally located thermostat controls room temperatures in smaller houses or build- ings. When the thermostat calls for heat, the boiler or water heater heats the water and sends it to the room convectors or radiant heating panels where it is released and distributed through the rooms by natural convection. In larger houses or buildings, the heating sys- tem is zoned with individual thermostats controlling the temperatures in each zone.
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 250°F, 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 250°F and is used in residences and small buildings.