1042 All Water Systems

Air is not a convenient medium for transporting heat. A cubic foot of air weighs only about 0.074 pounds (0.34 kg) at standard conditions (70°F; 1 atm.). With a specific heat of about 0.24 Btu/lb°F (0.14 joule/C), one cubic foot can carry less than 0.02 Btu per degree Fahrenheit temperature difference. By comparison, a cubic foot of water weighs 62.4 pounds and can carry 62.4 Btu/ft3.

Water can be used for transporting heat energy in both heating and cooling systems. It can be heated in a boiler to a temperature of 140 to 250°F (60-120°C) or cooled by a chiller to 40 to 50°F (4-10°C), and piped throughout a building to terminal devices which take in or extract heat energy typically through finned coils.

Steam can also be used to transport heat energy. Steam provides most of its energy by releasing the latent heat of vaporization (about 1000 Btu/lb or 2.3 joules/kg). Thus one mass unit of steam provides as much heating as fifty units of water which undergo a 20°F (11°C) temperature change. However, when water vaporizes, it expands in volume more than 1600 times. Consequently liquid water actually carries more energy per cubic foot than steam and therefore requires the least space for piping.

All-water distribution systems provide flexible zoning for comfort heating and cooling and have a relatively low installed cost when compared to all-air systems. The minimal space required for distribution piping makes them an excellent choice for retrofit installation in existing buildings or in buildings with significant spatial constraints. The disadvantage to these systems is that since no ventilation air is supplied, all-water distribution systems provide little or no control over air quality or humidity and cannot avail themselves of some of the energy conservation approaches of all-air systems.

Water distribution piping systems are described in terms of the number of pipes which are attached to each terminal device:

One-pipe systems use the least piping by connecting all of the terminal units in a series loop. Since the water passes through each terminal in the system, its ability to heat or cool becomes progressively less at great distances from the boiler or chiller. Thermal control is poor and system efficiency is low.

Two-pipe systems provide a supply pipe and a return pipe to each terminal unit, connected in parallel so that each unit (zone) can draw from the supply as needed. Efficiency and thermal control are both high, but the system cannot provide heating in one zone while cooling another.

Four-pipe systems provide a supply and return pipe for both hot water and chilled water, allowing simultaneous heating and cooling along with relatively high efficiency and excellent thermal control. They are, of course, the most expensive to install, but are still inexpensive compared to all-air systems.

Three-pipe systems employ separate supply pipes for heating and cooling but provide only a single, common return pipe. Mixing the returned hot water, at perhaps 140°F (60°C), with the chilled water return, at 55°F (13°C), is highly inefficient and wastes energy required to reheat or recool this water. Such systems should be avoided.

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