Single Duct Systems

Single-duct systems may serve one or many zones. In small buildings, a single-zone air-distribution system with a master thermostat regulates the temperature for the whole building. Such a system has a very low first cost.

A multizone single-duct system is a collection of up to eight single-zone systems served by one supply fan. Separate ducts from the central air-handling unit serve each zone with its own centrally conditioned air stream. Multizone systems have very large distribution trees. Energy is always being expended for ventilation, even if there is no need for heating or cooling. Multizone systems are used in medium-sized buildings or in large buildings with central equipment on each floor.

The constant air volume (CAV) system is the simplest central air-conditioning system. Single-duct CAV systems supply conditioned air at a constant temperature through a low-velocity duct system. Fans circulate air past heating and cooling coils through a duct system that delivers it to all rooms. Heated coils are active only when the building requires heat. Small systems use a firebox burning gas or oil instead. Cooling coils in residential systems can be the cool side of a refrigerant system connected directly to a compressor. In larger buildings, cooling coils carry water chilled by refrigerant coils from a nearby compression or absorption chiller. Electrically controlled dampers exhaust a percentage of air from the return ductwork, and admit an equal amount of fresh air from outdoors to ventilate the building. A single thermostat controls the entire system.

In the 1990s, single-duct, variable air volume (VAV) systems (Fig. 26-3) became the most popular. Each zone has its own thermostat that operates a damper. The dampers at terminal outlets control the flow of conditioned air to the local ductwork in that zone according to temperature requirements for each zone or space.

Single-duct VAV systems are the most common system for new institutional and office buildings where precise control is not critical. Older buildings with less

Figure 26-3 Single-duct variable air volume (VAV) system.

efficient systems can be converted to VAV to reduce energy consumption. The single duct takes up less space than multiple-duct systems. Single-duct VAV systems work better for stable interior spaces than for more variable perimeter zones. They do not work well where the internal zones generate a lot of heat. Fan-powered VAV systems use individual units that can supply heat while the main supply system is providing cooling elsewhere, and these systems improve ventilation and circulation of air. They work well with the variations in temperatures at less stable perimeter zones. The fan draws air from the ceiling or floor plenum, heating it as required. When new, cool outdoor air is reduced to a minimum, energy is saved but air quality suffers.

Terminal reheat systems offer more flexibility in meeting changing space requirements. A single duct supplies a central air stream at around 13°C (55°F) to terminals with electric or hot water reheat coils. The reheat coils regulate the temperature of the air to each individually controlled zone or space. Where water is used in the reheat coils, the system must circulate both the air and hot water to each zone. Most of the time, the 13°C air stream has been cooled from warmer air temperatures, and this wastes a lot of energy. Single-duct reheat systems have small duct networks that save space. Terminal reheat systems are more expensive to install and operate than CAV or VAV systems. They are restricted by codes and by American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) standards to use in critical areas like laboratories, electronics factories, and hospital operating rooms.

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