A single-zone AHU is intended to serve only one room, or a group of rooms which are contiguous and which have similar load and exposure characteristics. The maximum area served by a single-zone AHU should not exceed 10,000 ft2.
The typical single-zone AHU arrangement is shown in Fig. 11.3. This is a draw-through system, with the heating coil in the preheat position to protect the cooling coil from freezing air.
It is important to sequence the operation of the control valves to avoid simultaneous heating and cooling. When one or more of the rooms served by a single-zone AHU has a load characteristic different from the other rooms, zone reheat must be provided by means of coils in the zone branch ducts, by radiation, or by fan-coil units.
Because reheat is potentially energy wasteful, it may be preferable to use a different type of AHU, as described.
A single-zone unit may be used to control humidity in the room. The cooling coil precedes the heating coil, which is therefore in the reheat position.
Humidity control always requires additional energy—as reheat or in other ways. The cooling coil valve is controlled by either the space temperature or the space humidity, whichever creates the greater demand.
If humidity controls, the temperature will tend to fall and the space thermostat will control the heating coil valve to provide reheat. The humidifier is used when required.