A common humidity problem of direct-expansion systems can be resolved by having two coils in the system on separate refrigerant circuits. The system is controlled by a two-stage thermostat. One coil is activated first. If the room temperature cannot be decreased to setpoint, the second coil is activated. This allows further control of the supply-air temperature than is possible with a simple DDX control system serving both coils.
This is a standard arrangement with water-source heat-pump systems and other units with multiple compressors (for redundancy and staging). Many such systems have split coils, wherein half of the air passes across each coil surface. This works fine for most applications of heating and cooling if the supply-air temperature is measured after full mixing of the two airstreams. If the system is designed to dehumidify, however, split coils will not work well because only half of the air is dehumidified. Such systems work much better in the two-stage arrangement with the coils in parallel across the airstream.
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Excerpt from Energy Conservation Projects.