Most HVAC systems cool air to a space temperature of 75°F or thereabouts, with a temperature leaving the coils of 55°F. This has been the standard for many years, though now systems are being developed to cool air even lower. There are potential problems with this new system; fortunately, they are design rather than equipment problems. A qualified consultant can build a proper cold-air system using off-the-shelf equipment that operates under a new set of parameters.
The advantages of the cold-air system are: (1) the ability to use smaller air handlers, fan motors, pumps, and ductwork; (2) a reduction in electricity use by these smaller motors; (3) the need to have less ceiling space for the smaller ductwork, a savings in building steel and other construction materials.
Another advantage is that distribution of air at cooler temperatures requires a six- or eight-row cooling coil at the air handler, versus the usual three- or four-row coil for conventional units. The multi-row coil extracts much more water from the air, thus dehumidifying the air with each pass. Finally, air at a lower humidity can be kept a few degrees warmer without noticeable effect to the occupants, so additional savings are available by increasing the room thermostat setpoint.
Another bonus is that dryer air tends to cause fewer indoor air quality problems. When humid air returns to the coils, where the water is extracted, the air is purified of many contaminants, much as a fresh rain clears the atmosphere. Also, dry air is less receptive to molds and spores that can grow in the ductwork and otherwise inhabit the conditioned space of the building.
One final requirement applies to using this low-humidity system in hot and humid climates. In such weather, the high outside humidity migrates into the building, sometimes condensing in outside walls or beneath wallpaper or other impermeable wall coverings. This migration increases along with the humidity difference between the conditioned space and the outside, so maintaining a dry environment inside can lead to problems in older buildings with a below-par vapor barrier. A variable air volume system will pre-treat air before ducting it to each zone control box. On a signal from the zone thermostat, the two-way valve opens to supply hot water to the coil in the VAV box.
The purpose served by the heating coil in the VAV box in the summer is to allow a single air handler to serve a diversity of zones requiring heating and cooling. The air handler delivers cool air to the entire distribution system, while the reheat coils temper the air to interior zones that have a lower cooling load. In the winter, the air handler delivers air at a relatively low temperature, which is then heated at the remote coils to satisfy the high heating load for exterior zones.
Another useful purpose of reheat coils is dehumidification during the cooling season. If a humidistat in the conditioned space served by a VAV box calls for dehumidification, the heating coil will be activated. Warmer, drier air will be delivered to the space. Subsequent air changes will cool the space to the setpoint of the thermostat.
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Excerpt from Energy Conservation Projects.