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Supply-air temperature

As mentioned previously, for an entire latent load to be handled, supply air must be dehumidified to a dew point lower than that of the space. Designers must decide whether to supply air off of a cooling coil at “space-neutral” or “cold-air” conditions. With the former, dehumidified air is reheated to room conditions to introduce fresh air into a space without affecting thermal loads, and all of the sensible cooling done during dehumidification is wasted. With the latter, the air coming from a unit is left at the temperature coming off of the cooling coil, and the outdoor air is able to handle some of the sensible cooling in the space. By decreasing the sensible loads in the space, the cooling capacity and annual energy consumption of the local HVAC unit are reduced significantly. In the case of a typical wing of classrooms in a school, supplying cold, rather than space-neutral, air reduces local-HVAC-unit size by up to 40 percent.⁴

Supply-air-delivery location

Designers must decide whether to deliver outdoor air directly into individual spaces or into the local HVAC unit.

Delivering air directly into a space is the more economical option and the easier way to verify the required volume of outdoor air is being supplied. Also, it has a greater effect on thermal loads. Because a local HVAC unit needs to be energized only on a call for cooling or heat from a space, it does not need to be running when there is no call for tempering. This can greatly reduce the fan-energy consumption of the unit. Additionally, the unit will need to be sized only for the air volume required to satisfy the space sensible load, rather than the entire supply-air volume, saving on first cost, reducing physical size, and potentially reducing sound levels.

When outdoor air is ducted directly to a local HVAC unit, the cooling capacity of the unit can be decreased, but the unit needs to be sized to handle the entire supply-air volume. In such cases, it is highly recommended that the dedicated outdoor-air unit not reheat the outdoor air to avoid the local unit having to cool air that already has been cooled once.

Control strategies

When a building is occupied, a dedicated outdoor-air unit is energized and supplying conditioned outdoor air to a space. Local HVAC units are energized upon a call for cooling or heat from the space. This allows the precise volume of outdoor air to be delivered independent of space cooling or heating status. Several measures can be implemented to further enhance system performance:

• Resetting discharge temperature based on outdoor air

  When outdoor-air conditions are such that overcooling is likely, supply air is reheated to a neutral temperature. This can be accomplished using a microprocessor controller that monitors ambient temperature and resets the discharge temperature of the outdoor-air unit accordingly. A more advanced approach is to implement a building-management system (BMS) that can monitor multiple spaces and determine the “critical space.” Once the critical space is determined, the BMS can adjust the reheat capacity of the outdoor-air unit to prevent overcooling or overheating of the critical space.

• Implementing a dehumidistat

  If a room becomes too humid, the dehumidistat sends a signal to the microprocessor controller that more dehumidification is needed. The controller then lowers the discharge dew point of the outdoor-air unit. Once the room humidity reaches an acceptable level, the dew point is reset to normal operation.