3.1.3 Humidity control — dehumidification, sub-cooling, and reheat to SAT

When working with a humid climate, it’s important to understand that the pre-defined systems are set up so that any zone with excessive RH can vote for a cooler off-coil temperature (down to the user input for Cooling coil min LAT) to dry the air at the AHU. If the System AHU Heating coil LAT value is set lower than the Cooling coil min LAT, this will for immediate reheat at the AHU when, and only when, the cooling coil LAT is driven below the heating coil LAT by demand for dryer air.

·       The controller MC2: Cooling SAT reset per zone dehumidification demand for each zone votes on the system AHU cooling coil LAT with respect to zone humidity, and the default setting is a 55% RH midband with 10% bandwidth, such that this controller will vote for the min cooling coil LAT if RH in the zone it is attached to gets up to 60% or higher. The Max. Percent Saturation (%), which is the top end of the zone RH control band, assuming a 10% bandwidth, is set in the Space Data or Thermal Template for each zone in the model (within Apache Thermal view) and this value is picked up by the Loads Data spreadsheet at the time of zone-level sizing. You can also adjust the RH midband value in the Loads data spreadsheet for the system or within the controller itself (if you do the latter, you might want to remove the “MC2:” from the controller reference name to avoid overwriting manual changes with subsequent updates from the spreadsheet.

·       For cooling and dehumidification controls, it’s helpful to understand how the System Parameters inputs are used in the controls for cooling temperature reset and dehumidification at the AHU.

Example

In the following example from the System side of the System Parameters dialog, the inputs are set so that:

·       AHU cooling coil LAT can be as low as 50°F for dehumidification purposes; however, the cooling coil LAT will be constrained to not less than the AHU Heating coil LAT when not driven to a lower value specifically for the purpose of dehumidification.

·       AHU heating coil LAT will be 54°F. Because this determines the minimum supply air temperature leaving the AHU, this sets the lower bound for the AHU cooling coil LAT, except when overridden for dehumidification, thus forcing immediate reheat at the AHU only when justified by need to dry the air. (This parameters should really come before the Cooling SAT reset in the dialog.)

·       Design cooling SAT for calculating zone airflows will be 55°F, and this independent of setting for coil LAT values, which can be lower if desired for dehumidification and/or cooling supply temperature allowing for heat pickup in the ductwork. In this example, setting this value at 55°F and the AHU Heating coil LAT to 54°F suggests that you anticipate at least 1°F temperature rise in the ducts when the AHU is at its minimum SAT. For this, ductwork heat pickup should be enabled by indicating (within the Ductwork Heat Pickup component on the system) an approximate duct U-value, surface area, and location (e.g., a return plenum zone) for the supply ducts. When duct heat gain/loss is not modeled, the Design Cooling SAT is normally set the same at the AHU Heating coil LAT, assuming this is the coldest air temperature that will ever be supplied to the zones.

·       The cooling SAT reset of 10°F will allow the SAT at the AHU to be raised up to 10°F above the Design value when all zones on the system are at or below the zone Cooling setpoint. In this example, this value would allow an SAT up to 64°F leaving the AHU. If the assumed minimum heat gain is present in ductwork, this would result in an SAT of 55 to 65°F at the zone terminal unit. (This parameters should really come last among these four in the dialog.)

 

These four parameters will be more logically arranged in a future version. A display-only field will also be added to indicate the Min Cooling SAT from the AHU (uneditable) as the Min Cooling Coil LAT for zone cooling purposes that is coordinated with and thus determined by the AHU heating coil LAT.

 

 

A subsequent release will introduce revised means of translating system setpoints to controller inputs, and this will include user input parameters for control throttling range. This will facilitate modeling unusually tight control of space temperatures without having to manually customize the HVAC profiles.