Humidity Control

It is usually easiest to select an appropriate HVAC Template as the starting point when setting up humidity control for Detailed HVAC systems. However in some cases you may need to define humidity control manually and this page describes the steps involved.

 

The process depends on the base system type. The instructions below are for the Simple HVAC activity data option. When the Detailed HVAC activity data is used the process is similar but humidistat setpoints come from schedules defined on the HVAC zone dialog instead of setpoint values defined on the Activity tab and demand schedules on the HVAC zone dialog.

VAV with Reheat

To add humidification/dehumidification control to a VAV system, follow these steps:

Humidification

  1. Add a humidifier to the AHU, typically as the rightmost component in the AHU (nearest to the supply end).
  2. Set any sizing and capacity parameters on the humidifier dialog.
  3. On the supply loop level of the air loop add a setpoint manager immediately downstream of the humidifier and edit the setpoint manager type to use one of the "humidity minimum" setpoint managers. For example the 6-Multi-zone humidity minimum controller calculates the supply air minimum humidity ratio based on the zone with the highest humidity ratio setpoint to maintain the zone RH levels near their respective humidifying setpoints. Setting up the setpoint manager in this way ensures that air leaving the humidifier has a minimum level of humidity required to meet the setpoint in the zone with the lowest RH.The screenshot below illustrates the components involved.



  4. When using one of the "humidity minimum" controllers you must switch on the humidistat option on the HVAC zone dialog and define RH setpoint schedules. Humidistats can provide a fine degree of humidification control within each zone. To define RH setpoint schedules enter the zone Humidification setpoint model data and Humidifying demand schedule on the HVAC zone dialog (when using Simple HVAC Activity Data) or the HVAC zone Humidifying RH setpoint schedule (when using Detailed HVAC Activity Data).
  5. An alternative approach which does not provide close control of zone humidity is to use a 1-Scheduled Setpoint manager type and the 6-Minimum humidity ratio Control variable. This simply ensures that the supply air has a minimum RH. In this case the HVAC zone humidifying setpoint settings are not used.

Dehumidification

  1. On the AHU cooling coil dialog, to control both temperature and humidity set the Controller control variable to 3-Temperature and Humidity Ratio, or if no temperature control is required, select the 2-Humidity Ratio option.
  2. Where a cooling coil is to conduct both temperature and humidity control the temperature and humidity Setpoint managers must both be located on the same node. Assuming that the default temperature Setpoint manager is already placed downstream of the AHU, add a new setpoint manager downstream of the AHU and edit the setpoint manager Type to use one of the "humidity maximum" setpoint managers. For example the 7-Multi-zone humidity maximum controller calculates the supply air humidity ratio based on the zone with the lowest humidity ratio setpoint to maintain the zone RH levels near their respective humidity setpoints. Setting up the setpoint manager in this way ensures that air leaving the cooling coil has a maximum level of humidity, but on its own does not provide close control humidity in the zones. The
  3. To provide a finer degree of dehumidification control within each zone, you must switch on the humidistat option on the HVAC zone dialog and define any time-varying demand schedules.
  4. For all dehumidification systems currently supported by DesignBuilder it is essential to include some form of zone reheat to ensure that "overcooled" air from the AHU does not cause the zone to be overcooled. The zone reheat can be a standard reheat ADU or it can be some other form of zone heater such as a convector or radiator.

 

Note: In cases where a VAV system uses a water cooling coil for dehumidification in the AHU , it is sometimes not possible to control upper zone humidity levels accurately. This situation is more likely under low load conditions when the VAV terminal box dampers are at minimum position and the water cooling coil cannot drop the air stream temperature to the level required for high humidity control. In such cases you may find that swapping the water coil for a Cooling Coil - Two Stage with Humidity Control DX helps because the lower coil temperatures and humidity control capability of this DX coil allow it to drop out more moisture from the air stream, helping to maintain the zone maximum humidity setpoint. Another possible solution to this problem is to increase the minimum flow fraction for the ADU to a higher than normal value , e.g. 0.5. This allows a greater volume of dry air from the AHU to be introduced into the zones, even when sensible loads are low.

 

The Humidification and Dehumidification set ups described above can be used together to achieve full humidity control.

CAV with Reheat

The procedure for setting up humidity control in CAV systems is very similar to that described for VAV systems above, except that there is less likely to be issues with achieving dehumidification setpoints during times of low sensible load. This is because there is no damper to reduce flow at the terminal unit so zones get access to the full flow of air from the AHU.

Unitary Heat Cool and Unitary Heat Pump Humidity Control

To add humidification/dehumidification control to a unitary heat cool system, follow these steps:

 

  1. For each zone supplied by the unitary heat cool AHU, switch humidity control on by checking the Humidistat control check box on the HVAC zone dialog and define any time-varying demand schedules.

Dehumidification

  1. Go down to the air loop supply sub-loop and edit the AHU, changing the Dehumidification control type to 2-CoolReheat.
  2. Add a Maximum humidity setpoint manager downstream of the AHU (5-Multi-zone maximum humidity average or 7-Multi-zone humidity maximum or 9-Single zone humidity maximum).

Humidification

  1. Add a Humidifier component to the AHU.
  2. Add a Minimum humidity setpoint manager immediately downstream of the Humidifier (4-Multi-zone minimum humidity average or 6-Multi-zone humidity minimum or 8-Single zone humidity minimum).