Humidistat control not working

asked 2018-11-20 10:30:17 -0600

tanushree
63 ●2

updated 2019-01-22 14:19:57 -0600

Hi, I have an AHU unit with Reheat and a setpoint manager for the cooling coil that is set to 'single zone humidity maximum' in DesignBuilder (between 0.002 to 0.02 humidity ratio). At the zone level, I have a humidistat control that has a de-humidifying RH setpoint schedule for 50% always. However from my simulation results, there is no dehumdification in the zones, as the RH is more than 50% for all summer months.

I was wondering how I could correct the humidity control within the zones. I tried reducing the timestep of the simulation to 10.

edit retag flag offensive close merge delete

add a comment

answered 2018-11-20 11:02:01 -0600

rraustad

13804 ●79 ●12 http://www.fsec.ucf.edu/

You have to describe your AHU. Some can control humidity, others cannot. Are you using AirloopHVAC:UnitarySystem? ZoneHVAC:FourPipeFanCoil? etc. Those systems that can control humidity will also require a setting to be changed. For example, for CoilSystem;* there is an input called Dehumidification Control Type which can be set to None, MultiMode, or CoolReheat. Most HVAC models will use this type of control (and input).

edit flag offensive delete link

Comments

It is an AHU system with Reheat that is serving VAV air terminal units- ZoneHVAC:SingleDuct:VAV:NoReheat. The controltype includes a setpoint manager for the cooling coil set to single zone humidity maximum at the AHU level. The AHU is fed by a district chilled water and hot water loop. At the zone level a humidistat with dehumidication at 50%RH is added. The actual building contains a Desiccant wheel for moisture removal. The humidistat control doesn't seem to be working since the RH is beyond 50%.

tanushree ( 2018-11-21 22:51:50 -0600 )edit

OK, so air loop water coils. You have to set the control in the Controller:WaterCoil object to control both temperature and humidity ratio.

Controller:WaterCoil,
  TemperatureAndHumidityRatio,        !- Control Variable

Then when zone RH > 50 look at the water coil and check if the water node water flow rate is at the max. If at the max, then no more dehumidification is possible (i.e., a larger cooling coil would be needed to get more dehumidification).

rraustad ( 2018-11-23 12:40:20 -0600 )edit

I am using the OS 2.7.

There is DX with humidity control, but there is no where that we can select the dehumidification control type, so when I open the in.idf. I can see that nothing is selected for Dehumidification control type, right? does it make it the DXwithdehumidification control only behave as light a simple DX?