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Humidistat control not working

asked 2018-11-20 10:30:17 -0500

tanushree gravatar image

updated 2019-01-22 14:19:57 -0500

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.

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answered 2018-11-20 11:02:01 -0500

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).

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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 gravatar imagetanushree ( 2018-11-21 22:51:50 -0500 )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 gravatar imagerraustad ( 2018-11-23 12:40:20 -0500 )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?

s_pourm gravatar images_pourm ( 2018-12-20 13:05:35 -0500 )edit

There may be a measure that can set this parameter?

rraustad gravatar imagerraustad ( 2019-01-03 09:17:03 -0500 )edit

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Asked: 2018-11-20 10:30:17 -0500

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Last updated: Nov 20 '18