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Relative Humidity control in DesignBuilder

asked 2021-07-21 02:02:37 -0500

Debayan93's avatar

updated 2021-07-22 02:08:15 -0500


I am new user of DesignBuilder (v 6.1). For a project, I am trying to identifying suitable HVAC systems from predefined templates in DesignBuilder. The conditioned space is a single zone archive where temperature and humidity needs to be controlled (16-20°C and 45-55%). The idea is to understand which HVAC systems can realistically achieve closer to this targets. However, I tried with following HVAC template (Fan Coil Unit, 4 pipe, Air cooled Chiller) as shown below: FCU

However, the system fails to control the Relative humidity despite the humidity control is activated in "Activity" Tab with appropriate values.


I was wondering if such space can be conditioned with any specific localized humidification control system in the zone.

Edit1: HVAC diagram of Unitary Heat Cool system after adding Humidifier in AHU

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answered 2021-07-21 06:57:16 -0500

mdedoes's avatar


I don't believe DesignBuilder can do humidity control with only an FCU in the zone. Usually it requires an Air Loop with setpoint managers to control humidity. So, to simulate a full recirculating system like this, just be sure to set all your outside air values to zero (at both the Air Loop and AHU). I always find the humidity control takes a little trial and error to get it to work how you want, but the following link to DesignBuilder's online help walks you through the base process pretty well.


I'd recommend referring to the online help as you work through other issues in DesignBuilder. I've been a user for about 3 years and I still refer to it regularly.

Hope this helps.

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Thanks mdedoes, it seems like that all-air HVAC systems are more suitable for humidity control. I tried one of the solution (VAV with Reheat) which worked.

However, I am trying to make a list of different type of HVAC systems which can condition closer to the setpoints. I was wondering if other system which doesn't include an air loop (radiative system like heated floor, chilled ceiling or PTAC) can be modified for such control with additional adjustments or combination of different equipment.

Let me know if you have other idea like a local humidification control in zone.

Best, Debayan

Debayan93's avatar Debayan93  ( 2021-07-21 09:05:36 -0500 )edit

Any type of radiant system will only help meet sensible loads, so you'll still need another system for humidity control. Sometimes separating these two can be a good strategy.

From a design perspective, I think your options for humidification are more limited with a PTAC or FCU, rather than a dedicated air handler with a built-in or duct-mounted humidifier.

However, to test these systems for your modeling exercise, I would recommend creating an air loop to act how you want your PTAC/FCU to operate, just because DesignBuilder offers more humidity control options with an air loop.

mdedoes's avatar mdedoes  ( 2021-07-21 09:24:21 -0500 )edit

Thanks mdedoes, I will try out the air loop addition to FCU/PTAC solution.

For the first point, can you let me know which additional humidity control system can be added along with a radiant system (ex: heated floor, boiler hw, nat vent in DB).

Debayan93's avatar Debayan93  ( 2021-07-21 09:50:02 -0500 )edit

You'd need an air system to go along with any radiant-type system (heated floor, chilled ceiling, etc.). Depending on your plant equipment (boilers, chillers, etc.), you could do chilled water cooling with hot water reheat like you mentioned, DX cooling with hot gas reheat, or a different combination. But you're not going to be able to get away from having to wring the water out of the air stream and then reheat it for dehumidification and add water to the air stream for humidification.

mdedoes's avatar mdedoes  ( 2021-07-21 10:07:33 -0500 )edit

Thanks mdedoes, I tried humidity control solution of Unitary Heat Cool system from

However, after doing all the steps it produces the following error: "Humidifier missing setpoint manager: Air Loop Unitary HeatCool Steam Humidifier". I added the HVAC system in the original question.

Let me know if you know this.

Debayan93's avatar Debayan93  ( 2021-07-22 02:01:11 -0500 )edit

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Asked: 2021-07-21 02:02:37 -0500

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Last updated: Jul 22 '21