Scheduled %RH with simple HVAC settings? (DesignBuilder)

asked 2024-07-12 05:24:50 -0600

FabioFH97
41 ●2

updated 2024-07-13 07:49:28 -0600

Hello, My task is to find out, if Design Builder is capable of correctly simulating the hygric and thermal behaviour of certain building materials. For this reason, I am trying to recreate a real-life experiment in Design Builder. To do this, I need to be able to control the temperature and humidity in a room. The temperature should remain constant at 23°C, while the relative humidity should be 45% for 48 hours and then suddenly rise to 95% for an hour. The humidity supply should then be stopped. What is the best way to do this? Until now, I have only been able to control the humidity via the heating and cooling system. However, as this has to work to maintain the 23°C, the humidity supply also changes. So far, I have been trying to make this work with the simple HVAC settings. Is implementing such a schedule even possible with these settings, or do I have to change them to Detailed HVAC?

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answered 2024-07-12 15:08:07 -0600

Denis Bourgeois
2981 ●111 ●8 http://www.rd2.ca

updated 2024-07-13 06:27:16 -0600

Not a DB user: just a few questions and hints ...

By "real-life experiment", do you mean a closely-controlled lab experiment akin to a guarded hot box setting (e.g. no equipment gains, minimal envelope gains)? It will likely be harder to maintain constant temperature and humidity in a more conventional setting. And in either case, I suspect "constant" is short for a narrow control of heating/cooling: 23°C +/- 0.5°C (or similar)? Same for relative humidity control?

By controlling "humidity via the heating and cooling system", have you added AHU humidity control (based on EnergyPlus' de/humidifiers and suitable humidistat control)? You can certainly define minimum (humidifying) and maximum (dehumidifying) relative humidity setpoint schedules. That's the first thing I'd like into (EDIT: applicable only for Detailed HVAC mode).

If ~instantaneously achieving a relative humidity of 95% (!) is beyond the scope of the humidifier model, then my second thought would be to explore Process Gains, which harnesses one of EnergyPlus' Equipment classes (not sure which one), e.g. Electric, Steam or Other. You would want to maximize the latent fraction, minimize the radiant fraction and zero the fraction lost. You can associate a schedule to such process gains to match the sudden humidification/dehumidification events you're seeking to model.

You'll likely need a few trial-and-errors iterations to nail down the expected behaviour.

EDIT: Based on your follow-up comments, I suggest the following edits to your initial question:

The challenges you're facing with humidity control have more to do with setting the right variables in DesignBuilder than having to deal with a virtual test bed setup, or even temperature control. I suggest you revise the title of your question, e.g. "Scheduled %RH? (DesignBuilder)". Hopefully this will attract (more) experienced DesignBuilder users, maybe even DesignBuilder developers.
If you run an UMH search for "humidity designbuilder", you'll come across quite a few relevant posts. I suspect a few are worth the read.
From your description, I take it you've opted for Simple HVAC (i.e. IdealLoadsAirSystem) - not Detailed HVAC (plant/loop)? This should be made very clear in your initial question, even your title e.g. "Scheduled RH% : DesignBuilder (SimpleHVAC)?". The available options and subsequent steps to follow for humidity control are quite different between both options.
From the previous link and the following, I take it that one can't schedule de/humidification setpoints using the Simple HVAC option: one can only define a constant minimum %RH for humidification and a constant maximum %RH for dehumidification. That may be why DesignBuilder doesn't import the de/humidification schedules you've provided in your edited IDF. If you upgrade to Detailed HVAC, you should be able to schedule both de/humidfication setpoints.

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Thank you for your answer

Yes, the experiment was conducted under the conditions that you describe. About the schedules, I defined these in the IDF file. The schedules for heating and cooling work, but the schedules for humidification and dehumidification are being overwritten by Design Builder when I import the IDF file. And in Design Builder I don't see a possibility to create a schedule for the humidification process. I'm new to this software, so I try to keep the whole model as simple as possible. That's why I don't use a HVAC template, for example. Also getting to 95% isn't the problem.

FabioFH97 ( 2024-07-13 03:45:43 -0600 )edit

I edited my initial answer, based on your follow-up comments.

You've posted a related question. For both questions, I suggest you clearly state whether you're opting for Simple HVAC (limited humidity settings) or Detailed HVAC (flexible humidity control).