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UFAD modeling best practice?

asked 2015-02-24 12:11:18 -0500

updated 2015-11-12 15:32:03 -0500

I am modeling a UFAD system in IES-VE. IES has the capability of modeling several zones, so I have followed the ApacheHVAC Manual Appendix G: UFAD Systems guidance to have a common supply air plenum, a 6' high occupied zone, a stratified zone, and a common return air plenum. The guide also recommends using a 55F coil SAT, but modeling heat gain through the supply plenum by raising the temperature of air coming from the diffusers. My question has a few parts: 1) is this zoning method the best practice 2) is accounting for heat gain through the plenum necessary and 3) do most UFAD systems use 55F SAT at the coil or an elevated SAT at the coil? Any suggestions of the best approach for splitting up the zone and the supply air temperature setpoints would be appreciated, or references that outline the approach.

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I'd like to clarify: I have had no issues completing my model and have not gone into great detail on how it is modeled in IES because I am looking for more generic feedback on the best approach regardless of software type. Thanks.

Anna Osborne Brannon gravatar imageAnna Osborne Brannon ( 2015-03-02 10:21:48 -0500 )edit

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answered 2015-03-01 14:06:02 -0500

crduggin gravatar image

I have recently modeled UFAD in the VE. When the user guide refers to raising the SAT for account for the SA plenum, it is actually telling you to change the SAT used in the spreadsheet to calculate the required airflow. Not the SAT on the coil controls. It's a bit of an iterative process to get the airflow and SAT just right. Run it with the standard 55F SAT. Then figure out what the actual SAT to the zone is after it goes through the SA plenum. Then you can adjust the SAT value in the Room Design Airflows sheet of the spreadsheet, which will increase the airflow to the zones. You may need to repeat the process if the increased airflows significantly changes the resulting SAT to the zone.

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Thanks for the feedback, but I am modeling 55F SAT, and am already adjusting the SAT in the spreadsheet to simulate higher temperature at the diffuser as the guide says. My question is whether this is actually best practice that most modelers use, or best system design?

Anna Osborne Brannon gravatar imageAnna Osborne Brannon ( 2015-03-02 10:32:19 -0500 )edit

It is what i do, but i did it for the same reason you did (the user guide said to); however, it does make sense to me though. One of the main parts of modeling UFAD is to account for both the thermal mass and heat gain/loss effect of the SA and RA plenums. It seems this approach does a good job of that.

crduggin gravatar imagecrduggin ( 2015-03-04 10:12:57 -0500 )edit
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answered 2015-02-24 16:22:55 -0500

vasiliy gravatar image

Well, the best way to model UFAD system that I know is using the EnergyPlus. The two main differences between conventional systems and UFAD systems, are plenum thermal decay and the room air stratification. The thermal decay in the plenum or the difference between discharge air temperature after the cooling coil and the temperature of air leaving the diffuser can be as high as 10F depending on the construction and operational parameters.

Simply increasing the supply temperature will not model many actual system behaviors that are present. First the peak load will be underestimated since portion of the heat gain in the plenum goes into the slab below the raised floor instead of the zone. Some of the cooling from plenum decay that does make it into the zone will not contribute to cooling the zone, but will exchange heat with the ceiling via radiation. Both of those behaviors can be beneficial or harmful to energy consumption depending on operational parameters of the system, such as minimum flow rate.

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answered 2015-02-24 20:30:22 -0500

nkegel gravatar image

updated 2015-02-25 08:27:08 -0500

I'd encourage you to submit your question to support@iesve.com or visit the user forums over at iesve.com

There you'll find the best answers to your questions - and many FAQ's about how to model certain system types.

You'll note that the VE currently has several UFAD and displacement ventilation prototypes in it that you can start with and modify as needed. All the prototypes include the necessary components for accurate UFAD/DV modeling including stratification of the zones, variation of where the load is, detailed solar load assignment at each time step, etc.

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Nathan- I do utilize all of those resources- I frequently email support. I also am using a prototype system and have successfully completed my model. What I am looking for is feedback on IES' documented procedure and opinions on the best approach. Thanks!

Anna Osborne Brannon gravatar imageAnna Osborne Brannon ( 2015-03-02 10:20:34 -0500 )edit

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Asked: 2015-02-24 12:11:18 -0500

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Last updated: Mar 02 '15