Comm/Proto Hospital Reference Building: Inaccurate energy use due to unrealistic geometry?

asked 2017-10-10 10:12:38 -0500

jfirrantello's avatar

updated 2018-07-02 12:44:29 -0500

EDIT 10/12/2017 I am going to leave this post here in its entirety, but I do realize that my "question" is probably outside the intent of a StackExchange-type setup (to my understanding, non-speculative, hard technical questions with answers that can be well-defined). Thank you, mods, for humoring me. This is possibly part of a larger set of questions that may be more discussion oriented, and I will choose a more appropriate general discussion venue for that.

I have been looking into using the Hospital Commercial/Prototype Reference Buildings to get rough ideas of energy conservation measure savings potentials. I am noticing some things that seem inaccurate, even incorrect, based on what I’m seeing in actual hospitals.

Hospital ACH-Based Air Flows

Hospital spaces related to healthcare (e.g., patient rooms, not general offices) tend to follow OA ACH and total ACH requirements established by ASHRAE Standard 170 (formerly published as AIA guidelines, currently referenced verbatim in FGI-2014). Minimum total ACH requirements can contribute heavily to fan, cooling, and reheat energy due to requiring more air than is needed for comfort conditioning and ventilation. For energy use estimation, this means that both the ACH required for the space type and space geometry need to be correct.

For an example, I will use room PatRoom2_Flr_3 as used in both the New Commercial Reference Buildings 90.1-2004 and Commercial Prototype Buildings 90.1-2013.

Ceiling (Floor-to-Floor) Height = 4.2683 m (14 ft)
Volume = 148.71 m3 (5,247.6 ft3)
Confirmed for both versions either through geometry entries or explicit definition

6 ACH has been the minimum total ACH for patient rooms for some time now (this changed recently to 4 ACH for “general” patient rooms, but that is not specifically related to this issue). Using the geometry mentioned above:
Minimum Flow - 90.1-2004 = 0.24785 m3/s = 6 ACH
Minimum Flow - 90.1-2013 = 0.007111113 m3/(s-m2) = 6 ACH

Similarly, both have 2 ACH of OA specified, the correct amount for a patient room. So far, everything looks good.

The issue is the ceiling height. I don’t have years of hospital design experience, but I've been working with ACH calculations for multiple hospitals over the past 6 months. I have yet to see any situation where there is no dropped ceiling. From what I've seen, ceiling heights tend to range from 9 ft to 10 ft for all of the space types where ASHRAE 170 is applicable. 14 ft ceiling height vs 9 ft or 10 ft can make a significant difference in OA ACH and total minimum ACH.

Other spaces in the Hospital Reference Buildings with ACH requirements appear to define airflow in the same way, using the ceiling (floor-to-floor) height, which is 14 ft in all levels except the basement.

In short, the question is:

Are all of the implementations of the Hospital Reference Building over-estimating fan, cooling, and reheat energy use due to an unrealistic representation of room volume?

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You didn't tag this as an OpenStudio question, but if you're using the OpenStudio Measure to generate your prototype model you can essentially generate a "prototype" with customized geometry using this Measure instead. The difference between the two Measures is the geometry.

ljbrackney's avatar ljbrackney  ( 2017-10-10 12:53:41 -0500 )edit

You're correct that no ceiling height is defined in the thermal zones, and the hard entered ACH airflows (converted to m3/s or CFM) equate to 2 and 6 ACH for the entire zone volume at 14 feet. I've never seen a patient room with this ceiling height, most are at 9 ft or 10 ft as you mention. I would agree with you that this is overestimating the ventilation required based on an unrealistic zone height and volume.

Lyle K's avatar Lyle K  ( 2017-10-10 14:48:41 -0500 )edit

Thank you both for your comments.

Larry: Currently I have just been using vanilla EnergyPlus. Almost zero experience with OpenStudio, but I've been meaning to check it out for rough evaluation of energy efficiency measures (current work mainly involves capital/controls upgrades at large facilities, so there is no opportunity for a detailed energy model). I was completely unaware of the functionality you've mentioned, and it seems like it may be in line with what I'm looking to do.

Lyle: Thanks for the check on my math, it's appreciated.

jfirrantello's avatar jfirrantello  ( 2017-10-12 09:17:28 -0500 )edit